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<H1><a name="Ruby"></a>27 SWIG and Ruby</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Ruby_nn2">Preliminaries</a>
<ul>
<li><a href="#Ruby_nn3">Running SWIG</a>
<li><a href="#Ruby_nn4">Getting the right header files</a>
<li><a href="#Ruby_nn5">Compiling a dynamic module</a>
<li><a href="#Ruby_nn6">Using your module</a>
<li><a href="#Ruby_nn7">Static linking</a>
<li><a href="#Ruby_nn8">Compilation of C++ extensions</a>
</ul>
<li><a href="#Ruby_nn9">Building Ruby Extensions under Windows 95/NT</a>
<ul>
<li><a href="#Ruby_nn10">Running SWIG from Developer Studio</a>
</ul>
<li><a href="#Ruby_nn11">The Ruby-to-C/C++ Mapping</a>
<ul>
<li><a href="#Ruby_nn12">Modules</a>
<li><a href="#Ruby_nn13">Functions</a>
<li><a href="#Ruby_nn14">Variable Linking</a>
<li><a href="#Ruby_nn15">Constants</a>
<li><a href="#Ruby_nn16">Pointers</a>
<li><a href="#Ruby_nn17">Structures</a>
<li><a href="#Ruby_nn18">C++ classes</a>
<li><a href="#Ruby_nn19">C++ Inheritance</a>
<li><a href="#Ruby_nn20">C++ Overloaded Functions</a>
<li><a href="#Ruby_nn21">C++ Operators</a>
<li><a href="#Ruby_nn22">C++ namespaces</a>
<li><a href="#Ruby_nn23">C++ templates</a>
<li><a href="#ruby_cpp_smart_pointers">C++ Smart Pointers</a>
<li><a href="#Ruby_nn25">Cross-Language Polymorphism</a>
<ul>
<li><a href="#Ruby_nn26">Exception Unrolling</a>
</ul>
</ul>
<li><a href="#Ruby_nn27">Input and output parameters</a>
<li><a href="#Ruby_nn29">Typemaps</a>
<ul>
<li><a href="#Ruby_nn30">What is a typemap?</a>
<li><a href="#Ruby_nn31">Ruby typemaps</a>
<li><a href="#Ruby_nn32">Typemap variables</a>
<li><a href="#Ruby_nn33">Useful Functions</a>
<ul>
<li><a href="#Ruby_nn34">C Datatypes to Ruby Objects</a>
<li><a href="#Ruby_nn35">Ruby Objects to C Datatypes</a>
<li><a href="#Ruby_nn36">Macros for VALUE</a>
<li><a href="#Ruby_nn37">Exceptions</a>
<li><a href="#Ruby_nn38">Iterators</a>
</ul>
<li><a href="#ruby_typemap_examples">Typemap Examples</a>
<li><a href="#Ruby_nn40">Converting a Ruby array to a char **</a>
<li><a href="#Ruby_nn41">Collecting arguments in a hash</a>
<li><a href="#Ruby_nn42">Pointer handling</a>
<ul>
<li><a href="#Ruby_nn43">Ruby Datatype Wrapping</a>
</ul>
</ul>
<li><a href="#ruby_operator_overloading">Operator overloading</a>
<ul>
<li><a href="#Ruby_nn45">Example: STL Vector to Ruby Array</a>
</ul>
<li><a href="#Ruby_nn46">Advanced Topics</a>
<ul>
<li><a href="#Ruby_nn47">Creating Multi-Module Packages</a>
<li><a href="#Ruby_nn48">Defining Aliases</a>
<li><a href="#Ruby_nn49">Predicate Methods</a>
<li><a href="#Ruby_nn50">Specifying Mixin Modules</a>
</ul>
<li><a href="#Ruby_nn51">Memory Management</a>
<ul>
<li><a href="#Ruby_nn53">Object Ownership</a>
<li><a href="#Ruby_nn54">Object Tracking</a>
<li><a href="#Ruby_nn55">Mark Functions</a>
<li><a href="#Ruby_nn56">Free Functions</a>
</ul>
</ul>
</div>
<!-- INDEX -->


		<div class="sectiontoc">
			<ul>
				<li>
					<a href="#Ruby_nn2">Preliminaries</a>
					<ul>
						<li>
							<a href="#Ruby_nn3">Running SWIG</a>
						<li>
							<a href="#Ruby_nn4">Getting the right header files</a>
						<li>
							<a href="#Ruby_nn5">Compiling a dynamic module</a>
						<li>
							<a href="#Ruby_nn6">Using your module</a>
						<li>
							<a href="#Ruby_nn7">Static linking</a>
						<li>
							<a href="#Ruby_nn8">Compilation of C++ extensions</a>
					</ul>
				<li>
					<a href="#Ruby_nn9">Building Ruby Extensions under Windows 95/NT</a>
					<ul>
						<li>
							<a href="#Ruby_nn10">Running SWIG from Developer Studio</a>
					</ul>
				<li>
					<a href="#Ruby_nn11">The Ruby-to-C/C++ Mapping</a>
					<ul>
						<li>
							<a href="#Ruby_nn12">Modules</a>
						<li>
							<a href="#Ruby_nn13">Functions</a>
						<li>
							<a href="#Ruby_nn14">Variable Linking</a>
						<li>
							<a href="#Ruby_nn15">Constants</a>
						<li>
							<a href="#Ruby_nn16">Pointers</a>
						<li>
							<a href="#Ruby_nn17">Structures</a>
						<li>
							<a href="#Ruby_nn18">C++ classes</a>
						<li>
							<a href="#Ruby_nn19">C++ Inheritance</a>
						<li>
							<a href="#Ruby_nn20">C++ Overloaded Functions</a>
						<li>
							<a href="#Ruby_nn21">C++ Operators</a>
						<li>
							<a href="#Ruby_nn22">C++ namespaces</a>
						<li>
							<a href="#Ruby_nn23">C++ templates</a>
						<li>
							<a href="#ruby_cpp_smart_pointers">C++ Smart Pointers</a>
						<li>
							<a href="#Ruby_nn25">Cross-Language Polymorphism</a>
							<ul>
								<li>
									<a href="#Ruby_nn26">Exception Unrolling</a>
							</ul>
					</ul>
				<li>
					<a href="#Ruby_nn27">Input and output parameters</a>
				<li>
					<a href="#Ruby_nn28">Simple exception handling </a>
				<li>
					<a href="#Ruby_nn29">Typemaps</a>
					<ul>
						<li>
							<a href="#Ruby_nn30">What is a typemap?</a>
						<li>
							<a href="#Ruby_nn31">Ruby typemaps</a>
						<li>
							<a href="#Ruby_nn32">Typemap variables</a>
						<li>
							<a href="#Ruby_nn33">Useful Functions</a>
							<ul>
								<li>
									<a href="#Ruby_nn34">C Datatypes to Ruby Objects</a>
								<li>
									<a href="#Ruby_nn35">Ruby Objects to C Datatypes</a>
								<li>
									<a href="#Ruby_nn36">Macros for VALUE</a>
								<li>
									<a href="#Ruby_nn37">Exceptions</a>
								<li>
									<a href="#Ruby_nn38">Iterators</a>
							</ul>
						<li>
							<a href="#ruby_typemap_examples">Typemap Examples</a>
						<li>
							<a href="#Ruby_nn40">Converting a Ruby array to a char **</a>
						<li>
							<a href="#Ruby_nn41">Collecting arguments in a hash</a>
						<li>
							<a href="#Ruby_nn42">Pointer handling</a>
							<ul>
								<li>
									<a href="#Ruby_nn43">Ruby Datatype Wrapping</a>
							</ul>
					</ul>
				<li>
					<a href="#ruby_operator_overloading">Operator overloading</a>
					<ul>
						<li>
							<a href="#Ruby_nn45">Example: STL Vector to Ruby Array</a>
					</ul>
				<li>
					<a href="#Ruby_nn46">Advanced Topics</a>
					<ul>
						<li>
							<a href="#Ruby_nn47">Creating Multi-Module Packages</a>
						<li>
							<a href="#Ruby_nn48">Defining Aliases</a>
						<li>
							<a href="#Ruby_nn49">Predicate Methods</a>
						<li>
							<a href="#Ruby_nn50">Specifying Mixin Modules</a>
					</ul>
				<li>
					<a href="#Ruby_nn51">Memory Management</a>
					<ul>
						<li>
							<a href="#Ruby_nn52">Mark and Sweep Garbage Collector </a>
						<li>
							<a href="#Ruby_nn53">Object Ownership</a>
						<li>
							<a href="#Ruby_nn54">Object Tracking</a>
						<li>
							<a href="#Ruby_nn55">Mark Functions</a>
						<li>
							<a href="#Ruby_nn56">Free Functions</a>
					</ul>
			</ul>
		</div> <!-- INDEX -->
		<p>This chapter describes SWIG's support of Ruby.</p>
<H2><a name="Ruby_nn2"></a>27.1 Preliminaries</H2>


		<p>
			SWIG 1.3 is known to work with Ruby versions 1.6 and later. Given the choice, 
			you should use the latest stable version of Ruby. You should also determine if 
			your system supports shared libraries and dynamic loading. SWIG will work with 
			or without dynamic loading, but the compilation process will vary.
		</p>
		<p>This chapter covers most SWIG features, but in less depth than is found in 
			earlier chapters. At the very least, make sure you also read the "<a href="SWIG.html#SWIG">SWIG 
				Basics</a>" chapter. It is also assumed that the reader has a basic 
			understanding of Ruby.
		</p>
<H3><a name="Ruby_nn3"></a>27.1.1 Running SWIG</H3>


		<p>
			To build a Ruby module, run SWIG using the <tt>-ruby</tt> option:</p>
		<div class="code">
			<pre>$ <b>swig -ruby example.i</b>
</pre>
		</div>
		<p>
			If building a C++ extension, add the <tt>-c++</tt> option:
		</p>
		<div class="code">
			<pre>$ <b>swig -c++ -ruby example.i</b>
</pre>
		</div>
		<p>
			This creates a file <tt>example_wrap.c</tt> (<tt>example_wrap.cxx</tt> if 
			compiling a C++ extension) that contains all of the code needed to build a Ruby 
			extension module. To finish building the module, you need to compile this file 
			and link it with the rest of your program.
		</p>
<H3><a name="Ruby_nn4"></a>27.1.2 Getting the right header files</H3>


		<p>
			In order to compile the wrapper code, the compiler needs the <tt>ruby.h</tt> header 
			file. This file is usually contained in a directory such as
		</p>
		<div class="code">
			<pre>/usr/local/lib/ruby/1.6/i686-linux/ruby.h<br></pre>
		</div>
		<p>
			The exact location may vary on your machine, but the above location is typical. 
			If you are not entirely sure where Ruby is installed, you can run Ruby to find 
			out. For example:
		</p>
		<div class="code">
			<pre>
$ <b>ruby -e 'puts $:.join("\n")'</b><br>/usr/local/lib/ruby/site_ruby/1.6 /usr/local/lib/ruby/site_ruby/1.6/i686-linux
/usr/local/lib/ruby/site_ruby /usr/local/lib/ruby/1.6 /usr/local/lib/ruby/1.6/i686-linux .
 </pre>
		</div>
<H3><a name="Ruby_nn5"></a>27.1.3 Compiling a dynamic module</H3>


		<p>
			Ruby extension modules are typically compiled into shared libraries that the 
			interpreter loads dynamically at runtime. Since the exact commands for doing 
			this vary from platform to platform, your best bet is to follow the steps 
			described in the <tt>README.EXT</tt> file from the Ruby distribution:
		</p>
		<ol>
			<li>
				<p>Create a file called <tt>extconf.rb</tt> that looks like the following:</p>
				<div class="code">
					<pre>require 'mkmf'<br>create_makefile('example')<br></pre>
				</div>
			</li>
			<li>
				<p>Type the following to build the extension:</p>
				<div class="code">
					<pre>$ <b>ruby extconf.rb</b><br>$ <b>make</b><br>$ <b>make install</b>
    </pre>
				</div>
			</li>
		</ol>
		<p>
			Of course, there is the problem that mkmf does not work correctly on all 
			platforms, e.g, HPUX. If you need to add your own make rules to the file that <tt>extconf.rb</tt>
			produces, you can add this:
		</p>
		<div class="code">
			<pre>open("Makefile", "a") { |mf|<br> puts &lt;&lt;EOM<br> # Your make rules go here<br> EOM<br>}<br></pre>
		</div>
		<p>
			to the end of the <tt>extconf.rb</tt> file. If for some reason you don't want 
			to use the standard approach, you'll need to determine the correct compiler and 
			linker flags for your build platform. For example, a typical sequence of 
			commands for the Linux operating system would look something like this:
		</p>
		<div class="code">
			<pre>$ <b>swig -ruby example.i</b><br>$ <b>gcc -c example.c</b><br>$ <b>gcc -c example_wrap.c -I/usr/local/lib/ruby/1.6/i686-linux</b> <br>$ <b>gcc -shared example.o example_wrap.o -o example.so</b>
</pre>
		</div>
		<p>
			For other platforms it may be necessary to compile with the <tt>-fPIC</tt> option 
			to generate position-independent code. If in doubt, consult the manual pages 
			for your compiler and linker to determine the correct set of options. You might 
			also check the <a href="http://swig.cs.uchicago.edu/cgi-bin/wiki.pl">SWIG Wiki</a>
			for additional information.
		</p>
		<p>
			<a name="n6"></a>
		</p>
<H3><a name="Ruby_nn6"></a>27.1.4 Using your module</H3>


		<p>
			Ruby <i>module</i> names must be capitalized, but the convention for Ruby <i>feature</i>
			names is to use lowercase names. So, for example, the <b>Etc</b> extension 
			module is imported by requiring the <b>etc</b> feature:
		</p>
		<div class="code">
			<pre># The feature name begins with a lowercase letter...<br>require 'etc'<br><br># ... but the module name begins with an uppercase letter<br>puts "Your login name: #{Etc.getlogin}"<br></pre>
		</div>
		<p>
			To stay consistent with this practice, you should always specify a <b>lowercase</b>
			module name with SWIG's <tt>%module</tt> directive. SWIG will automatically 
			correct the resulting Ruby module name for your extension. So for example, a 
			SWIG interface file that begins with:
		</p>
		<div class="code">
			<pre>%module example<br></pre>
		</div>
		<p>
			will result in an extension module using the feature name "example" and Ruby 
			module name "Example".
		</p>
<H3><a name="Ruby_nn7"></a>27.1.5 Static linking</H3>


		<p>
			An alternative approach to dynamic linking is to rebuild the Ruby interpreter 
			with your extension module added to it. In the past, this approach was 
			sometimes necessary due to limitations in dynamic loading support on certain 
			machines. However, the situation has improved greatly over the last few years 
			and you should not consider this approach unless there is really no other 
			option.
		</p>
		<p>The usual procedure for adding a new module to Ruby involves finding the Ruby 
			source, adding an entry to the <tt>ext/Setup</tt> file, adding your directory 
			to the list of extensions in the file, and finally rebuilding Ruby.
		</p>
		<p><a name="n8"></a></p>
<H3><a name="Ruby_nn8"></a>27.1.6 Compilation of C++ extensions</H3>


		<p>
			On most machines, C++ extension modules should be linked using the C++ 
			compiler. For example:
		</p>
		<div class="code">
			<pre>$ <b>swig -c++ -ruby example.i</b><br>$ <b>g++ -c example.cxx</b><br>$ <b>g++ -c example_wrap.cxx -I/usr/local/lib/ruby/1.6/i686-linux</b><br>$ <b>g++ -shared example.o example_wrap.o -o example.so</b>
</pre>
		</div>
		<p>
			If you've written an <tt>extconf.rb</tt> script to automatically generate a <tt>Makefile</tt>
			for your C++ extension module, keep in mind that (as of this writing) Ruby 
			still uses <tt>gcc</tt> and not <tt>g++</tt> as its linker. As a result, the 
			required C++ runtime library support will not be automatically linked into your 
			extension module and it may fail to load on some platforms. A workaround for 
			this problem is use the <tt>mkmf</tt> module's <tt>append_library()</tt> method 
			to add one of the C++ runtime libraries to the list of libraries linked into 
			your extension, e.g.
		</p>
		<div class="code">
			<pre>require 'mkmf'<br>$libs = append_library($libs, "supc++")<br>create_makefile('example')<br></pre>
		</div>
<H2><a name="Ruby_nn9"></a>27.2 Building Ruby Extensions under Windows 95/NT</H2>


