| 1 | '\" |
| 2 | '\" Copyright (c) 1996-1997 Sun Microsystems, Inc. |
| 3 | '\" |
| 4 | '\" See the file "license.terms" for information on usage and redistribution |
| 5 | '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. |
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| 7 | '\" RCS: @(#) $Id: Object.3,v 1.6 2002/10/22 12:16:53 dkf Exp $ |
| 8 | '\" |
| 9 | '\" The definitions below are for supplemental macros used in Tcl/Tk |
| 10 | '\" manual entries. |
| 11 | '\" |
| 12 | '\" .AP type name in/out ?indent? |
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| 31 | '\" .CS |
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| 59 | '\" End of list of standard options for a Tk widget. |
| 60 | '\" |
| 61 | '\" .OP cmdName dbName dbClass |
| 62 | '\" Start of description of a specific option. cmdName gives the |
| 63 | '\" option's name as specified in the class command, dbName gives |
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| 70 | '\" RCS: @(#) $Id: man.macros,v 1.4 2000/08/25 06:18:32 ericm Exp $ |
| 71 | '\" |
| 72 | '\" # Set up traps and other miscellaneous stuff for Tcl/Tk man pages. |
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| 133 | \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul' |
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| 180 | \kx\h'-\\nxu'\h'|\\n(^lu+3n'\ky\L'-\\n(^xu'\v'\\n(^xu'\h'|0u'\c |
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| 205 | '\" # SO - start of list of standard options |
| 206 | .de SO |
| 207 | .SH "STANDARD OPTIONS" |
| 208 | .LP |
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| 212 | .. |
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| 217 | .LP |
| 218 | See the \\fBoptions\\fR manual entry for details on the standard options. |
| 219 | .. |
| 220 | '\" # OP - start of full description for a single option |
| 221 | .de OP |
| 222 | .LP |
| 223 | .nf |
| 224 | .ta 4c |
| 225 | Command-Line Name: \\fB\\$1\\fR |
| 226 | Database Name: \\fB\\$2\\fR |
| 227 | Database Class: \\fB\\$3\\fR |
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| 243 | \\$1\l'|0\(ul'\\$2 |
| 244 | .. |
| 245 | .TH Tcl_Obj 3 8.1 Tcl "Tcl Library Procedures" |
| 246 | .BS |
| 247 | .SH NAME |
| 248 | Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl_InvalidateStringRep \- manipulate Tcl objects |
| 249 | .SH SYNOPSIS |
| 250 | .nf |
| 251 | \fB#include <tcl.h>\fR |
| 252 | .sp |
| 253 | Tcl_Obj * |
| 254 | \fBTcl_NewObj\fR() |
| 255 | .sp |
| 256 | Tcl_Obj * |
| 257 | \fBTcl_DuplicateObj\fR(\fIobjPtr\fR) |
| 258 | .sp |
| 259 | \fBTcl_IncrRefCount\fR(\fIobjPtr\fR) |
| 260 | .sp |
| 261 | \fBTcl_DecrRefCount\fR(\fIobjPtr\fR) |
| 262 | .sp |
| 263 | int |
| 264 | \fBTcl_IsShared\fR(\fIobjPtr\fR) |
| 265 | .sp |
| 266 | \fBTcl_InvalidateStringRep\fR(\fIobjPtr\fR) |
| 267 | .SH ARGUMENTS |
| 268 | .AS Tcl_Obj *objPtr in |
| 269 | .AP Tcl_Obj *objPtr in |
| 270 | Points to an object; |
| 271 | must have been the result of a previous call to \fBTcl_NewObj\fR. |
| 272 | .BE |
| 273 | |
| 274 | .SH INTRODUCTION |
| 275 | .PP |
| 276 | This man page presents an overview of Tcl objects and how they are used. |
| 277 | It also describes generic procedures for managing Tcl objects. |
| 278 | These procedures are used to create and copy objects, |
| 279 | and increment and decrement the count of references (pointers) to objects. |
| 280 | The procedures are used in conjunction with ones |
| 281 | that operate on specific types of objects such as |
| 282 | \fBTcl_GetIntFromObj\fR and \fBTcl_ListObjAppendElement\fR. |
| 283 | The individual procedures are described along with the data structures |
| 284 | they manipulate. |
| 285 | .PP |
| 286 | Tcl's \fIdual-ported\fR objects provide a general-purpose mechanism |
| 287 | for storing and exchanging Tcl values. |
| 288 | They largely replace the use of strings in Tcl. |
| 289 | For example, they are used to store variable values, |
| 290 | command arguments, command results, and scripts. |
| 291 | Tcl objects behave like strings but also hold an internal representation |