		<p>
			Building a SWIG extension to Ruby under Windows 95/NT is roughly similar to the 
			process used with Unix. Normally, you will want to produce a DLL that can be 
			loaded into the Ruby interpreter. For all recent versions of Ruby, the 
			procedure described above (i.e. using an <tt>extconf.rb</tt> script) will work 
			with Windows as well; you should be able to build your code into a DLL by 
			typing:
		</p>
		<div class="code">
			<pre>C:\swigtest&gt; <b>ruby extconf.rb</b><br>C:\swigtest&gt; <b>nmake</b><br>C:\swigtest&gt; <b>nmake install</b>
</pre>
		</div>
		<p>
			The remainder of this section covers the process of compiling SWIG-generated 
			Ruby extensions with Microsoft Visual C++ 6 (i.e. within the Developer Studio 
			IDE, instead of using the command line tools). In order to build extensions, 
			you may need to download the source distribution to the Ruby package, as you 
			will need the Ruby header files.
		</p>
		<p><a name="n10"></a></p>
<H3><a name="Ruby_nn10"></a>27.2.1 Running SWIG from Developer Studio</H3>


		<p>
			If you are developing your application within Microsoft developer studio, SWIG 
			can be invoked as a custom build option. The process roughly follows these 
			steps :
		</p>
		<ul>
			<li>
				Open up a new workspace and use the AppWizard to select a DLL project.
			</li>
			<li>
				Add both the SWIG interface file (the .i file), any supporting C files, and the 
				name of the wrapper file that will be created by SWIG (i.e.. <tt>example_wrap.c</tt>). 
				Note : If using C++, choose a different suffix for the wrapper file such as <tt>example_wrap.cxx</tt>. 
				Don't worry if the wrapper file doesn't exist yet--Developer Studio will keep a 
				reference to it around.
			</li>
			<li>
				Select the SWIG interface file and go to the settings menu. Under settings, 
				select the "Custom Build" option.
			</li>
			<li>
				Enter "SWIG" in the description field.
			</li>
			<li>
				Enter "<tt>swig -ruby -o $(ProjDir)\$(InputName)_wrap.c $(InputPath)</tt>" in 
				the "Build command(s) field". You may have to include the path to swig.exe.
			</li>
			<li>
				Enter "<tt>$(ProjDir)\$(InputName)_wrap.c</tt>" in the "Output files(s) field".
			</li>
			<li>
				Next, select the settings for the entire project and go to the C/C++ tab and 
				select the Preprocessor category. Add NT=1 to the Preprocessor definitions. 
				This must be set else you will get compilation errors. Also add IMPORT to the 
				preprocessor definitions, else you may get runtime errors. Also add the include 
				directories for your Ruby installation under "Additional include directories".
			</li>
			<li>
				Next, select the settings for the entire project and go to the Link tab and 
				select the General category. Set the name of the output file to match the name 
				of your Ruby module (i.e.. example.dll). Next add the Ruby library file to your 
				link libraries under Object/Library modules. For example "mswin32-ruby16.lib. 
				You also need to add the path to the library under the Input tab - Additional 
				library path.
			</li>
			<li>
				Build your project.
			</li>
		</ul>
		<p>
			Now, assuming all went well, SWIG will be automatically invoked when you build 
			your project. Any changes made to the interface file will result in SWIG being 
			automatically invoked to produce a new version of the wrapper file. To run your 
			new Ruby extension, simply run Ruby and use the <tt>require</tt> command as 
			normal. For example if you have this ruby file run.rb:</p>
		<div class="code">
			<pre># file: run.rb<br>require 'Example'<br><br># Call a c function<br>print "Foo = ", Example.Foo, "\n"<br></pre>
		</div>
		<p>
			Ensure the dll just built is in your path or current directory, then run the 
			Ruby script from the DOS/Command prompt:
		</p>
		<div class="code">
			<pre>C:\swigtest&gt; <b>ruby run.rb</b><br>Foo = 3.0<br></pre>
		</div>
<H2><a name="Ruby_nn11"></a>27.3 The Ruby-to-C/C++ Mapping</H2>


		<p>
			This section describes the basics of how SWIG maps C or C++ declarations in 
			your SWIG interface files to Ruby constructs.
		</p>
<H3><a name="Ruby_nn12"></a>27.3.1 Modules</H3>


		<p>
			The SWIG <tt>%module</tt> directive specifies the name of the Ruby module. If 
			you specify:
		</p>
		<div class="code">
			<pre>%module example</pre>
		</div>
		<p>
			then everything is wrapped into a Ruby module named <tt>Example</tt> that is 
			nested directly under the global module. You can specify a more deeply nested 
			module by specifying the fully-qualified module name in quotes, e.g.
		</p>
		<div class="code">
			<pre>%module "foo::bar::spam"</pre>
		</div>
		<p>
			An alternate method of specifying a nested module name is to use the
			<span style="font-family: monospace;">-prefix</span>
			option on the SWIG command line. The prefix that you specify with this option 
			will be prepended to the module name specified with the
			<span style="font-family: monospace;">%module</span>
			directive in your SWIG interface file. So for example, this declaration at the 
			top of your SWIG interface file:<br>
		</p>
		<div class="code">
			<pre>%module "foo::bar::spam"</pre>
		</div>
		<p>
			will result in a nested module name of
			<span style="font-family: monospace;">Foo::Bar::Spam</span>, but you can 
			achieve the
			<span style="font-style: italic;">same</span>
			effect by specifying:<br>
		</p>
		<div class="code">
			<pre>%module spam</pre>
		</div>
		<p>
			and then running SWIG with the
			<span style="font-family: monospace;">-prefix</span>
			command line option:<br>
		</p>
		<div class="code">
			<pre>$ <b>swig -ruby -prefix "foo::bar::" example.i</b></pre>
		</div>
		<p>
			Starting with SWIG 1.3.20, you can also choose to wrap everything into the 
			global module by specifying the <tt>-globalmodule</tt> option on the SWIG 
			command line, i.e.
		</p>
		<div class="code">
			<pre>$ <b>swig -ruby -globalmodule example.i</b></pre>
		</div>
		<p>
			Note that this does not relieve you of the requirement of specifying the SWIG 
			module name with the <tt>%module</tt> directive (or the <tt>-module</tt> command-line 
			option) as described earlier.
		</p>
		<p>When choosing a module name, do not use the same name as a built-in Ruby command 
			or standard module name, as the results may be unpredictable. Similarly, if 
			you're using the <tt>-globalmodule</tt> option to wrap everything into the 
			global module, take care that the names of your constants, classes and methods 
			don't conflict with any of Ruby's built-in names.
		</p>
<H3><a name="Ruby_nn13"></a>27.3.2 Functions</H3>


		<p>
			Global functions are wrapped as Ruby module methods. For example, given the 
			SWIG interface file <tt>example.i</tt>:
		</p>
		<div class="code">
			<pre>%module example<br><br>int fact(int n);<br></pre>
		</div>
		<p>
			and C source file <tt>example.c</tt>:
		</p>
		<div class="code">
			<pre>int fact(int n) {<br> if (n == 0)<br> return 1;<br> return (n * fact(n-1));<br>}<br></pre>
		</div>
		<p>
			SWIG will generate a method <i>fact</i> in the <i>Example</i> module that can 
			be used like so:
		</p>
		<div class="code">
			<pre>$ <b>irb</b><br>irb(main):001:0&gt; <b>require 'example'</b><br>true<br>irb(main):002:0&gt; <b>Example.fact(4)</b><br>24<br></pre>
		</div>
<H3><a name="Ruby_nn14"></a>27.3.3 Variable Linking</H3>


		<p>
			C/C++ global variables are wrapped as a pair of singleton methods for the 
			module: one to get the value of the global variable and one to set it. For 
			example, the following SWIG interface file declares two global variables:
		</p>
		<div class="code">
			<pre>// SWIG interface file with global variables<br>%module example<br>...<br>%inline %{<br>extern int variable1;<br>extern double Variable2;<br>%}<br>...<br></pre>
		</div>
		<p>
			Now look at the Ruby interface:</p>
		<div class="code">
			<pre>$ <b>irb</b><br>irb(main):001:0&gt; <b>require 'Example'</b><br>true<br>irb(main):002:0&gt; <b>Example.variable1 = 2</b><br>2<br>irb(main):003:0&gt; <b>Example.Variable2 = 4 * 10.3</b><br>41.2<br>irb(main):004:0&gt; <b>Example.Variable2</b><br>41.2<br></pre>
		</div>
		<p>
			If you make an error in variable assignment, you will receive an error message. 
			For example:
		</p>
		<div class="code">
			<pre>irb(main):005:0&gt; <b>Example.Variable2 = "hello"</b><br>TypeError: no implicit conversion to float from string<br>from (irb):5:in `Variable2='<br>from (irb):5<br></pre>
		</div>
		<p>
			If a variable is declared as <tt>const</tt>, it is wrapped as a read-only 
			variable. Attempts to modify its value will result in an error.
		</p>
		<p>To make ordinary variables read-only, you can also use the <tt>%immutable</tt> directive. 
			For example:
		</p>
		<div class="code">
			<pre>%immutable;<br>%inline %{<br>extern char *path;<br>%}<br>%mutable;<br></pre>
		</div>
		<p>
			The <tt>%immutable</tt> directive stays in effect until it is explicitly 
			disabled using <tt>%mutable</tt>.
		</p>
<H3><a name="Ruby_nn15"></a>27.3.4 Constants</H3>


		<p>
			C/C++ constants are wrapped as module constants initialized to the appropriate 
			value. To create a constant, use <tt>#define</tt> or the <tt>%constant</tt> directive. 
			For example:
		</p>
		<div class="code">
			<pre>#define PI 3.14159<br>#define VERSION "1.0"<br><br>%constant int FOO = 42;<br>%constant const char *path = "/usr/local";<br><br>const int BAR = 32;<br></pre>
		</div>
		<p>
			Remember to use the :: operator in Ruby to get at these constant values, e.g.
		</p>
		<div class="code">
			<pre>$ <b>irb</b><br>irb(main):001:0&gt; <b>require 'Example'</b><br>true<br>irb(main):002:0&gt; <b>Example::PI</b><br>3.14159<br></pre>
		</div>
<H3><a name="Ruby_nn16"></a>27.3.5 Pointers</H3>


		<p>
			"Opaque" pointers to arbitrary C/C++ types (i.e. types that aren't explicitly 
			declared in your SWIG interface file) are wrapped as data objects. So, for 
			example, consider a SWIG interface file containing only the declarations:
		</p>
		<div class="code">
			<pre>Foo *get_foo();<br>void set_foo(Foo *foo);<br></pre>
		</div>
		<p>
			For this case, the <i>get_foo()</i> method returns an instance of an internally 
			generated Ruby class:
		</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>foo = Example::get_foo()</b><br>#&lt;SWIG::TYPE_p_Foo:0x402b1654&gt;<br></pre>
		</div>
		<p>
			A <tt>NULL</tt> pointer is always represented by the Ruby <tt>nil</tt> object.
		</p>
<H3><a name="Ruby_nn17"></a>27.3.6 Structures</H3>


		<p>
			C/C++ structs are wrapped as Ruby classes, with accessor methods (i.e. 
			"getters" and "setters") for all of the struct members. For example, this 
			struct declaration:
		</p>
		<div class="code">
			<pre>struct Vector {<br> double x, y;<br>};<br></pre>
		</div>
		<p>
			gets wrapped as a <tt>Vector</tt> class, with Ruby instance methods <tt>x</tt>, <tt>
				x=</tt>, <tt>y</tt> and <tt>y=</tt>. These methods can be used to access 
			structure data from Ruby as follows:
		</p>
		<div class="code">
			<pre>$ <b>irb</b><br>irb(main):001:0&gt; <b>require 'Example'</b><br>true<br>irb(main):002:0&gt; <b>f = Example::Vector.new</b><br>#&lt;Example::Vector:0x4020b268&gt;<br>irb(main):003:0&gt; <b>f.x = 10</b><br>nil<br>irb(main):004:0&gt; <b>f.x</b><br>10.0<br></pre>
		</div>
		<p>
			Similar access is provided for unions and the public data members of C++ 
			classes.</p>
		<p><tt>const</tt> members of a structure are read-only. Data members can also be 
			forced to be read-only using the <tt>%immutable</tt> directive (in C++, <tt>private</tt>
			may also be used). For example:
		</p>
		<div class="code">
			<pre>struct Foo {<br> ...<br> %immutable;<br> int x; /* Read-only members */<br> char *name;<br> %mutable;<br> ...<br>};<br></pre>
		</div>
		<p>
			When <tt>char *</tt> members of a structure are wrapped, the contents are 
			assumed to be dynamically allocated using <tt>malloc</tt> or <tt>new</tt> (depending 
			on whether or not SWIG is run with the <tt>-c++</tt> option). When the 
			structure member is set, the old contents will be released and a new value 
			created. If this is not the behavior you want, you will have to use a typemap 
			(described shortly).
		</p>
		<p>Array members are normally wrapped as read-only. For example, this code:
		</p>
		<div class="code">
			<pre>struct Foo {<br> int x[50];<br>};<br></pre>
		</div>
		<p>
			produces a single accessor function like this:
		</p>
		<div class="code">
			<pre>int *Foo_x_get(Foo *self) {<br> return self-&gt;x;<br>};<br></pre>
		</div>
		<p>
			If you want to set an array member, you will need to supply a "memberin" 
			typemap described in the <a href="#ruby_cpp_smart_pointers">section on typemaps</a>. 
			As a special case, SWIG does generate code to set array members of type <tt>char</tt>
			(allowing you to store a Ruby string in the structure).
		</p>
		<p>When structure members are wrapped, they are handled as pointers. For example,
		</p>
		<div class="code">
			<pre>struct Foo {<br> ...<br>};<br><br>struct Bar {<br> Foo f;<br>};<br></pre>
		</div>
		<p>
			generates accessor functions such as this:
		</p>
		<div class="code">
			<pre>Foo *Bar_f_get(Bar *b) {<br> return &amp;b-&gt;f;<br>}<br><br>void Bar_f_set(Bar *b, Foo *val) {<br> b-&gt;f = *val;<br>}<br></pre>
		</div>
<H3><a name="Ruby_nn18"></a>27.3.7 C++ classes</H3>


		<p>
			Like structs, C++ classes are wrapped by creating a new Ruby class of the same 
			name with accessor methods for the public class member data. Additionally, 
			public member functions for the class are wrapped as Ruby instance methods, and 
			public static member functions are wrapped as Ruby singleton methods. So, given 
			the C++ class declaration:
		</p>
		<div class="code">
			<pre>class List {<br>public:<br> List();<br> ~List();<br> int search(char *item);<br> void insert(char *item);<br> void remove(char *item);<br> char *get(int n);<br> int length;<br> static void print(List *l);<br>};<br></pre>
		</div>
		<p>
			SWIG would create a <tt>List</tt> class with:
		</p>
		<ul>
			<li>
				instance methods <i>search</i>, <i>insert</i>, <i>remove</i>, and <i>get</i>;
			</li>
			<li>
				instance methods <i>length</i> and <i>length=</i> (to get and set the value of 
				the <i>length</i> data member); and,
			</li>
			<li>
				a <i>print</i> singleton method for the class.
			</li>
		</ul>
		<p>
			In Ruby, these functions are used as follows:
		</p>
		<div class="code">
			<pre>require 'Example'<br><br>l = Example::List.new<br><br>l.insert("Ale")<br>l.insert("Stout")<br>l.insert("Lager")<br>Example.print(l)<br>l.length()<br>----- produces the following output <br>Lager<br>Stout<br>Ale<br>3<br></pre>
		</div>
<H3><a name="Ruby_nn19"></a>27.3.8 C++ Inheritance</H3>