| 292 | that can be manipulated more efficiently. |
| 293 | For example, a Tcl list is now represented as an object |
| 294 | that holds the list's string representation |
| 295 | as well as an array of pointers to the objects for each list element. |
| 296 | Dual-ported objects avoid most runtime type conversions. |
| 297 | They also improve the speed of many operations |
| 298 | since an appropriate representation is immediately available. |
| 299 | The compiler itself uses Tcl objects to |
| 300 | cache the instruction bytecodes resulting from compiling scripts. |
| 301 | .PP |
| 302 | The two representations are a cache of each other and are computed lazily. |
| 303 | That is, each representation is only computed when necessary, |
| 304 | it is computed from the other representation, |
| 305 | and, once computed, it is saved. |
| 306 | In addition, a change in one representation invalidates the other one. |
| 307 | As an example, a Tcl program doing integer calculations can |
| 308 | operate directly on a variable's internal machine integer |
| 309 | representation without having to constantly convert |
| 310 | between integers and strings. |
| 311 | Only when it needs a string representing the variable's value, |
| 312 | say to print it, |
| 313 | will the program regenerate the string representation from the integer. |
| 314 | Although objects contain an internal representation, |
| 315 | their semantics are defined in terms of strings: |
| 316 | an up-to-date string can always be obtained, |
| 317 | and any change to the object will be reflected in that string |
| 318 | when the object's string representation is fetched. |
| 319 | Because of this representation invalidation and regeneration, |
| 320 | it is dangerous for extension writers to access |
| 321 | \fBTcl_Obj\fR fields directly. |
| 322 | It is better to access Tcl_Obj information using |
| 323 | procedures like \fBTcl_GetStringFromObj\fR and \fBTcl_GetString\fR. |
| 324 | .PP |
| 325 | Objects are allocated on the heap |
| 326 | and are referenced using a pointer to their \fBTcl_Obj\fR structure. |
| 327 | Objects are shared as much as possible. |
| 328 | This significantly reduces storage requirements |
| 329 | because some objects such as long lists are very large. |
| 330 | Also, most Tcl values are only read and never modified. |
| 331 | This is especially true for procedure arguments, |
| 332 | which can be shared between the caller and the called procedure. |
| 333 | Assignment and argument binding is done by |
| 334 | simply assigning a pointer to the value. |
| 335 | Reference counting is used to determine when it is safe to |
| 336 | reclaim an object's storage. |
| 337 | .PP |
| 338 | Tcl objects are typed. |
| 339 | An object's internal representation is controlled by its type. |
| 340 | Seven types are predefined in the Tcl core |
| 341 | including integer, double, list, and bytecode. |
| 342 | Extension writers can extend the set of types |
| 343 | by using the procedure \fBTcl_RegisterObjType\fR . |
| 344 | |
| 345 | .SH "THE TCL_OBJ STRUCTURE" |
| 346 | .PP |
| 347 | Each Tcl object is represented by a \fBTcl_Obj\fR structure |
| 348 | which is defined as follows. |
| 349 | .CS |
| 350 | typedef struct Tcl_Obj { |
| 351 | int \fIrefCount\fR; |
| 352 | char *\fIbytes\fR; |
| 353 | int \fIlength\fR; |
| 354 | Tcl_ObjType *\fItypePtr\fR; |
| 355 | union { |
| 356 | long \fIlongValue\fR; |
| 357 | double \fIdoubleValue\fR; |
| 358 | VOID *\fIotherValuePtr\fR; |
| 359 | struct { |
| 360 | VOID *\fIptr1\fR; |
| 361 | VOID *\fIptr2\fR; |
| 362 | } \fItwoPtrValue\fR; |
| 363 | } \fIinternalRep\fR; |
| 364 | } Tcl_Obj; |
| 365 | .CE |
| 366 | The \fIbytes\fR and the \fIlength\fR members together hold |
| 367 | .VS 8.1 |
| 368 | an object's UTF-8 string representation, |
| 369 | which is a \fIcounted string\fR not containing null bytes (UTF-8 null |
| 370 | characters should be encoded as a two byte sequence: 192, 128.) |
| 371 | \fIbytes\fR points to the first byte of the string representation. |
| 372 | The \fIlength\fR member gives the number of bytes. |
| 373 | The byte array must always have a null byte after the last data byte, |
| 374 | at offset \fIlength\fR; |
| 375 | this allows string representations |
| 376 | to be treated as conventional null-terminated C strings. |
| 377 | .VE 8.1 |
| 378 | C programs use \fBTcl_GetStringFromObj\fR and \fBTcl_GetString\fR to get |
| 379 | an object's string representation. |
| 380 | If \fIbytes\fR is NULL, |
| 381 | the string representation is invalid. |
| 382 | .PP |
| 383 | An object's type manages its internal representation. |
| 384 | The member \fItypePtr\fR points to the Tcl_ObjType structure |
| 385 | that describes the type. |
| 386 | If \fItypePtr\fR is NULL, |
| 387 | the internal representation is invalid. |
| 388 | .PP |
| 389 | The \fIinternalRep\fR union member holds |
| 390 | an object's internal representation. |
| 391 | This is either a (long) integer, a double-precision floating point number, |
| 392 | a pointer to a value containing additional information |
| 393 | needed by the object's type to represent the object, |
| 394 | or two arbitrary pointers. |
| 395 | .PP |
| 396 | The \fIrefCount\fR member is used to tell when it is safe to free |
| 397 | an object's storage. |
| 398 | It holds the count of active references to the object. |
| 399 | Maintaining the correct reference count is a key responsibility |
| 400 | of extension writers. |
| 401 | Reference counting is discussed below |
| 402 | in the section \fBSTORAGE MANAGEMENT OF OBJECTS\fR. |
| 403 | .PP |
| 404 | Although extension writers can directly access |
| 405 | the members of a Tcl_Obj structure, |
| 406 | it is much better to use the appropriate procedures and macros. |
| 407 | For example, extension writers should never |
| 408 | read or update \fIrefCount\fR directly; |
| 409 | they should use macros such as |
| 410 | \fBTcl_IncrRefCount\fR and \fBTcl_IsShared\fR instead. |
| 411 | .PP |
| 412 | A key property of Tcl objects is that they hold two representations. |
| 413 | An object typically starts out containing only a string representation: |
| 414 | it is untyped and has a NULL \fItypePtr\fR. |
| 415 | An object containing an empty string or a copy of a specified string |
| 416 | is created using \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR respectively. |
| 417 | An object's string value is gotten with |
| 418 | \fBTcl_GetStringFromObj\fR or \fBTcl_GetString\fR |
| 419 | and changed with \fBTcl_SetStringObj\fR. |
| 420 | If the object is later passed to a procedure like \fBTcl_GetIntFromObj\fR |
| 421 | that requires a specific internal representation, |
| 422 | the procedure will create one and set the object's \fItypePtr\fR. |
| 423 | The internal representation is computed from the string representation. |
| 424 | An object's two representations are duals of each other: |
| 425 | changes made to one are reflected in the other. |
| 426 | For example, \fBTcl_ListObjReplace\fR will modify an object's |
| 427 | internal representation and the next call to \fBTcl_GetStringFromObj\fR |
| 428 | or \fBTcl_GetString\fR will reflect that change. |
| 429 | .PP |
| 430 | Representations are recomputed lazily for efficiency. |
| 431 | A change to one representation made by a procedure |
| 432 | such as \fBTcl_ListObjReplace\fR is not reflected immediately |
| 433 | in the other representation. |
| 434 | Instead, the other representation is marked invalid |
| 435 | so that it is only regenerated if it is needed later. |
| 436 | Most C programmers never have to be concerned with how this is done |
| 437 | and simply use procedures such as \fBTcl_GetBooleanFromObj\fR or |
| 438 | \fBTcl_ListObjIndex\fR. |
| 439 | Programmers that implement their own object types |
| 440 | must check for invalid representations |
| 441 | and mark representations invalid when necessary. |
| 442 | The procedure \fBTcl_InvalidateStringRep\fR is used |
| 443 | to mark an object's string representation invalid and to |