		<p>
			The SWIG type-checker is fully aware of C++ inheritance. Therefore, if you have 
			classes like this:
		</p>
		<div class="code">
			<pre>class Parent {<br> ...<br>};<br><br>class Child : public Parent {<br> ...<br>};<br></pre>
		</div>
		<p>
			those classes are wrapped into a hierarchy of Ruby classes that reflect the 
			same inheritance structure. All of the usual Ruby utility methods work 
			normally:
		</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>c = Child.new</b><br>#&lt;Bar:0x4016efd4&gt;<br>irb(main):002:0&gt; <b>c.instance_of? Child</b><br>true<br>irb(main):003:0&gt; <b>b.instance_of? Parent</b><br>false<br>irb(main):004:0&gt; <b>b.is_a? Child</b><br>true<br>irb(main):005:0&gt; <b>b.is_a? Parent</b><br>true<br>irb(main):006:0&gt; <b>Child &lt; Parent</b><br>true<br>irb(main):007:0&gt; <b>Child &gt; Parent</b><br>false<br></pre>
		</div>
		<p>
			Furthermore, if you have a function like this:
		</p>
		<div class="code">
			<pre>void spam(Parent *f);<br></pre>
		</div>
		<p>
			then the function <tt>spam()</tt> accepts <tt>Parent</tt>* or a pointer to any 
			class derived from <tt>Parent</tt>.
		</p>
		<p>Until recently, the Ruby module for SWIG didn't support multiple inheritance, 
			and this is still the default behavior. This doesn't mean that you can't wrap 
			C++ classes which inherit from multiple base classes; it simply means that only 
			the <b>first</b> base class listed in the class declaration is considered, and 
			any additional base classes are ignored. As an example, consider a SWIG 
			interface file with a declaration like this:
		</p>
		<div class="code">
			<pre>class Derived : public Base1, public Base2<br>{<br> ...<br>};<br></pre>
		</div>
		<p>
			For this case, the resulting Ruby class (<tt>Derived</tt>) will only consider <tt>Base1</tt>
			as its superclass. It won't inherit any of <tt>Base2</tt>'s member functions or 
			data and it won't recognize <tt>Base2</tt> as an "ancestor" of <tt>Derived</tt> 
			(i.e. the <em>is_a?</em> relationship would fail). When SWIG processes this 
			interface file, you'll see a warning message like:
		</p>
		<div class="code">
			<pre>example.i:5: Warning(802): Warning for Derived: Base Base2 ignored.<br>Multiple inheritance is not supported in Ruby.<br></pre>
		</div>
		<p>
			Starting with SWIG 1.3.20, the Ruby module for SWIG provides limited support 
			for multiple inheritance. Because the approach for dealing with multiple 
			inheritance introduces some limitations, this is an optional feature that you 
			can activate with the <tt>-minherit</tt> command-line option:
		</p>
		<div class="code">
			<pre>$ <b>swig -c++ -ruby -minherit example.i</b></pre>
		</div>
		<p>
			Using our previous example, if your SWIG interface file contains a declaration 
			like this:
		</p>
		<div class="code">
			<pre>class Derived : public Base1, public Base2<br>{<br> ...<br>};<br></pre>
		</div>
		<p>
			and you run SWIG with the <tt>-minherit</tt> command-line option, then you will 
			end up with a Ruby class <tt>Derived</tt> that appears to "inherit" the member 
			data and functions from both <tt>Base1</tt> and <tt>Base2</tt>. What actually 
			happens is that three different top-level classes are created, with Ruby's <tt>Object</tt>
			class as their superclass. Each of these classes defines a nested module named <tt>Impl</tt>, 
			and it's in these nested <tt>Impl</tt> modules that the actual instance methods 
			for the classes are defined, i.e.
		</p>
		<div class="code">
			<pre>class Base1<br> module Impl<br> # Define Base1 methods here<br> end<br> include Impl<br>end<br><br>class Base2<br> module Impl<br> # Define Base2 methods here<br> end<br> include Impl<br>end<br><br>class Derived<br> module Impl<br> include Base1::Impl<br> include Base2::Impl<br> # Define Derived methods here<br> end<br> include Impl<br>end<br></pre>
		</div>
		<p>
			Observe that after the nested <tt>Impl</tt> module for a class is defined, it 
			is mixed-in to the class itself. Also observe that the <tt>Derived::Impl</tt> module 
			first mixes-in its base classes' <tt>Impl</tt> modules, thus "inheriting" all 
			of their behavior.
		</p>
		<p>The primary drawback is that, unlike the default mode of operation, neither <tt>Base1</tt>
			nor <tt>Base2</tt> is a true superclass of <tt>Derived</tt> anymore:
		</p>
		<div class="code">
			<pre>obj = Derived.new<br>obj.is_a? Base1 # this will return false...<br>obj.is_a? Base2 # ... and so will this<br></pre>
		</div>
		<p>
			In most cases, this is not a serious problem since objects of type <tt>Derived</tt>
			will otherwise behave as though they inherit from both <tt>Base1</tt> and <tt>Base2</tt>
			(i.e. they exhibit <a href="http://c2.com/cgi/wiki?DuckTyping">"Duck Typing"</a>).
		</p>
<H3><a name="Ruby_nn20"></a>27.3.9 C++ Overloaded Functions</H3>


		<p>
			C++ overloaded functions, methods, and constructors are mostly supported by 
			SWIG. For example, if you have two functions like this:
		</p>
		<div class="code">
			<pre>void foo(int);<br>void foo(char *c);<br></pre>
		</div>
		<p>
			You can use them in Ruby in a straightforward manner:
		</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>foo(3)</b> # foo(int)<br>irb(main):002:0&gt; <b>foo("Hello")</b> # foo(char *c)<br></pre>
		</div>
		<p>Similarly, if you have a class like this,</p>
		<div class="code">
			<pre>class Foo {<br>public:<br> Foo();<br> Foo(const Foo &amp;);<br> ...<br>};<br></pre>
		</div>
		<p>you can write Ruby code like this:</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>f = Foo.new</b> # Create a Foo<br>irb(main):002:0&gt; <b>g = Foo.new(f)</b> # Copy f<br></pre>
		</div>
		<p>
			Overloading support is not quite as flexible as in C++. Sometimes there are 
			methods that SWIG can't disambiguate. For example:
		</p>
		<div class="code">
			<pre>void spam(int);<br>void spam(short);<br></pre>
		</div>
		<p>or</p>
		<div class="code">
			<pre>void foo(Bar *b);<br>void foo(Bar &amp;b);<br></pre>
		</div>
		<p>
			If declarations such as these appear, you will get a warning message like this:
		</p>
		<div class="code">
			<pre>example.i:12: Warning(509): Overloaded spam(short) is shadowed by spam(int)<br>at example.i:11.<br> </pre>
		</div>
		<p>
			To fix this, you either need to ignore or rename one of the methods. For 
			example:
		</p>
		<div class="code">
			<pre>%rename(spam_short) spam(short);<br>...<br>void spam(int); <br>void spam(short); // Accessed as spam_short<br></pre>
		</div>
		<p>or</p>
		<div class="code">
			<pre>%ignore spam(short);<br>...<br>void spam(int); <br>void spam(short); // Ignored<br></pre>
		</div>
		<p>
			SWIG resolves overloaded functions and methods using a disambiguation scheme 
			that ranks and sorts declarations according to a set of type-precedence rules. 
			The order in which declarations appear in the input does not matter except in 
			situations where ambiguity arises--in this case, the first declaration takes 
			precedence.
		</p>
		<p>Please refer to the <a href="SWIGPlus.html#SWIGPlus">"SWIG and C++"</a> chapter 
			for more information about overloading. <a name="n21"></a>
		</p>
<H3><a name="Ruby_nn21"></a>27.3.10 C++ Operators</H3>


		<p>
			For the most part, overloaded operators are handled automatically by SWIG and 
			do not require any special treatment on your part. So if your class declares an 
			overloaded addition operator, e.g.
		</p>
		<div class="code">
			<pre>class Complex {<br> ...<br> Complex operator+(Complex &amp;);<br> ...<br>};<br></pre>
		</div>
		<p>
			the resulting Ruby class will also support the addition (+) method correctly.
		</p>
		<p>For cases where SWIG's built-in support is not sufficient, C++ operators can be 
			wrapped using the <tt>%rename</tt> directive (available on SWIG 1.3.10 and 
			later releases). All you need to do is give the operator the name of a valid 
			Ruby identifier. For example:
		</p>
		<div class="code">
			<pre>%rename(add_complex) operator+(Complex &amp;, Complex &amp;);<br>...<br>Complex operator+(Complex &amp;, Complex &amp;);<br></pre>
		</div>
		<p>Now, in Ruby, you can do this:</p>
		<div class="code">
			<pre>a = Example::Complex.new(2, 3)<br>b = Example::Complex.new(4, -1)<br>c = Example.add_complex(a, b)<br></pre>
		</div>
		<p>
			More details about wrapping C++ operators into Ruby operators is discussed in 
			the <a href="#ruby_operator_overloading">section on operator overloading</a>.
		</p>
<H3><a name="Ruby_nn22"></a>27.3.11 C++ namespaces</H3>


		<p>
			SWIG is aware of C++ namespaces, but namespace names do not appear in the 
			module nor do namespaces result in a module that is broken up into submodules 
			or packages. For example, if you have a file like this,
		</p>
		<div class="code">
			<pre>%module example<br><br>namespace foo {<br> int fact(int n);<br> struct Vector {<br> double x,y,z;<br> };<br>};<br></pre>
		</div>
		<p>it works in Ruby as follows:</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>require 'example'</b><br>true<br>irb(main):002:0&gt; <b>Example.fact(3)</b><br>6<br>irb(main):003:0&gt; <b>v = Example::Vector.new</b><br>#&lt;Example::Vector:0x4016f4d4&gt;<br>irb(main):004:0&gt; <b>v.x = 3.4</b><br>3.4<br>irb(main):004:0&gt; <b>v.y</b><br>0.0<br></pre>
		</div>
		<p>
			If your program has more than one namespace, name conflicts (if any) can be 
			resolved using <tt>%rename</tt> For example:
		</p>
		<div class="code">
			<pre>%rename(Bar_spam) Bar::spam;<br><br>namespace Foo {<br> int spam();<br>}<br><br>namespace Bar {<br> int spam();<br>}<br></pre>
		</div>
		<p>
			If you have more than one namespace and your want to keep their symbols 
			separate, consider wrapping them as separate SWIG modules. For example, make 
			the module name the same as the namespace and create extension modules for each 
			namespace separately. If your program utilizes thousands of small deeply nested 
			namespaces each with identical symbol names, well, then you get what you 
			deserve.
		</p>
<H3><a name="Ruby_nn23"></a>27.3.12 C++ templates</H3>


		<p>
			C++ templates don't present a huge problem for SWIG. However, in order to 
			create wrappers, you have to tell SWIG to create wrappers for a particular 
			template instantiation. To do this, you use the <tt>%template</tt> directive. 
			For example:
		</p>
		<div class="code">
			<pre>%module example<br><br>%{<br>#include "pair.h"<br>%}<br><br>template&lt;class T1, class T2&gt;<br>struct pair {<br> typedef T1 first_type;<br> typedef T2 second_type;<br> T1 first;<br> T2 second;<br> pair();<br> pair(const T1&amp;, const T2&amp;);<br> ~pair();<br>};<br><br>%template(Pairii) pair&lt;int,int&gt;;<br></pre>
		</div>
		<p>In Ruby:</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>require 'example'</b><br>true<br>irb(main):002:0&gt; <b>p = Example::Pairii.new(3, 4)</b><br>#&lt;Example:Pairii:0x4016f4df&gt;<br>irb(main):003:0&gt; <b>p.first</b><br>3<br>irb(main):004:0&gt; <b>p.second</b><br>4<br></pre>
		</div>
		<p>
			On a related note, the standard SWIG library contains a number of modules that 
			provide typemaps for standard C++ library classes (such as <tt>std::pair</tt>, <tt>std::string</tt>
			and <tt>std::vector</tt>). These library modules don't provide wrappers around 
			the templates themselves, but they do make it convenient for users of your 
			extension module to pass Ruby objects (such as arrays and strings) to wrapped 
			C++ code that expects instances of standard C++ templates. For example, suppose 
			the C++ library you're wrapping has a function that expects a vector of floats:
		</p>
		<div class="code">
			<pre>%module example<br><br>float sum(const std::vector&lt;float&gt;&amp; values);<br></pre>
		</div>
		<p>
			Rather than go through the hassle of writing an "in" typemap to convert an 
			array of Ruby numbers into a std::vector&lt;float&gt;, you can just use the <tt>std_vector.i</tt>
			module from the standard SWIG library:
		</p>
		<div class="code">
			<pre>%module example<br><br><b>%include std_vector.i</b><br>float sum(const std::vector&lt;float&gt;&amp; values);<br></pre>
		</div>
		<p>
			Obviously, there is a lot more to template wrapping than shown in these 
			examples. More details can be found in the <a href="SWIGPlus.html#SWIGPlus">SWIG 
				and C++</a> chapter.
		</p>
<H3><a name="ruby_cpp_smart_pointers"></a>27.3.13 C++ Smart Pointers</H3>


		<p>
			In certain C++ programs, it is common to use classes that have been wrapped by 
			so-called "smart pointers." Generally, this involves the use of a template 
			class that implements <tt>operator-&gt;()</tt> like this:
		</p>
		<div class="code">
			<pre>template&lt;class T&gt; class SmartPtr {<br> ...<br> T *operator-&gt;();<br> ...<br>}<br></pre>
		</div>
		<p>Then, if you have a class like this,</p>
		<div class="code">
			<pre>class Foo {<br>public:<br> int x;<br> int bar();<br>};<br></pre>
		</div>
		<p>A smart pointer would be used in C++ as follows:</p>
		<div class="code">
			<pre>SmartPtr&lt;Foo&gt; p = CreateFoo(); // Created somehow (not shown)<br>...<br>p-&gt;x = 3; // Foo::x<br>int y = p-&gt;bar(); // Foo::bar<br></pre>
		</div>
		<p>
			To wrap this in Ruby, simply tell SWIG about the <tt>SmartPtr</tt> class and 
			the low-level <tt>Foo</tt> object. Make sure you instantiate <tt>SmartPtr</tt> using
			<tt>%template</tt> if necessary. For example:
		</p>
		<div class="code">
			<pre>%module example<br>...<br>%template(SmartPtrFoo) SmartPtr&lt;Foo&gt;;<br>...<br></pre>
		</div>
		<p>Now, in Ruby, everything should just "work":</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>p = Example::CreateFoo()</b> # Create a smart-pointer somehow<br>#&lt;Example::SmartPtrFoo:0x4016f4df&gt;<br>irb(main):002:0&gt; <b>p.x = 3</b> # Foo::x<br>3<br>irb(main):003:0&gt; <b>p.bar()</b> # Foo::bar<br></pre>
		</div>
		<p>
			If you ever need to access the underlying pointer returned by <tt>operator-&gt;()</tt>
			itself, simply use the <tt>__deref__()</tt> method. For example:
		</p>
		<div class="code">
			<pre>irb(main):004:0&gt; <b>f = p.__deref__()</b> # Returns underlying Foo *<br></pre>
		</div>
<H3><a name="Ruby_nn25"></a>27.3.14 Cross-Language Polymorphism</H3>


		<p>
			SWIG's Ruby module supports cross-language polymorphism (a.k.a. the "directors" 
			feature) similar to that for SWIG's Python module. Rather than duplicate the 
			information presented in the <a href="Python.html#Python">Python</a> chapter, 
			this section just notes the differences that you need to be aware of when using 
			this feature with Ruby.
		</p>
<H4><a name="Ruby_nn26"></a>27.3.14.1 Exception Unrolling</H4>


		<p>
			Whenever a C++ director class routes one of its virtual member function calls 
			to a Ruby instance method, there's always the possibility that an exception 
			will be raised in the Ruby code. By default, those exceptions are ignored, 
			which simply means that the exception will be exposed to the Ruby interpreter. 
			If you would like to change this behavior, you can use the <tt>%feature("director:except")</tt>
			directive to indicate what action should be taken when a Ruby exception is 
			raised. The following code should suffice in most cases:
		</p>
		<div class="code">
			<pre>%feature("director:except") {<br> throw Swig::DirectorMethodException($error);<br>}<br></pre>
		</div>
		<p>
			When this feature is activated, the call to the Ruby instance method is 
			"wrapped" using the <tt>rb_rescue2()</tt> function from Ruby's C API. If any 
			Ruby exception is raised, it will be caught here and a C++ exception is raised 
			in its place.
		</p>
<H2><a name="Ruby_nn27"></a>27.4 Input and output parameters</H2>


		<p>
			A common problem in some C programs is handling parameters passed as simple 
			pointers. For example:
		</p>
		<div class="code">
			<pre>void add(int x, int y, int *result) {<br> *result = x + y;<br>}<br>or<br>int sub(int *x, int *y) {<br> return *x-*y;<br>}<br></pre>
		</div>
		<p>
			The easiest way to handle these situations is to use the <tt>typemaps.i</tt> file. 
			For example:
		</p>
		<div class="code">
			<pre>%module Example<br>%include "typemaps.i"<br><br>void add(int, int, int *OUTPUT);<br>int sub(int *INPUT, int *INPUT);<br></pre>
		</div>
		<p>In Ruby, this allows you to pass simple values. For example:</p>
		<div class="code">
			<pre>a = Example.add(3,4)<br>puts a<br>7<br>b = Example.sub(7,4)<br>puts b<br>3<br></pre>
		</div>
		<p>
			Notice how the <tt>INPUT</tt> parameters allow integer values to be passed 
			instead of pointers and how the <tt>OUTPUT</tt> parameter creates a return 
			result.
		</p>
		<p>If you don't want to use the names <tt>INPUT</tt> or <tt>OUTPUT</tt>, use the <tt>%apply</tt>
			directive. For example:
		</p>
		<div class="code">
			<pre>%module Example<br>%include "typemaps.i"<br><br>%apply int *OUTPUT { int *result };<br>%apply int *INPUT { int *x, int *y};<br><br>void add(int x, int y, int *result);<br>int sub(int *x, int *y);<br></pre>
		</div>
		<p>
			If a function mutates one of its parameters like this,
		</p>
		<div class="code">
			<pre>void negate(int *x) {<br> *x = -(*x);<br>}<br></pre>
		</div>
		<p>you can use <tt>INOUT</tt> like this:</p>
		<div class="code">
			<pre>%include "typemaps.i"<br>...<br>void negate(int *INOUT);<br></pre>
		</div>
		<p>In Ruby, a mutated parameter shows up as a return value. For example:</p>
		<div class="code">
			<pre>a = Example.negate(3)<br>print a<br>-3<br><br></pre>
		</div>
		<p>
			The most common use of these special typemap rules is to handle functions that 
			return more than one value. For example, sometimes a function returns a result 
			as well as a special error code:
		</p>
		<div class="code">
			<pre>/* send message, return number of bytes sent, success code, and error_code */<br>int send_message(char *text, int *success, int *error_code);<br></pre>
		</div>
		<p>
			To wrap such a function, simply use the <tt>OUTPUT</tt> rule above. For 
			example:
		</p>
		<div class="code">
			<pre>%module example<br>%include "typemaps.i"<br>...<br>int send_message(char *, int *OUTPUT, int *OUTPUT);<br></pre>
		</div>
		<p>
			When used in Ruby, the function will return an array of multiple values.
		</p>
		<div class="code">
			<pre>bytes, success, error_code = send_message("Hello World")<br>if not success<br> print "error #{error_code} : in send_message"<br>else<br> print "Sent", bytes<br>end<br></pre>
		</div>
		<p>
			Another way to access multiple return values is to use the <tt>%apply</tt> rule. 
			In the following example, the parameters rows and columns are related to SWIG 
			as <tt>OUTPUT</tt> values through the use of <tt>%apply</tt>
		</p>
		<div class="code">
			<pre>%module Example<br>%include "typemaps.i"<br>%apply int *OUTPUT { int *rows, int *columns };<br>...<br>void get_dimensions(Matrix *m, int *rows, int*columns);<br></pre>
		</div>
		<p>In Ruby:</p>
		<div class="code">
			<pre>r, c = Example.get_dimensions(m)<br></pre>
		</div>
		<H2><a name="Ruby_nn28"></a>27.5 Simple exception handling
		</H2>
		<p>
			The SWIG <tt>%exception</tt> directive can be used to define a user-definable 
			exception handler that can convert C/C++ errors into Ruby exceptions. The 
			chapter on <a href="Customization.html#Customization">Customization Features</a>
			contains more details, but suppose you have a C++ class like the following :
		</p>
		<div class="code">
			<pre>class DoubleArray {<br> private:<br> int n;<br> double *ptr;<br> public:<br> // Create a new array of fixed size<br> DoubleArray(int size) {<br> ptr = new double[size];<br> n = size;<br> }<br> // Destroy an array<br> ~DoubleArray() {<br> delete ptr;<br> }<br> // Return the length of the array<br> int length() {<br> return n;<br> }<br><br> // Get an array item and perform bounds checking.<br> double getitem(int i) {<br> if ((i &gt;= 0) &amp;&amp; (i &lt; n))<br> return ptr[i];<br> else<br> throw RangeError();<br> }<br> // Set an array item and perform bounds checking.<br> void setitem(int i, double val) {<br> if ((i &gt;= 0) &amp;&amp; (i &lt; n))<br> ptr[i] = val;<br> else {<br> throw RangeError();<br> }<br> }<br> };<br></pre>
		</div>
		<p>
			Since several methods in this class can throw an exception for an out-of-bounds 
			access, you might want to catch this in the Ruby extension by writing the 
			following in an interface file:
		</p>
		<div class="code">
			<pre>%exception {<br> try {<br> $action<br> }<br> catch (const RangeError&amp;) {<br> static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);<br> rb_raise(cpperror, "Range error.");<br> }<br>}<br><br>class DoubleArray {<br> ...<br>};<br></pre>
		</div>
		<p>
			The exception handling code is inserted directly into generated wrapper 
			functions. When an exception handler is defined, errors can be caught and used 
			to gracefully raise a Ruby exception instead of forcing the entire program to 
			terminate with an uncaught error.
		</p>
		<p>As shown, the exception handling code will be added to every wrapper function. 
			Because this is somewhat inefficient, you might consider refining the exception 
			handler to only apply to specific methods like this:
		</p>
		<div class="code">
			<pre>%exception getitem {<br> try {<br> $action<br> }<br> catch (const RangeError&amp;) {<br> static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);<br> rb_raise(cpperror, "Range error in getitem.");<br> }<br>}<br><br>%exception setitem {<br> try {<br> $action<br> }<br> catch (const RangeError&amp;) {<br> static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);<br> rb_raise(cpperror, "Range error in setitem.");<br> }<br>}<br></pre>
		</div>
		<p>
			In this case, the exception handler is only attached to methods and functions 
			named <tt>getitem</tt> and <tt>setitem</tt>.
		</p>
		<p>Since SWIG's exception handling is user-definable, you are not limited to C++ 
			exception handling. See the chapter on <a href="Customization.html#Customization">Customization 
				Features</a> for more examples.
		</p>
		<p>When raising a Ruby exception from C/C++, use the <tt>rb_raise()</tt> function 
			as shown above. The first argument passed to <tt>rb_raise()</tt> is the 
			exception type. You can raise a custom exception type (like the <tt>cpperror</tt>
			example shown above) or one of the built-in Ruby exception types. For a list of 
			the standard Ruby exception classes, consult a Ruby reference such as <a href="http://www.rubycentral.com/book">
				<em>Programming Ruby</em></a>.
		</p>
<H2><a name="Ruby_nn29"></a>27.5 Typemaps</H2>


		<p>
			This section describes how you can modify SWIG's default wrapping behavior for 
			various C/C++ datatypes using the <tt>%typemap</tt> directive. This is an 
			advanced topic that assumes familiarity with the Ruby C API as well as the 
			material in the "<a href="Typemaps.html#Typemaps">Typemaps</a>" chapter.
		</p>
		<p>Before proceeding, it should be stressed that typemaps are not a required part 
			of using SWIG---the default wrapping behavior is enough in most cases. Typemaps 
			are only used if you want to change some aspect of the primitive C-Ruby 
			interface.</p>
<H3><a name="Ruby_nn30"></a>27.5.1 What is a typemap?</H3>


		<p>
			A typemap is nothing more than a code generation rule that is attached to a 
			specific C datatype. For example, to convert integers from Ruby to C, you might 
			define a typemap like this:
		</p>
		<div class="code">
			<pre>%module example<br><br>%typemap(in) int {<br> $1 = (int) NUM2INT($input);<br> printf("Received an integer : %d\n",$1);<br>}<br><br>%inline %{<br>extern int fact(int n);<br>%}<br></pre>
		</div>
		<p>
			Typemaps are always associated with some specific aspect of code generation. In 
			this case, the "in" method refers to the conversion of input arguments to 
			C/C++. The datatype <tt>int</tt> is the datatype to which the typemap will be 
			applied. The supplied C code is used to convert values. In this code a number 
			of special variables prefaced by a <tt>$</tt> are used. The <tt>$1</tt> variable 
			is placeholder for a local variable of type <tt>int</tt>. The <tt>$input</tt> variable 
			is the input Ruby object.
		</p>
		<p>When this example is compiled into a Ruby module, the following sample code:
		</p>
		<div class="code">
			<pre>require 'example'<br><br>puts Example.fact(6)<br></pre>
		</div>
		<p>prints the result:</p>
		<div class="code">
			<pre>Received an integer : 6<br>720<br></pre>
		</div>
		<p>
			In this example, the typemap is applied to all occurrences of the <tt>int</tt> datatype. 
			You can refine this by supplying an optional parameter name. For example:
		</p>
		<div class="code">
			<pre>%module example<br><br>%typemap(in) int n {<br> $1 = (int) NUM2INT($input);<br> printf("n = %d\n",$1);<br>}<br><br>%inline %{<br>extern int fact(int n);<br>%}<br></pre>
		</div>
		<p>
			In this case, the typemap code is only attached to arguments that exactly match 
			"<tt>int n</tt>".
		</p>
		<p>The application of a typemap to specific datatypes and argument names involves 
			more than simple text-matching--typemaps are fully integrated into the SWIG 
			type-system. When you define a typemap for <tt>int</tt>, that typemap applies 
			to <tt>int</tt> and qualified variations such as <tt>const int</tt>. In 
			addition, the typemap system follows <tt>typedef</tt> declarations. For 
			example:
		</p>
		<div class="code">
			<pre>%typemap(in) int n {<br> $1 = (int) NUM2INT($input);<br> printf("n = %d\n",$1);<br>}<br><br>typedef int Integer;<br>extern int fact(Integer n); // Above typemap is applied<br></pre>
		</div>
		<p>
			However, the matching of <tt>typedef</tt> only occurs in one direction. If you 
			defined a typemap for <tt>Integer</tt>, it is not applied to arguments of type <tt>int</tt>.
		</p>
		<p>Typemaps can also be defined for groups of consecutive arguments. For example:
		</p>
		<div class="code">
			<pre>%typemap(in) (char *str, int len) {<br> $1 = STR2CSTR($input);<br> $2 = (int) RSTRING($input)-&gt;len;<br>};<br><br>int count(char c, char *str, int len);<br></pre>
		</div>
		<p>
			When a multi-argument typemap is defined, the arguments are always handled as a 
			single Ruby object. This allows the function <tt>count</tt> to be used as 
			follows (notice how the length parameter is omitted):
		</p>
		<div class="code">
			<pre>puts Example.count('o','Hello World')<br>2<br></pre>
		</div>
<H3><a name="Ruby_nn31"></a>27.5.2 Ruby typemaps</H3>


		<p>
			The previous section illustrated an "in" typemap for converting Ruby objects to 
			C. A variety of different typemap methods are defined by the Ruby module. For 
			example, to convert a C integer back into a Ruby object, you might define an 
			"out" typemap like this:
		</p>
		<div class="code">
			<pre>%typemap(out) int {<br> $result = INT2NUM($1);<br>}<br></pre>
		</div>
		<p>
			The following list details all of the typemap methods that can be used by the 
			Ruby module:
		</p>
		<p><tt>%typemap(in) </tt>
		</p>
		<div class="indent">Converts Ruby objects to input function arguments
		</div>
		<p><tt>%typemap(out)</tt></p>
		<div class="indent">Converts return value of a C function to a Ruby object
		</div>
		<p><tt>%typemap(varin)</tt></p>
		<div class="indent">Assigns a C global variable from a Ruby object
		</div>
		<p><tt>%typemap(varout)</tt></p>
		<div class="indent">Returns a C global variable as a Ruby object
		</div>
		<p><tt>%typemap(freearg)</tt></p>
		<div class="indent">Cleans up a function argument (if necessary)
		</div>
		<p><tt>%typemap(argout)</tt></p>
		<div class="indent">Output argument processing
		</div>
		<p><tt>%typemap(ret)</tt></p>
		<div class="indent">Cleanup of function return values
		</div>
		<p><tt>%typemap(memberin)</tt></p>
		<div class="indent">Setting of structure/class member data
		</div>
		<p><tt>%typemap(globalin)</tt></p>
		<div class="indent">Setting of C global variables
		</div>
		<p><tt>%typemap(check)</tt></p>
		<div class="indent">Checks function input values.
		</div>
		<p><tt>%typemap(default)</tt></p>
		<div class="indent">Set a default value for an argument (making it optional).
		</div>
		<p><tt>%typemap(arginit)</tt></p>
		<div class="indent">Initialize an argument to a value before any conversions occur.
		</div>
		<p>
			Examples of these typemaps appears in the <a href="#ruby_typemap_examples">section 
				on typemap examples</a>
		</p>
<H3><a name="Ruby_nn32"></a>27.5.3 Typemap variables</H3>


		Within a typemap, a number of special variables prefaced with a <tt>$</tt> may 
		appear. A full list of variables can be found in the "<a href="Typemaps.html#Typemaps">Typemaps</a>" 
		chapter. This is a list of the most common variables:
		<p><tt>$1</tt>
		</p>
		<div class="indent">A C local variable corresponding to the actual type specified 
			in the <tt>%typemap</tt> directive. For input values, this is a C local 
			variable that is supposed to hold an argument value. For output values, this is 
			the raw result that is supposed to be returned to Ruby.
		</div>
		<p><tt>$input</tt></p>
		<div class="indent">A <tt>VALUE</tt> holding a raw Ruby object with an argument or 
			variable value.
		</div>
		<p><tt>$result</tt></p>
		<div class="indent">A <tt>VALUE</tt> that holds the result to be returned to Ruby.
		</div>
		<p><tt>$1_name</tt></p>
		<div class="indent">The parameter name that was matched.
		</div>
		<p><tt>$1_type</tt></p>
		<div class="indent">The actual C datatype matched by the typemap.
		</div>
		<p><tt>$1_ltype</tt></p>
		<div class="indent">An assignable version of the datatype matched by the typemap (a 
			type that can appear on the left-hand-side of a C assignment operation). This 
			type is stripped of qualifiers and may be an altered version of <tt>$1_type</tt>. 
			All arguments and local variables in wrapper functions are declared using this 
			type so that their values can be properly assigned.
		</div>
		<p><tt>$symname</tt></p>
		<div class="indent">The Ruby name of the wrapper function being created.
		</div>
<H3><a name="Ruby_nn33"></a>27.5.4 Useful Functions</H3>


		<p>
			When you write a typemap, you usually have to work directly with Ruby objects. 
			The following functions may prove to be useful. (These functions plus many more 
			can be found in <a href="http://www.rubycentral.com/book"><em>Programming Ruby</em></a>, 
			by David Thomas and Andrew Hunt.)
		</p>
		<p><a name="n34"></a></p>
<H4><a name="Ruby_nn34"></a>27.5.4.1 C Datatypes to Ruby Objects</H4>


		<div class="code">
			<pre>INT2NUM(long or int) - int to Fixnum or Bignum<br>INT2FIX(long or int) - int to Fixnum (faster than INT2NUM)<br>CHR2FIX(char) - char to Fixnum<br>rb_str_new2(char*) - char* to String<br>rb_float_new(double) - double to Float<br></pre>
		</div>
<H4><a name="Ruby_nn35"></a>27.5.4.2 Ruby Objects to C Datatypes</H4>


		<div class="code">
			<pre> int NUM2INT(Numeric)<br> int FIX2INT(Numeric)<br> unsigned int NUM2UINT(Numeric)<br> unsigned int FIX2UINT(Numeric)<br> long NUM2LONG(Numeric)<br> long FIX2LONG(Numeric)<br>unsigned long FIX2ULONG(Numeric)<br> char NUM2CHR(Numeric or String)<br> char * STR2CSTR(String)<br> char * rb_str2cstr(String, int*length)<br> double NUM2DBL(Numeric)<br><br></pre>
		</div>
<H4><a name="Ruby_nn36"></a>27.5.4.3 Macros for VALUE</H4>


		<p>
			<tt>RSTRING(str)-&gt;len</tt>
		</p>
		<div class="indent">length of the Ruby string</div>
		<p><tt>RSTRING(str)-&gt;ptr</tt></p>
		<div class="indent">pointer to string storage</div>
		<p><tt>RARRAY(arr)-&gt;len</tt></p>
		<div class="indent">length of the Ruby array</div>
		<p><tt>RARRAY(arr)-&gt;capa</tt></p>
		<div class="indent">capacity of the Ruby array</div>
		<p><tt>RARRAY(arr)-&gt;ptr</tt></p>
		<div class="indent">pointer to array storage</div>
<H4><a name="Ruby_nn37"></a>27.5.4.4 Exceptions</H4>


		<p>
			<tt>void rb_raise(VALUE exception, const char *fmt, ...)</tt>
		</p>
		<div class="indent">
			Raises an exception. The given format string <i>fmt</i> and remaining arguments 
			are interpreted as with <tt>printf()</tt>.
		</div>
		<p><tt>void rb_fatal(const char *fmt, ...)</tt></p>
		<div class="indent">
			Raises a fatal exception, terminating the process. No rescue blocks are called, 
			but ensure blocks will be called. The given format string <i>fmt</i> and 
			remaining arguments are interpreted as with <tt>printf()</tt>.
		</div>
		<p><tt>void rb_bug(const char *fmt, ...)</tt></p>
		<div class="indent">
			Terminates the process immediately -- no handlers of any sort will be called. 
			The given format string <i>fmt</i> and remaining arguments are interpreted as 
			with <tt>printf()</tt>. You should call this function only if a fatal bug has 
			been exposed.
		</div>
		<p><tt>void rb_sys_fail(const char *msg)</tt></p>
		<div class="indent">
			Raises a platform-specific exception corresponding to the last known system 
			error, with the given string <i>msg</i>.
		</div>
		<p><tt>VALUE rb_rescue(VALUE (*body)(VALUE), VALUE args, VALUE(*rescue)(VALUE, VALUE), 
				VALUE rargs)</tt></p>
		<div class="indent">
			Executes <i>body</i> with the given <i>args</i>. If a <tt>StandardError</tt> exception 
			is raised, then execute <i>rescue</i> with the given <i>rargs</i>.
		</div>
		<p><tt>VALUE rb_ensure(VALUE(*body)(VALUE), VALUE args, VALUE(*ensure)(VALUE), VALUE 
				eargs)</tt></p>
		<div class="indent">
			Executes <i>body</i> with the given <i>args</i>. Whether or not an exception is 
			raised, execute <i>ensure</i> with the given <i>rargs</i> after <i>body</i> has 
			completed.
		</div>
		<p><tt>VALUE rb_protect(VALUE (*body)(VALUE), VALUE args, int *result)</tt></p>
		<div class="indent">
			Executes <i>body</i> with the given <i>args</i> and returns nonzero in result 
			if any exception was raised.
		</div>
		<p><tt>void rb_notimplement()</tt></p>
		<div class="indent">
			Raises a <tt>NotImpError</tt> exception to indicate that the enclosed function 
			is not implemented yet, or not available on this platform.
		</div>
		<p><tt>void rb_exit(int status)</tt></p>
		<div class="indent">
			Exits Ruby with the given <i>status</i>. Raises a <tt>SystemExit</tt> exception 
			and calls registered exit functions and finalizers.
		</div>
		<p><tt>void rb_warn(const char *fmt, ...)</tt></p>
		<div class="indent">
			Unconditionally issues a warning message to standard error. The given format 
			string <i>fmt</i> and remaining arguments are interpreted as with <tt>printf()</tt>.
		</div>
		<p><tt>void rb_warning(const char *fmt, ...)</tt></p>
		<div class="indent">
			Conditionally issues a warning message to standard error if Ruby was invoked 
			with the <tt>-w</tt> flag. The given format string <i>fmt</i> and remaining 
			arguments are interpreted as with <tt>printf()</tt>.
		</div>
<H4><a name="Ruby_nn38"></a>27.5.4.5 Iterators</H4>


		<p>
			<tt>void rb_iter_break()</tt>
		</p>
		<div class="indent">
			Breaks out of the enclosing iterator block.
		</div>
		<p><tt>VALUE rb_each(VALUE obj)</tt></p>
		<div class="indent">
			Invokes the <tt>each</tt> method of the given <i>obj</i>.
		</div>
		<p><tt>VALUE rb_yield(VALUE arg)</tt></p>
		<div class="indent">
			Transfers execution to the iterator block in the current context, passing <i>arg</i>
			as an argument. Multiple values may be passed in an array.
		</div>
		<p><tt>int rb_block_given_p()</tt></p>
		<div class="indent">
			Returns <tt>true</tt> if <tt>yield</tt> would execute a block in the current 
			context; that is, if a code block was passed to the current method and is 
			available to be called.
		</div>
		<p><tt>VALUE rb_iterate(VALUE (*method)(VALUE), VALUE args, VALUE (*block)(VALUE, 
				VALUE), VALUE arg2)</tt></p>
		<div class="indent">
			Invokes <i>method</i> with argument <i>args</i> and block <i>block</i>. A <tt>yield</tt>
			from that method will invoke <i>block</i> with the argument given to <tt>yield</tt>, 
			and a second argument <i>arg2</i>.
		</div>
		<p><tt>VALUE rb_catch(const char *tag, VALUE (*proc)(VALUE, VALUE), VALUE value)</tt></p>
		<div class="indent">
			Equivalent to Ruby's <tt>catch</tt>.
		</div>
		<p><tt>void rb_throw(const char *tag, VALUE value)</tt></p>
		<div class="indent">
			Equivalent to Ruby's <tt>throw</tt>.
		</div>
<H3><a name="ruby_typemap_examples"></a>27.5.5 Typemap Examples</H3>


		<p>
			This section includes a few examples of typemaps. For more examples, you might 
			look at the examples in the <tt>Example/ruby</tt> directory.
		</p>
<H3><a name="Ruby_nn40"></a>27.5.6 Converting a Ruby array to a char **</H3>


		<p>
			A common problem in many C programs is the processing of command line 
			arguments, which are usually passed in an array of <tt>NULL</tt> terminated 
			strings. The following SWIG interface file allows a Ruby Array instance to be 
			used as a <tt>char **</tt> object.
		</p>
		<div class="code">
			<pre>%module argv<br><br>// This tells SWIG to treat char ** as a special case<br>%typemap(in) char ** {<br> /* Get the length of the array */<br> int size = RARRAY($input)-&gt;len; <br> int i;<br> $1 = (char **) malloc((size+1)*sizeof(char *));<br> /* Get the first element in memory */<br> VALUE *ptr = RARRAY($input)-&gt;ptr; <br> for (i=0; i &lt; size; i++, ptr++)<br> /* Convert Ruby Object String to char* */<br> $1[i]= STR2CSTR(*ptr); <br> $1[i]=NULL; /* End of list */<br>}<br><br>// This cleans up the char ** array created before <br>// the function call<br><br>%typemap(freearg) char ** {<br> free((char *) $1);<br>}<br><br>// Now a test function<br>%inline %{<br>int print_args(char **argv) {<br> int i = 0;<br> while (argv[i]) {<br> printf("argv[%d] = %s\n", i,argv[i]);<br> i++;<br> }<br> return i;<br>}<br>%}<br><br></pre>
		</div>
		<p>
			When this module is compiled, the wrapped C function now operates as follows :
		</p>
		<div class="code">
			<pre>require 'Argv'<br>Argv.print_args(["Dave","Mike","Mary","Jane","John"])<br>argv[0] = Dave<br>argv[1] = Mike<br>argv[2] = Mary<br>argv[3] = Jane<br>argv[4] = John<br></pre>
		</div>
		<p>
			In the example, two different typemaps are used. The "in" typemap is used to 
			receive an input argument and convert it to a C array. Since dynamic memory 
			allocation is used to allocate memory for the array, the "freearg" typemap is 
			used to later release this memory after the execution of the C function.
		</p>
<H3><a name="Ruby_nn41"></a>27.5.7 Collecting arguments in a hash</H3>


		<p>
			Ruby's solution to the "keyword arguments" capability of some other languages 
			is to allow the programmer to pass in one or more key-value pairs as arguments 
			to a function. All of those key-value pairs are collected in a single <tt>Hash</tt>
			argument that's presented to the function. If it makes sense, you might want to 
			provide similar functionality for your Ruby interface. For example, suppose 
			you'd like to wrap this C function that collects information about people's 
			vital statistics:
		</p>
		<div class="code">
			<pre>void setVitalStats(const char *person, int nattributes, const char **names, int *values);<br></pre>
		</div>
		<p>
			and you'd like to be able to call it from Ruby by passing in an arbitrary 
			number of key-value pairs as inputs, e.g.
		</p>
		<div class="code">
			<pre>setVitalStats("Fred",<br> 'weight' =&gt; 270,<br>	'age' =&gt; 42<br>	)<br></pre>
		</div>
		<p>
			To make this work, you need to write a typemap that expects a Ruby <tt>Hash</tt>
			as its input and somehow extracts the last three arguments (<i>nattributes</i>, <i>names</i>
			and <i>values</i>) needed by your C function. Let's start with the basics:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br>}<br> </pre>
		</div>
		<p>
			This <tt>%typemap</tt> directive tells SWIG that we want to match any function 
			declaration that has the specified types and names of arguments somewhere in 
			the argument list. The fact that we specified the argument names (<i>nattributes</i>,
			<i>names</i> and <i>values</i>) in our typemap is significant; this ensures 
			that SWIG won't try to apply this typemap to <i>other</i> functions it sees 
			that happen to have a similar declaration with different argument names. The 
			arguments that appear in the second set of parentheses (<i>keys_arr</i>, <i>i</i>,
			<i>key</i> and <i>val</i>) define local variables that our typemap will need.
		</p>
		<p>Since we expect the input argument to be a <tt>Hash</tt>, let's next add a check 
			for that:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br> <b>Check_Type($input, T_HASH);</b><br>}<br></pre>
		</div>
		<p>
			<tt>Check_Type()</tt> is just a macro (defined in the Ruby header files) that 
			confirms that the input argument is of the correct type; if it isn't, an 
			exception will be raised.
		</p>
		<p>The next task is to determine how many key-value pairs are present in the hash; 
			we'll assign this number to the first typemap argument (<tt>$1</tt>). This is a 
			little tricky since the Ruby/C API doesn't provide a public function for 
			querying the size of a hash, but we can get around that by calling the hash's <i>size</i>
			method directly and converting its result to a C <tt>int</tt> value:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br> Check_Type($input, T_HASH);<br> <b>$1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));</b><br>}<br></pre>
		</div>
		<p>
			So now we know the number of attributes. Next we need to initialize the second 
			and third typemap arguments (i.e. the two C arrays) to <tt>NULL</tt> and set 
			the stage for extracting the keys and values from the hash:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br> Check_Type($input, T_HASH);<br> $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));<br> <b>$2 = NULL;<br> $3 = NULL;<br> if ($1 &gt; 0) {<br> $2 = (char **) malloc($1*sizeof(char *));<br> $3 = (int *) malloc($1*sizeof(int));<br> }</b><br>}<br></pre>
		</div>
		<p>
			There are a number of ways we could extract the keys and values from the input 
			hash, but the simplest approach is to first call the hash's <i>keys</i> method 
			(which returns a Ruby array of the keys) and then start looping over the 
			elements in that array:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br> Check_Type($input, T_HASH);<br> $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));<br> $2 = NULL;<br> $3 = NULL;<br> if ($1 &gt; 0) {<br> $2 = (char **) malloc($1*sizeof(char *));<br> $3 = (int *) malloc($1*sizeof(int));<br> <b>keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);<br> for (i = 0; i &lt; $1; i++) {<br> }</b><br>}<br>}<br></pre>
		</div>
		<p>
			Recall that <i>keys_arr</i> and <i>i</i> are local variables for this typemap. 
			For each element in the <i>keys_arr</i> array, we want to get the key itself, 
			as well as the value corresponding to that key in the hash:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br> Check_Type($input, T_HASH);<br> $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));<br> $2 = NULL;<br> $3 = NULL;<br> if ($1 &gt; 0) {<br> $2 = (char **) malloc($1*sizeof(char *));<br> $3 = (int *) malloc($1*sizeof(int));<br> keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);<br> for (i = 0; i &lt; $1; i++) {<br> <b>key = rb_ary_entry(keys_arr, i);<br> val = rb_hash_aref($input, key);</b><br>}<br>}<br>}<br></pre>
		</div>
		<p>
			To be safe, we should again use the <tt>Check_Type()</tt> macro to confirm that 
			the key is a <tt>String</tt> and the value is a <tt>Fixnum</tt>:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br> Check_Type($input, T_HASH);<br> $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));<br> $2 = NULL;<br> $3 = NULL;<br> if ($1 &gt; 0) {<br> $2 = (char **) malloc($1*sizeof(char *));<br> $3 = (int *) malloc($1*sizeof(int));<br> keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);<br> for (i = 0; i &lt; $1; i++) {<br> key = rb_ary_entry(keys_arr, i);<br> val = rb_hash_aref($input, key);<br> <b>Check_Type(key, T_STRING);<br> Check_Type(val, T_FIXNUM);</b><br>}<br>}<br>}<br></pre>
		</div>
		<p>
			Finally, we can convert these Ruby objects into their C equivalents and store 
			them in our local C arrays:
		</p>
		<div class="code">
			<pre>%typemap(in) (int nattributes, const char **names, const int *values)<br> (VALUE keys_arr, int i, VALUE key, VALUE val) {<br> Check_Type($input, T_HASH);<br> $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));<br> $2 = NULL;<br> $3 = NULL;<br> if ($1 &gt; 0) {<br> $2 = (char **) malloc($1*sizeof(char *));<br> $3 = (int *) malloc($1*sizeof(int));<br> keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);<br> for (i = 0; i &lt; $1; i++) {<br> key = rb_ary_entry(keys_arr, i);<br> val = rb_hash_aref($input, key);<br> Check_Type(key, T_STRING);<br> Check_Type(val, T_FIXNUM);<br> <b>$2[i] = STR2CSTR(key);<br> $3[i] = NUM2INT(val);</b><br>}<br>}<br>}<br></pre>
		</div>
		<p>
			We're not done yet. Since we used <tt>malloc()</tt> to dynamically allocate the 
			memory used for the <i>names</i> and <i>values</i> arguments, we need to 
			provide a corresponding "freearg" typemap to free that memory so that there is 
			no memory leak. Fortunately, this typemap is a lot easier to write:
		</p>
		<div class="code">
			<pre>%typemap(freearg) (int nattributes, const char **names, const int *values) {<br> free((void *) $2);<br> free((void *) $3);<br>}<br></pre>
		</div>
		<p>
			All of the code for this example, as well as a sample Ruby program that uses 
			the extension, can be found in the <tt>Examples/ruby/hashargs</tt> directory of 
			the SWIG distribution.
		</p>
<H3><a name="Ruby_nn42"></a>27.5.8 Pointer handling</H3>


		<p>
			Occasionally, it might be necessary to convert pointer values that have been 
			stored using the SWIG typed-pointer representation. Since there are several 
			ways in which pointers can be represented, the following two functions are used 
			to safely perform this conversion:
		</p>
		<p><tt>int SWIG_ConvertPtr(VALUE obj, void **ptr, swig_type_info *ty, int flags)</tt>
		</p>
		<div class="indent">Converts a Ruby object <i>obj</i> to a C pointer whose address 
			is <i>ptr</i> (i.e. <i>ptr</i> is a pointer to a pointer). The third argument, <i>ty</i>, 
			is a pointer to a SWIG type descriptor structure. If <i>ty</i> is not <tt>NULL</tt>, 
			that type information is used to validate type compatibility and other aspects 
			of the type conversion. If <i>flags</i> is non-zero, any type errors 
			encountered during this validation result in a Ruby <tt>TypeError</tt> exception 
			being raised; if <i>flags</i> is zero, such type errors will cause <tt>SWIG_ConvertPtr()</tt>
			to return -1 but not raise an exception. If <i>ty</i> is <tt>NULL</tt>, no 
			type-checking is performed.
		</div>
		<p>
			<tt>VALUE SWIG_NewPointerObj(void *ptr, swig_type_info *ty, int own)</tt>
		</p>
		<div class="indent">Creates a new Ruby pointer object. Here, <i>ptr</i> is the 
			pointer to convert, <i>ty</i> is the SWIG type descriptor structure that 
			describes the type, and <i>own</i> is a flag that indicates whether or not Ruby 
			should take ownership of the pointer (i.e. whether Ruby should free this data 
			when the corresponding Ruby instance is garbage-collected).
		</div>
		<p>
			Both of these functions require the use of a special SWIG type-descriptor 
			structure. This structure contains information about the mangled name of the 
			datatype, type-equivalence information, as well as information about converting 
			pointer values under C++ inheritance. For a type of <tt>Foo *</tt>, the type 
			descriptor structure is usually accessed as follows:
		</p>
		<div class="indent">
			<pre>Foo *foo;<br>SWIG_ConvertPtr($input, (void **) &amp;foo, SWIGTYPE_p_Foo, 1);<br><br>VALUE obj;<br>obj = SWIG_NewPointerObj(f, SWIGTYPE_p_Foo, 0);<br></pre>
		</div>
		<p>
			In a typemap, the type descriptor should always be accessed using the special 
			typemap variable <tt>$1_descriptor</tt>. For example:
		</p>
		<div class="indent">
			<pre>%typemap(in) Foo * {<br> SWIG_ConvertPtr($input, (void **) &amp;$1, $1_descriptor, 1);<br>}<br></pre>
		</div>
<H4><a name="Ruby_nn43"></a>27.5.8.1 Ruby Datatype Wrapping</H4>


		<p>
			<tt>VALUE Data_Wrap_Struct(VALUE class, void (*mark)(void *), void (*free)(void *), 
				void *ptr)</tt>
		</p>
		<div class="indent">Given a pointer <i>ptr</i> to some C data, and the two garbage 
			collection routines for this data (<i>mark</i> and <i>free</i>), return a <tt>VALUE</tt>
			for the Ruby object.
		</div>
		<p><tt>VALUE Data_Make_Struct(VALUE class, <i>c-type</i>, void (*mark)(void *), void 
				(*free)(void *), <i>c-type</i> *ptr)</tt></p>
		<div class="indent">Allocates a new instance of a C data type <i>c-type</i>, 
			assigns it to the pointer <i>ptr</i>, then wraps that pointer with <tt>Data_Wrap_Struct()</tt>
			as above.
		</div>
		<p><tt>Data_Get_Struct(VALUE obj, <i>c-type</i>, <i>c-type</i> *ptr)</tt></p>
		<div class="indent">Retrieves the original C pointer of type <i>c-type</i> from the 
			data object <i>obj</i> and assigns that pointer to <i>ptr</i>.
		</div>
<H2><a name="ruby_operator_overloading"></a>27.6 Operator overloading</H2>


		<p>
			SWIG allows operator overloading with, by using the <tt>%extend</tt> or <tt>%rename</tt>
			commands in SWIG and the following operator names (derived from Python):
		</p>
		<div class="code">
			<pre><b> General</b> <br>__repr__ - inspect<br>__str__ - to_s<br>__cmp__ - &lt;=&gt;<br>__hash__ - hash<br>__nonzero__ - nonzero?<br><br><b> Callable</b> <br>__call__ - call<br><br><b> Collection</b> <br>__len__ - length<br>__getitem__ - []<br>__setitem__ - []=<br><br><b> Numeric</b> <br>__add__ - +<br>__sub__ - -<br>__mul__ - *<br>__div__ - /<br>__mod__ - %<br>__divmod__ - divmod<br>__pow__ - **<br>__lshift__ - &lt;&lt;<br>__rshift__ - &gt;&gt;<br>__and__ - &amp;<br>__xor__ - ^<br>__or__ - |<br>__neg__ - -@<br>__pos__ - +@<br>__abs__ - abs<br>__invert__ - ~<br>__int__ - to_i<br>__float__ - to_f<br>__coerce__ - coerce<br><br><b>Additions in 1.3.13 </b> <br>__lt__ - &lt; <br>__le__ - &lt;=<br>__eq__ - ==<br>__gt__ - &gt;<br>__ge__ - &gt;=<br><br></pre>
		</div>
		<p>
			Note that although SWIG supports the <tt>__eq__</tt> magic method name for 
			defining an equivalence operator, there is no separate method for handling <i>inequality</i>
			since Ruby parses the expression <i>a != b</i> as <i>!(a == b)</i>.
		</p>
<H3><a name="Ruby_nn45"></a>27.6.1 Example: STL Vector to Ruby Array</H3>


		<p>
			<em><b>FIXME: This example is out of place here!</b></em>
		</p>
		<p>Another use for macros and type maps is to create a Ruby array from a STL vector 
			of pointers. In essence, copy of all the pointers in the vector into a Ruby 
			array. The use of the macro is to make the typemap so generic that any vector 
			with pointers can use the type map. The following is an example of how to 
			construct this type of macro/typemap and should give insight into constructing 
			similar typemaps for other STL structures:
		</p>
		<div class="code">
			<pre>%define PTR_VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)<br>%typemap(ruby, out) vectorclassname &amp;, const vectorclassname &amp; {<br> VALUE arr = rb_ary_new2($1-&gt;size());<br> vectorclassname::iterator i = $1-&gt;begin(), iend = $1-&gt;end();<br> for ( ; i!=iend; i++ )<br> rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));<br> $result = arr;<br>}<br>%typemap(ruby, out) vectorclassname, const vectorclassname {<br> VALUE arr = rb_ary_new2($1.size());<br> vectorclassname::iterator i = $1.begin(), iend = $1.end();<br> for ( ; i!=iend; i++ )<br> rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));<br> $result = arr;<br>}<br>%enddef<br></pre>
		</div>
		<p>
			Note, that the "<tt>c ## classname.klass"</tt> is used in the preprocessor step 
			to determine the actual object from the class name.
		</p>
		<p>To use the macro with a class Foo, the following is used:
		</p>
		<div class="code">
			<pre>PTR_VECTOR_TO_RUBY_ARRAY(vector&lt;foo *=""&gt;, Foo)<br></pre>
		</div>
		<p>
			It is also possible to create a STL vector of Ruby objects:
		</p>
		<div class="code">
			<pre>%define RUBY_ARRAY_TO_PTR_VECTOR(vectorclassname, classname)<br>%typemap(ruby, in) vectorclassname &amp;, const vectorclassname &amp; {<br> Check_Type($input, T_ARRAY);<br> vectorclassname *vec = new vectorclassname;<br> int len = RARRAY($input)-&gt;len;<br> for (int i=0; i!=len; i++) {<br> VALUE inst = rb_ary_entry($input, i);<br> //The following _should_ work but doesn't on HPUX<br> // Check_Type(inst, T_DATA);<br> classname *element = NULL;<br> Data_Get_Struct(inst, classname, element);<br> vec-&gt;push_back(element);<br> }<br> $1 = vec;<br>}<br><br>%typemap(ruby, freearg) vectorclassname &amp;, const vectorclassname &amp; {<br> delete $1;<br>}<br>%enddef<br></pre>
		</div>
		<p>
			It is also possible to create a Ruby array from a vector of static data types:
		</p>
		<div class="code">
			<pre>%define VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)<br>%typemap(ruby, out) vectorclassname &amp;, const vectorclassname &amp; {<br> VALUE arr = rb_ary_new2($1-&gt;size()); <br> vectorclassname::iterator i = $1-&gt;begin(), iend = $1-&gt;end();<br> for ( ; i!=iend; i++ )<br> rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &amp;(*i)));<br> $result = arr;<br>}<br>%typemap(ruby, out) vectorclassname, const vectorclassname {<br> VALUE arr = rb_ary_new2($1.size()); <br> vectorclassname::iterator i = $1.begin(), iend = $1.end();<br> for ( ; i!=iend; i++ )<br> rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &amp;(*i)));<br> $result = arr;<br>}<br>%enddef<br></pre>
		</div>
<H2><a name="Ruby_nn46"></a>27.7 Advanced Topics</H2>


<H3><a name="Ruby_nn47"></a>27.7.1 Creating Multi-Module Packages</H3>


		<p>
			The chapter on <a href="Modules.html">Working with Modules</a> discusses the 
			basics of creating multi-module extensions with SWIG, and in particular the 
			considerations for sharing runtime type information among the different 
			modules.
		</p>
		<p>As an example, consider one module's interface file (<tt>shape.i</tt>) that 
			defines our base class:
		</p>
		<div class="code">
			<pre>%module shape<br><br>%{<br>#include "Shape.h"<br>%}<br><br>class Shape {<br>protected:<br> double xpos;<br> double ypos;<br>protected:<br> Shape(double x, double y);<br>public:<br> double getX() const;<br> double getY() const;<br>};<br></pre>
		</div>
		<p>
			We also have a separate interface file (<tt>circle.i</tt>) that defines a 
			derived class:
		</p>
		<div class="code">
			<pre>%module circle<br><br>%{<br>#include "Shape.h"<br>#include "Circle.h"<br>%}<br><br>// Import the base class definition from Shape module<br>%import shape.i<br><br>class Circle : public Shape {<br>protected:<br> double radius;<br>public:<br> Circle(double x, double y, double r);<br> double getRadius() const;<br>};<br></pre>
		</div>
		<p>
			We'll start by building the <b>Shape</b> extension module:
		</p>
		<div class="code">
			<pre>$ <b>swig -c++ -ruby shape.i</b>
</pre>
		</div>
		<p>
			SWIG generates a wrapper file named <tt>shape_wrap.cxx</tt>. To compile this 
			into a dynamically loadable extension for Ruby, prepare an <tt>extconf.rb</tt> script 
			using this template:
		</p>
		<div class="code">
			<pre>require 'mkmf'<br><br># Since the SWIG runtime support library for Ruby<br># depends on the Ruby library, make sure it's in the list<br># of libraries.<br>$libs = append_library($libs, Config::CONFIG['RUBY_INSTALL_NAME'])<br><br># Create the makefile<br>create_makefile('shape')<br></pre>
		</div>
		<p>
			Run this script to create a <tt>Makefile</tt> and then type <tt>make</tt> to 
			build the shared library:
		</p>
		<div class="code">
			<pre>$ <b>ruby extconf.rb</b><br>creating Makefile<br>$ <b>make</b><br>g++ -fPIC -g -O2 -I. -I/usr/local/lib/ruby/1.7/i686-linux \<br>-I. -c shape_wrap.cxx<br>gcc -shared -L/usr/local/lib -o shape.so shape_wrap.o -L. \<br>-lruby -lruby -lc<br></pre>
		</div>
		<p>
			Note that depending on your installation, the outputs may be slightly 
			different; these outputs are those for a Linux-based development environment. 
			The end result should be a shared library (here, <tt>shape.so</tt>) containing 
			the extension module code. Now repeat this process in a separate directory for 
			the <b>Circle</b> module:
		</p>
		<ol>
			<li>
				Run SWIG to generate the wrapper code (<tt>circle_wrap.cxx</tt>);
			</li>
			<li>
				Write an <tt>extconf.rb</tt> script that your end-users can use to create a 
				platform-specific <tt>Makefile</tt> for the extension;
			</li>
			<li>
				Build the shared library for this extension by typing <tt>make</tt>.
			</li>
		</ol>
		<p>
			Once you've built both of these extension modules, you can test them 
			interactively in IRB to confirm that the <tt>Shape</tt> and <tt>Circle</tt> modules 
			are properly loaded and initialized:
		</p>
		<div class="code">
			<pre>$ <b>irb</b><br>irb(main):001:0&gt; <b>require 'shape'</b><br>true<br>irb(main):002:0&gt; <b>require 'circle'</b><br>true<br>irb(main):003:0&gt; <b>c = Circle::Circle.new(5, 5, 20)</b><br>#&lt;Circle::Circle:0xa097208&gt;<br>irb(main):004:0&gt; <b>c.kind_of? Shape::Shape</b><br>true<br>irb(main):005:0&gt; <b>c.getX()</b><br>5.0<br></pre>
		</div>
<H3><a name="Ruby_nn48"></a>27.7.2 Defining Aliases</H3>


		<p>
			It's a fairly common practice in the Ruby built-ins and standard library to 
			provide aliases for method names. For example, <em>Array#size</em> is an alias 
			for <em>Array#length</em>. If you'd like to provide an alias for one of your 
			class' instance methods, one approach is to use SWIG's <tt>%extend</tt> directive 
			to add a new method of the aliased name that calls the original function. For 
			example:
		</p>
		<div class="code">
			<pre>class MyArray {<br>public:<br> // Construct an empty array<br> MyArray();<br> <br> // Return the size of this array<br> size_t length() const;<br>};<br><br>%extend MyArray {<br> // MyArray#size is an alias for MyArray#length<br> size_t size() const {<br> return self-&gt;length();<br> }<br>}<br></pre>
		</div>
		<p>
			A better solution is to instead use the <tt>%alias</tt> directive (unique to 
			SWIG's Ruby module). The previous example could then be rewritten as:
		</p>
		<div class="code">
			<pre>// MyArray#size is an alias for MyArray#length<br>%alias MyArray::length "size";<br><br>class MyArray {<br>public:<br> // Construct an empty array<br> MyArray();<br> <br> // Return the size of this array<br> size_t length() const;<br>};<br></pre>
		</div>
		<p>
			Multiple aliases can be associated with a method by providing a comma-separated 
			list of aliases to the <tt>%alias</tt> directive, e.g.
		</p>
		<div class="code">
			<pre>%alias MyArray::length "amount,quantity,size";</pre>
		</div>
		<p>
			From an end-user's standpoint, there's no functional difference between these 
			two approaches; i.e. they should get the same result from calling either <em>MyArray#size</em>
			or <em>MyArray#length</em>. However, when the <tt>%alias</tt> directive is 
			used, SWIG doesn't need to generate all of the wrapper code that's usually 
			associated with added methods like our <em>MyArray::size()</em> example.
		</p>
		<p>Note that the <tt>%alias</tt> directive is implemented using SWIG's "features" 
			mechanism and so the same name matching rules used for other kinds of features 
			apply (see the chapter on <a href="Customization.html#Customization">"Customization 
				Features"</a>) for more details).</p>
<H3><a name="Ruby_nn49"></a>27.7.3 Predicate Methods</H3>


		<p>
			Predicate methods in Ruby are those which return either <tt>true</tt> or <tt>false</tt>. 
			By convention, these methods' names end in a question mark; some examples from 
			built-in Ruby classes include <em>Array#empty?</em> (which returns <tt>true</tt>
			for an array containing no elements) and <em>Object#instance_of?</em> (which 
			returns <tt>true</tt> if the object is an instance of the specified class). For 
			consistency with Ruby conventions you would also want your interface's 
			predicate methods' names to end in a question mark and return <tt>true</tt> or <tt>false</tt>.
		</p>
		<p>One cumbersome solution to this problem is to rename the method (using SWIG's <tt>%rename</tt>
			directive) and provide a custom typemap that converts the function's actual 
			return type to Ruby's <tt>true</tt> or <tt>false</tt>. For example:
		</p>
		<div class="code">
			<pre>%rename("is_it_safe?") is_it_safe();<br><br>%typemap(out) int is_it_safe <br> "$result = ($1 != 0) ? Qtrue : Qfalse;";<br><br>int is_it_safe();<br></pre>
		</div>
		<p>
			A better solution is to instead use the <tt>%predicate</tt> directive (unique 
			to SWIG's Ruby module) to designate certain methods as predicate methods. For 
			the previous example, this would look like:
		</p>
		<div class="code">
			<pre>%predicate is_it_safe();<br><br>int is_it_safe();<br></pre>
		</div>
		<p>and to use this method from your Ruby code:</p>
		<div class="code">
			<pre>irb(main):001:0&gt; <b>Example::is_it_safe?</b><br>true<br></pre>
		</div>
		<p>
			Note that the <tt>%predicate</tt> directive is implemented using SWIG's 
			"features" mechanism and so the same name matching rules used for other kinds 
			of features apply (see the chapter on <a href="Customization.html#Customization">"Customization 
				Features"</a>) for more details).
		</p>
<H3><a name="Ruby_nn50"></a>27.7.4 Specifying Mixin Modules</H3>


		<p>
			The Ruby language doesn't support multiple inheritance, but it does allow you 
			to mix one or more modules into a class using Ruby's <tt>include</tt> method. 
			For example, if you have a Ruby class that defines an <em>each</em> instance 
			method, e.g.
		</p>
		<div class="code">
			<pre>class Set<br> def initialize<br> @members = []<br> end<br> <br> def each<br> @members.each { |m| yield m }<br> end<br>end<br></pre>
		</div>
		<p>
			then you can mix-in Ruby's <tt>Enumerable</tt> module to easily add a lot of 
			functionality to your class:
		</p>
		<div class="code">
			<pre>class Set<br> <b>include Enumerable</b><br>def initialize<br>@members = []<br>end<br>def each<br>@members.each { |m| yield m }<br>end<br>end<br></pre>
		</div>
		<p>
			To get the same benefit for your SWIG-wrapped classes, you can use the <tt>%mixin</tt>
			directive to specify the names of one or more modules that should be mixed-in 
			to a class. For the above example, the SWIG interface specification might look 
			like this:
		</p>
		<div class="code">
			<pre>%mixin Set "Enumerable";<br><br>class Set {<br>public:<br> // Constructor<br> Set();<br> <br> // Iterates through set members<br> void each();<br>};<br></pre>
		</div>
		<p>
			Multiple modules can be mixed into a class by providing a comma-separated list 
			of module names to the <tt>%mixin</tt> directive, e.g.
		</p>
		<div class="code">
			<pre>%mixin Set "Fee,Fi,Fo,Fum";</pre>
		</div>
		<p>
			Note that the <tt>%mixin</tt> directive is implemented using SWIG's "features" 
			mechanism and so the same name matching rules used for other kinds of features 
			apply (see the chapter on <a href="Customization.html#Customization">"Customization 
				Features"</a>) for more details).
		</p>
<H2><a name="Ruby_nn51"></a>27.8 Memory Management</H2>


		<p>One of the most common issues in generating SWIG bindings for Ruby is proper 
			memory management. The key to proper memory management is clearly defining 
			whether a wrapper Ruby object owns the underlying C struct or C++ class. There 
			are two possibilities:</p>
		<ul>
			<li>
				The Ruby object is responsible for freeing the C struct or C++ object
			</li>
			<li>
				The Ruby object should not free the C struct or C++ object because it will be 
				freed by the underlying C or C++ code</li>
		</ul>
		<p>To complicate matters, object ownership may transfer from Ruby to C++ (or vice 
			versa) depending on what function or methods are invoked. Clearly, developing a 
			SWIG wrapper requires a thorough understanding of how the underlying library 
			manages memory.</p>
		<h3><a name="Ruby_nn52" id="Ruby_nn52"></a>27.9.1 Mark and Sweep Garbage Collector
		</h3>
		<p>Ruby uses a mark and sweep garbage collector. When the garbage collector runs, 
			it finds all the "root" objects, including local variables, global variables, 
			global constants, hardware registers and the C stack. For each root object, the 
			garbage collector sets its mark flag to true and calls <tt>rb_gc_mark</tt> on 
			the object. The job of <tt>rb_gc_mark</tt> is to recursively mark all the 
			objects that a Ruby object has a reference to (ignoring those objects that have 
			already been marked). Those objects, in turn, may reference other objects. This 
			process will continue until all active objects have been "marked." After the 
			mark phase comes the sweep phase. In the sweep phase, all objects that have not 
			been marked will be garbage collected. For more information about the Ruby 
			garbage collector please refer to <a href="http://rubygarden.org/ruby/ruby?GCAndExtensions">
				<span style="text-decoration: underline;">http://rubygarden.org/ruby/ruby?GCAndExtensions</span></a>.</p>
		<p>The Ruby C/API provides extension developers two hooks into the garbage 
			collector - a "mark" function and a "sweep" function. By default these functions 
			are set to NULL.</p>
		<p>If a C struct or C++ class references any other Ruby objects, then it must 
			provide a "mark" function. The "mark" function should identify any referenced 
			Ruby objects by calling the rb_gc_mark function for each one. Unsurprisingly, 
			this function will be called by the Ruby garbage during the "mark" phase.</p>
		<p>During the sweep phase, Ruby destroys any unused objects. If any memory has been 
			allocated in creating the underlying C struct or C++ struct, then a "free" 
			function must be defined that deallocates this memory.
		</p>
<H3><a name="Ruby_nn53"></a>27.8.1 Object Ownership</H3>


		<p>As described above, memory management depends on clearly defining who is 
			responsible for freeing the underlying C struct or C++ class. If the Ruby 
			object is responsible for freeing the C++ object, then a "free" function must 
			be registered for the object. If the Ruby object is not responsible for freeing 
			the underlying memory, then a "free" function must not be registered for the 
			object.</p>
		<p>For the most part, SWIG takes care of memory management issues. The rules it 
			uses are:</p>
		<ul>
			<li>
				When calling a C++ object's constructor from Ruby, SWIG will assign a "free" 
				function thereby making the Ruby object responsible for freeing the C++ object</li>
			<li>
				When calling a C++ member function that returns a pointer, SWIG will not assign 
				a "free" function thereby making the underlying library responsible for freeing 
				the object.</li>
		</ul>
		<p>To make this clearer, let's look at an example. Assume we have a Foo and a Bar 
			class.
		</p>
		<div class="code">
			<pre>/* File "RubyOwernshipExample.h" */<br><br>class Foo<br>{<br>public:<br> Foo() {}<br> ~Foo() {}<br>};<br><br>class Bar<br>{<br> Foo *foo_;<br>public:<br> Bar(): foo_(new Foo) {}<br> ~Bar() { delete foo_; }<br> Foo* get_foo() { return foo_; }<br> Foo* get_new_foo() { return new Foo; }<br> void set_foo(Foo *foo) { delete foo_; foo_ = foo; }<br>};<br>
</pre>
		</div>
		<p>First, consider this Ruby code:
		</p>
		<div class="code">
			<pre>foo = Foo.new</pre>
		</div>
		<p>In this case, the Ruby code calls the underlying <tt>Foo</tt> C++ constructor, 
			thus creating a new <tt>foo</tt> object. By default, SWIG will assign the new 
			Ruby object a "free" function. When the Ruby object is garbage collected, the 
			"free" function will be called. It in turn will call <tt>Foo's</tt> destructor.</p>
		<p>Next, consider this code:
		</p>
		<div class="code">
			<pre>bar = Bar.new<br>foo = bar.get_foo()</pre>
		</div>
		<p>In this case, the Ruby code calls a C++ member function, <tt>get_foo</tt>. By 
			default, SWIG will not assign the Ruby object a "free" function. Thus, when the 
			Ruby object is garbage collected the underlying C++ <tt>foo</tt> object is not 
			affected.</p>
		<p>Unfortunately, the real world is not as simple as the examples above. For 
			example:</p>
		<div class="code">
			<pre>bar = Bar.new<br>foo = bar.get_new_foo()</pre>
		</div>
		<p>In this case, the default SWIG behavior for calling member functions is 
			incorrect. The Ruby object should assume ownership of the returned object. This 
			can be done by using the %newobject directive. See <a href="file:///d:/msys/1.0/src/SWIG/Doc/Manual/Customization.html#ownership">
				Object ownership and %newobject</a> for more information.
		</p>
		<p>The SWIG default mappings are also incorrect in this case:</p>
		<div class="code">
			<pre>foo = Foo.new<br>bar = Bar.new<br>bar.set_foo(foo)</pre>
		</div>
		<p>Without modification, this code will cause a segmentation fault. When the Ruby <tt>foo</tt>
			object goes out of scope, it will free the underlying C++ <tt>foo</tt> object. 
			However, when the Ruby bar object goes out of scope, it will call the C++ bar 
			destructor which will also free the C++ <tt>foo</tt> object. The problem is 
			that object ownership is transferred from the Ruby object to the C++ object 
			when the <tt>set_foo</tt> method is called. This can be done by using the 
			special DISOWN type map, which was added to the Ruby bindings in SWIG-1.3.26.</p>
		<p>Thus, a correct SWIG interface file correct mapping for these classes is:</p>
		<div class="code">
			<pre>/* File RubyOwnershipExample.i */<br><br>%module RubyOwnershipExample<br><br>%{<br>#include "RubyOwnershipExample.h"<br>%}<br><br>class Foo<br>{<br>public:<br> Foo();<br> ~Foo();<br>};<br><br>class Bar<br>{<br> Foo *foo_;<br>public:<br> Bar();<br> ~Bar();<br> Foo* get_foo();<br><br><span style="font-weight: bold;"> %newobject get_new_foo;</span><br> Foo* get_new_foo();<br><br><span style="font-weight: bold;"> %apply SWIGTYPE *DISOWN {Foo *foo};</span><br> void set_foo(Foo *foo);<br><span style="font-weight: bold;"> %clear Foo *foo;</span><br>};<br>
</pre>
		</div>
		<br>
		<p>
			This code can be seen in swig/examples/ruby/tracking.</p>
		<br>
<H3><a name="Ruby_nn54"></a>27.8.2 Object Tracking</H3>


		<p>The remaining parts of this section will use the class library shown below to 
			illustrate different memory management techniques. The class library models a 
			zoo and the animals it contains.
		</p>
		<div class="code">
			<pre>%module zoo<br><br>%{<br>#include &lt;string&gt;<br>#include &lt;vector&gt;<br><br>#include "zoo.h"<br>%}<br><br>class Animal<br>{<br>private:<br> typedef std::vector&lt;Animal*&gt; AnimalsType;<br> typedef AnimalsType::iterator IterType;<br>protected:<br> AnimalsType animals;<br>protected:<br> std::string name_;<br>public:<br> // Construct an animal with this name<br> Animal(const char* name) : name_(name) {}<br> <br> // Return the animal's name<br> const char* get_name() const { return name.c_str(); }<br>};<br><br>class Zoo<br>{<br>protected:<br> std::vector&lt;animal *=""&gt; animals;<br> <br>public:<br> // Construct an empty zoo<br> Zoo() {}<br> <br> /* Create a new animal. */<br> static Animal* Zoo::create_animal(const char* name)<br> {<br> return new Animal(name);<br> }<br><br> // Add a new animal to the zoo<br> void add_animal(Animal* animal) {<br> animals.push_back(animal); <br> }<br><br> Animal* remove_animal(size_t i) {<br> Animal* result = this-&gt;animals[i];<br> IterType iter = this-&gt;animals.begin();<br> std::advance(iter, i);<br> this-&gt;animals.erase(iter);<br><br> return result;<br> }<br> <br> // Return the number of animals in the zoo<br> size_t get_num_animals() const {<br> return animals.size(); <br> }<br> <br> // Return a pointer to the ith animal<br> Animal* get_animal(size_t i) const {<br> return animals[i]; <br> }<br>};<br>
</pre>
		</div>
		<p>Let's say you SWIG this code and then run IRB:<br>
		</p>
		<div class="code">
			<pre>$ <span style="font-weight: bold;">irb</span><br>irb(main):001:0&gt; <span style="font-weight: bold;">require 'example'</span><br>=&gt; true<br><br>irb(main):002:0&gt; <span style="font-weight: bold;">tiger1 = Example::Animal.new("tiger1")</span><br>=&gt; #&lt;Example::Animal:0x2be3820&gt;<br><br>irb(main):004:0&gt; <span style="font-weight: bold;">tiger1.get_name()</span><br>=&gt; "tiger1"<br><br>irb(main):003:0&gt; <span style="font-weight: bold;">zoo = Example::Zoo.new()</span><br>=&gt; #&lt;Example::Zoo:0x2be0a60&gt;<br><br>irb(main):006:0&gt; <span style="font-weight: bold;">zoo.add_animal(tiger)</span><br>=&gt; nil<br><br>irb(main):007:0&gt; <span style="font-weight: bold;">zoo.get_num_animals()</span><br>=&gt; 1<br><br>irb(main):007:0&gt; <span style="font-weight: bold;">tiger2 = zoo.remove_animal(0)</span><br>=&gt; #&lt;Example::Animal:0x2bd4a18&gt;<br><br>irb(main):008:0&gt; <span style="font-weight: bold;">tiger2.get_name()</span><br>=&gt; "tiger1"<br><br>irb(main):009:0&gt; <span style="font-weight: bold;">tiger1.equal?(tiger2)</span><br>=&gt; false<br>
</pre>
		</div>
		<p>Pay particular attention to the code <tt>tiger1.equal?(tiger2)</tt>. Note that 
			the two Ruby objects are not the same - but they reference the same underlying 
			C++ object. This can cause problems. For example:<br>
		</p>
		<div class="code">
			<pre>irb(main):010:0&gt; <span style="font-weight: bold;">tiger1 = nil</span><br>=&gt; nil<br><br>irb(main):011:0&gt; <span style="font-weight: bold;">GC.start</span><br>=&gt; nil<br><br>irb(main):012:0&gt; <span style="font-weight: bold;">tiger2.get_name()</span><br>(irb):12: [BUG] Segmentation fault<br>
</pre>
		</div>
		<p>After the the garbage collector runs, as a result of our call to <tt>GC.start</tt>, 
			calling<tt>tiger2.get_name()</tt> causes a segmentation fault. The problem is 
			that when <tt>tiger1</tt> is garbage collected, it frees the underlying C++ 
			object. Thus, when <tt>tiger2</tt> calls the <tt>get_name()</tt> method it 
			invokes it on a destroyed object.</p>
		<p>This problem can be avoided if SWIG enforces a one-to-one mapping between Ruby 
			objects and C++ classes. This can be done via the use of the <tt>%trackobjects</tt>
			functionality available in SWIG-1.3.26. and later.</p>
		<p>When the <tt>%trackobjects</tt> is turned on, SWIG automatically keeps track of 
			mappings between C++ objects and Ruby objects. Note that enabling object 
			tracking causes a slight performance degradation. Test results show this 
			degradation to be about 3% to 5% when creating and destroying 100,000 animals 
			in a row.</p>
		<p>Since <tt>%trackobjects</tt> is implemented as a <tt>%feature</tt>, it uses the same name matching 
			rules as other kinds of features (see the chapter on <a href="Customization.html#Customization">
				"Customization Features"</a>) . Thus it can be applied on a class-by-class 
			basis if needed. To fix the example above:</p>
		<br>
		<div class="code">
			<pre>%module example<br><br>%{<br>#include "example.h"<br>%}<br><br><span style="font-weight: bold;">/* Tell SWIG that create_animal creates a new object */</span><br><span style="font-weight: bold;">%newobject Zoo::create_animal;</span><br><br><span style="font-weight: bold;">/* Tell SWIG to keep track of mappings between C/C++ structs/classes. */</span><br style="font-weight: bold;"><span style="font-weight: bold;">%trackobjects;</span><br><br>%include "example.h"</pre>
		</div>
		<p>When this code runs we see:<br>
			<br>
		</p>
		<div class="code">
			<pre>$ <span style="font-weight: bold;">irb</span><br>irb(main):001:0&gt; <span style="font-weight: bold;">require 'example'</span><br>=&gt; true<br><br>irb(main):002:0&gt; <span style="font-weight: bold;">tiger1 = Example::Animal.new("tiger1")</span><br>=&gt; #&lt;Example::Animal:0x2be37d8&gt;<br><br>irb(main):003:0&gt; <span style="font-weight: bold;">zoo = Example::Zoo.new()</span><br>=&gt; #&lt;Example::Zoo:0x2be0a18&gt;<br><br>irb(main):004:0&gt; <span style="font-weight: bold;">zoo.add_animal(tiger1)</span><br>=&gt; nil<br><br>irb(main):006:0&gt; <span style="font-weight: bold;">tiger2 = zoo.remove_animal(0)</span><br>=&gt; #&lt;Example::Animal:0x2be37d8&gt;<br><br>irb(main):007:0&gt; <span style="font-weight: bold;">tiger1.equal?(tiger2)</span><br>=&gt; true<br><br>irb(main):008:0&gt; <span style="font-weight: bold;">tiger1 = nil</span><br>=&gt; nil<br><br>irb(main):009:0&gt; <span style="font-weight: bold;">GC.start</span><br>=&gt; nil<br><br>irb(main):010:0&gt; <span style="font-weight: bold;">tiger.get_name()</span><br>=&gt; "tiger1"<br>irb(main):011:0&gt;<br>
</pre>
		</div>
		<p>For those who are interested, object tracking is implemented by storing Ruby 
			objects in a hash table and keying them on C++ pointers. The underlying API is:<br>
		</p>
		<div class="code">
			<pre>static void SWIG_RubyAddTracking(void* ptr, VALUE object);<br>static VALUE SWIG_RubyInstanceFor(void* ptr) ;<br>static void SWIG_RubyRemoveTracking(void* ptr);<br>static void SWIG_RubyUnlinkObjects(void* ptr);</pre>
		</div>
		<p>When an object is created, SWIG will automatically call the <tt>SWIG_RubyAddTracking</tt>
			method. Similarly, when an object is deleted, SWIG will call the <tt>SWIG_RubyRemoveTracking</tt>. 
			When an object is returned to Ruby from C++, SWIG will use the <tt>SWIG_RubyInstanceFor</tt>
			method to ensure a one-to-one mapping from Ruby to C++ objects. Last, the <tt>RubyUnlinkObjects</tt>
			method unlinks a Ruby object from its underlying C++ object.</p>
		<p>In general, you will only need to use the <tt>SWIG_RubyInstanceFor</tt>, which 
			is required for implementing mark functions as shown below. However, if you 
			implement your own free functions (see below) you may also have to call the<tt>SWIG_RubyRemoveTracking</tt>
			and <tt>RubyUnlinkObjects</tt> methods.</p>

<H3><a name="Ruby_nn55"></a>27.8.3 Mark Functions</H3>


		<p>With a bit more testing, we see that our class library still has problems. For 
			example:<br>
		</p>
		<div class="code">
			<pre>$ <b>irb</b><br>irb(main):001:0&gt; <span style="font-weight: bold;">require 'example'</span><br>=&gt; true<br><br>irb(main):002:0&gt; tiger1 = <span style="font-weight: bold;">Example::Animal.new("tiger1")</span><br>=&gt; #&lt;Example::Animal:0x2bea6a8&gt;<br><br>irb(main):003:0&gt; zoo = <span style="font-weight: bold;">Example::Zoo.new()</span><br>=&gt; #&lt;Example::Zoo:0x2be7960&gt;<br><br>irb(main):004:0&gt; <span style="font-weight: bold;">zoo.add_animal(tiger1)</span><br>=&gt; nil<br><br>irb(main):007:0&gt; <span style="font-weight: bold;">tiger1 = nil</span><br>=&gt; nil<br><br>irb(main):007:0&gt; <span style="font-weight: bold;">GC.start</span><br>=&gt; nil<br><br>irb(main):005:0&gt; <span style="font-weight: bold;">tiger2 = zoo.get_animal(0)</span><br>(irb):12: [BUG] Segmentation fault</pre>
		</div>
		<p>The problem is that Ruby does not know that the <tt>zoo</tt> object contains a 
			reference to a Ruby object. Thus, when Ruby garbage collects
			<span style="font-family: monospace;">tiger1</span>
			it frees the underlying C++ object.</p>
		<p>This can be fixed by implementing a
			<tt>mark</tt> function as described above in the <a href="Ruby.html#Ruby_nn52">Mark and Sweep Garbage 
				Collector</a> section. You can specify a mark function by using the <tt>%markfunc</tt>
			directive. Since the <tt>%markfunc</tt> directive is implemented using SWIG's' 
			"features" mechanism it uses the same name matching rules as other kinds of 
			features (see the chapter on <a href="Customization.html#Customization">"Customization 
				Features"</a> for more details).
		</p>
		<p>A <tt>mark</tt> function takes a single argument, which is a pointer to the C++ 
			object being marked; it should, in turn, call <tt>rb_gc_mark()</tt> for any 
			instances that are reachable from the current object. The mark function for our <tt>
				Zoo</tt> class should therefore loop over all of the C++ animal objects in 
			the zoo object, look up their Ruby object equivalent, and then call <tt>rb_gc_mark()</tt>. 
			One possible implementation is:</p>
		<div class="code">
			<pre>%module example<br><br>%{<br>#include "example.h"<br>%}<br><br>/* Keep track of mappings between C/C++ structs/classes<br> and Ruby objects so we can implement a mark function. */<br><span style="font-weight: bold;">%trackobjects;</span><br><br>/* Specify the mark function */<br><span style="font-weight: bold;">%markfunc Zoo "mark_Zoo";</span><br><br>%include "example.h"<br><br>%header %{<br><br>static void mark_Zoo(void* ptr) {<br> Zoo* zoo = (Zoo*) ptr;<br><br> /* Loop over each object and tell the garbage collector<br> that we are holding a reference to them. */<br> int count = zoo-&gt;get_num_animals();<br><br> for(int i = 0; i &lt; count; ++i) {<br>   Animal* animal = zoo-&gt;get_animal(i);<br>   VALUE object = SWIG_RubyInstanceFor(animal);<br><br>   if (object != Qnil) {<br>     rb_gc_mark(object);<br>   }<br> }<br>}<br>%}<br>
</pre>
		</div>
		<p>
			Note the <tt>mark</tt> function is dependent on the <tt>SWIG_RUBY_InstanceFor</tt>
			method, and thus requires that <tt>%trackobjects</tt>
			is enabled. For more information, please refer to the track_object.i test case in the SWIG test suite.</p>
		<p>When this code is compiled we now see:</p>
		<div class="code">
			<pre>$ <b>irb<br></b>irb(main):002:0&gt; <span style="font-weight: bold;">tiger1=Example::Animal.new("tiger1")</span><br>=&gt; #&lt;Example::Animal:0x2be3bf8&gt;<br><br>irb(main):003:0&gt; <span style="font-weight: bold;">Example::Zoo.new()</span><br>=&gt; #&lt;Example::Zoo:0x2be1780&gt;<br><br>irb(main):004:0&gt; <span style="font-weight: bold;">zoo = Example::Zoo.new()</span><br>=&gt; #&lt;Example::Zoo:0x2bde9c0&gt;<br><br>irb(main):005:0&gt; <span style="font-weight: bold;">zoo.add_animal(tiger1)</span><br>=&gt; nil<br><br>irb(main):009:0&gt; <span style="font-weight: bold;">tiger1 = nil</span><br>=&gt; nil<br><br>irb(main):010:0&gt; <span style="font-weight: bold;">GC.start</span><br>=&gt; nil<br>irb(main):014:0&gt; <span style="font-weight: bold;">tiger2 = zoo.get_animal(0)</span><br>=&gt; #&lt;Example::Animal:0x2be3bf8&gt;<br><br>irb(main):015:0&gt; <span style="font-weight: bold;">tiger2.get_name()</span><br>=&gt; "tiger1"<br>irb(main):016:0&gt;<br>
</pre>
		</div>
		<br>
		<p>This code can be seen in swig/examples/ruby/mark_function.</p>
<H3><a name="Ruby_nn56"></a>27.8.4 Free Functions</H3>


		<p>By default, SWIG creates a "free" function that is called when a Ruby object is 
			garbage collected. The free function simply calls the C++ object's destructor.</p>
		<p>However, sometimes an appropriate destructor does not exist or special 
			processing needs to be performed before the destructor is called. Therefore, 
			SWIG allows you to manually specify a "free" function via the use of the <tt>%freefunc</tt>
			directive. The <tt>%freefunc</tt> directive is implemented using SWIG's' 
			"features" mechanism and so the same name matching rules used for other kinds 
			of features apply (see the chapter on <a href="Customization.html#Customization">"Customization 
				Features"</a>) for more details).</p>
		<p>IMPORTANT ! - If you define your own free function, then you must ensure that 
			you call the underlying C++ object's destructor. In addition, if object 
			tracking is activated for the object's class, you must also call the <tt>SWIG_RubyRemoveTracking</tt>
			function (of course call this before you destroy the C++ object). Note that it 
			is harmless to call this method if object tracking if off so it is advised to 
			always call it.</p>
		<p>Note there is a subtle interaction between object ownership and free functions. 
			A custom defined free function will only be called if the Ruby object owns the 
			underlying C++ object. This also to Ruby objects which are created, but then 
			transfer ownership to C++ objects via the use of the <tt>disown</tt> typemap 
			described above.
		</p>
		<p>To show how to use the <tt>%freefunc</tt> directive, let's slightly change our 
			example. Assume that the zoo object is responsible for freeing animal that it 
			contains. This means that the
			<span style="font-family: monospace;">Zoo::add_animal</span>
			function should be marked with a
			<span style="font-family: monospace;">DISOWN</span>
			typemap and the destructor should be updated as below::</p>
		<div class="code">
			<pre>Zoo::~Zoo() {<br> IterType iter = this-&gt;animals.begin();<br> IterType end = this-&gt;animals.end();<br><br> for(iter; iter != end; ++iter) {<br> Animal* animal = *iter;<br> delete animal;<br> }<br>}</pre>
		</div>
		<p>When we use these objects in IRB we see:</p>
		<div class="code">
			<pre><span style="font-weight: bold;">$irb</span><br>irb(main):002:0&gt; <span style="font-weight: bold;">require 'example'</span><br>=&gt; true<br><br>irb(main):003:0&gt; <span style="font-weight: bold;">zoo = Example::Zoo.new()</span><br>=&gt; #&lt;Example::Zoo:0x2be0fe8&gt;<br><br>irb(main):005:0&gt; <span style="font-weight: bold;">tiger1 = Example::Animal.new("tiger1")</span><br>=&gt; #&lt;Example::Animal:0x2bda760&gt;<br><br>irb(main):006:0&gt; <span style="font-weight: bold;">zoo.add_animal(tiger1)</span><br>=&gt; nil<br><br>irb(main):007:0&gt; <span style="font-weight: bold;">zoo = nil</span><br>=&gt; nil<br><br>irb(main):008:0&gt; <span style="font-weight: bold;">GC.start</span><br>=&gt; nil<br><br>irb(main):009:0&gt; <span style="font-weight: bold;">tiger1.get_name()</span><br>(irb):12: [BUG] Segmentation fault<br>
		</pre>
		</div>
		<p>The error happens because the C++ <tt>animal</tt> object is freed when the <tt>zoo</tt>
			object is freed. Although this error is unavoidable, we can at least prevent 
			the segmentation fault. To do this requires enabling object tracking and 
			implementing a custom free function that calls the <tt>SWIG_RubyUnlinkObjects</tt>
			function for each animal object that is destroyed. The <tt>SWIG_RubyUnlinkObjects</tt>
			function notifies SWIG that a Ruby object's underlying C++ object is no longer 
			valid. Once notified, SWIG will intercept any calls from the existing Ruby 
			object to the destroyed C++ object and raise an exception.<br>
		</p>
		<div class="code">
			<pre>%module example<br><br>%{<br>#include "example.h"<br>%}<br><br>/* Specify that ownership is transferred to the zoo<br>	when calling add_animal */<br>%apply SWIGTYPE *DISOWN { Animal* animal };<br><br>/* Track objects */<br>%trackobjects;<br><br>/* Specify the mark function */<br>%freefunc Zoo "free_Zoo";<br><br>%include "example.h"<br><br>%header %{<br> static void free_Zoo(void* ptr) {<br> Zoo* zoo = (Zoo*) ptr;<br><br> /* Loop over each animal */<br> int count = zoo-&gt;get_num_animals();<br><br> for(int i = 0; i &lt; count; ++i) {<br> /* Get an animal */<br> Animal* animal = zoo-&gt;get_animal(i);<br><br> /* Unlink the Ruby object from the C++ object */<br> SWIG_RubyUnlinkObjects(animal);<br><br> /* Now remove the tracking for this animal */<br> SWIG_RubyRemoveTracking(animal);<br> }<br><br> /* Now call SWIG_RemoveMapping for the zoo */<br> SWIG_RemoveMapping(ptr);<br> <br> /* Now free the zoo which will free the animals it contains */<br> delete zoo;<br> }<br>%} </pre>
		</div>
		<p>Now when we use these objects in IRB we see:</p>
		<div class="code">
			<pre><span style="font-weight: bold;">$irb</span><br>irb(main):002:0&gt; <span style="font-weight: bold;">require 'example'</span><br>=&gt; true<br><br>irb(main):003:0&gt; <span style="font-weight: bold;">zoo = Example::Zoo.new()</span><br>=&gt; #&lt;Example::Zoo:0x2be0fe8&gt;<br><br>irb(main):005:0&gt; <span style="font-weight: bold;">tiger1 = Example::Animal.new("tiger1")</span><br>=&gt; #&lt;Example::Animal:0x2bda760&gt;<br><br>irb(main):006:0&gt; <span style="font-weight: bold;">zoo.add_animal(tiger1)</span><br>=&gt; nil<br><br>irb(main):007:0&gt; <span style="font-weight: bold;">zoo = nil</span><br>=&gt; nil<br><br>irb(main):008:0&gt; <span style="font-weight: bold;">GC.start</span><br>=&gt; nil<br><br>irb(main):009:0&gt; <span style="font-weight: bold;">tiger1.get_name()</span><br>RuntimeError: This Animal * already released<br> from (irb):10:in `get_name'<br> from (irb):10<br>irb(main):011:0&gt;</pre>
		</div>
		<p>Notice that SWIG can now detect the underlying C++ object has been freed, and 
			thus raises a runtime exception.</p>
		<p>This code can be seen in swig/examples/ruby/free_function.</p>
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