| 444 | free any storage associated with the old string representation. |
| 445 | .PP |
| 446 | Objects usually remain one type over their life, |
| 447 | but occasionally an object must be converted from one type to another. |
| 448 | For example, a C program might build up a string in an object |
| 449 | with repeated calls to \fBTcl_AppendToObj\fR, |
| 450 | and then call \fBTcl_ListObjIndex\fR to extract a list element from |
| 451 | the object. |
| 452 | The same object holding the same string value |
| 453 | can have several different internal representations |
| 454 | at different times. |
| 455 | Extension writers can also force an object to be converted from one type |
| 456 | to another using the \fBTcl_ConvertToType\fR procedure. |
| 457 | Only programmers that create new object types need to be concerned |
| 458 | about how this is done. |
| 459 | A procedure defined as part of the object type's implementation |
| 460 | creates a new internal representation for an object |
| 461 | and changes its \fItypePtr\fR. |
| 462 | See the man page for \fBTcl_RegisterObjType\fR |
| 463 | to see how to create a new object type. |
| 464 | |
| 465 | .SH "EXAMPLE OF THE LIFETIME OF AN OBJECT" |
| 466 | .PP |
| 467 | As an example of the lifetime of an object, |
| 468 | consider the following sequence of commands: |
| 469 | .CS |
| 470 | \fBset x 123\fR |
| 471 | .CE |
| 472 | This assigns to \fIx\fR an untyped object whose |
| 473 | \fIbytes\fR member points to \fB123\fR and \fIlength\fR member contains 3. |
| 474 | The object's \fItypePtr\fR member is NULL. |
| 475 | .CS |
| 476 | \fBputs "x is $x"\fR |
| 477 | .CE |
| 478 | \fIx\fR's string representation is valid (since \fIbytes\fR is non-NULL) |
| 479 | and is fetched for the command. |
| 480 | .CS |
| 481 | \fBincr x\fR |
| 482 | .CE |
| 483 | The \fBincr\fR command first gets an integer from \fIx\fR's object |
| 484 | by calling \fBTcl_GetIntFromObj\fR. |
| 485 | This procedure checks whether the object is already an integer object. |
| 486 | Since it is not, it converts the object |
| 487 | by setting the object's \fIinternalRep.longValue\fR member |
| 488 | to the integer \fB123\fR |
| 489 | and setting the object's \fItypePtr\fR |
| 490 | to point to the integer Tcl_ObjType structure. |
| 491 | Both representations are now valid. |
| 492 | \fBincr\fR increments the object's integer internal representation |
| 493 | then invalidates its string representation |
| 494 | (by calling \fBTcl_InvalidateStringRep\fR) |
| 495 | since the string representation |
| 496 | no longer corresponds to the internal representation. |
| 497 | .CS |
| 498 | \fBputs "x is now $x"\fR |
| 499 | .CE |
| 500 | The string representation of \fIx\fR's object is needed |
| 501 | and is recomputed. |
| 502 | The string representation is now \fB124\fR. |
| 503 | and both representations are again valid. |
| 504 | |
| 505 | .SH "STORAGE MANAGEMENT OF OBJECTS" |
| 506 | .PP |
| 507 | Tcl objects are allocated on the heap and are shared as much as possible |
| 508 | to reduce storage requirements. |
| 509 | Reference counting is used to determine when an object is |
| 510 | no longer needed and can safely be freed. |
| 511 | An object just created by \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR |
| 512 | has \fIrefCount\fR 0. |
| 513 | The macro \fBTcl_IncrRefCount\fR increments the reference count |
| 514 | when a new reference to the object is created. |
| 515 | The macro \fBTcl_DecrRefCount\fR decrements the count |
| 516 | when a reference is no longer needed and, |
| 517 | if the object's reference count drops to zero, frees its storage. |
| 518 | An object shared by different code or data structures has |
| 519 | \fIrefCount\fR greater than 1. |
| 520 | Incrementing an object's reference count ensures that |
| 521 | it won't be freed too early or have its value change accidently. |
| 522 | .PP |
| 523 | As an example, the bytecode interpreter shares argument objects |
| 524 | between calling and called Tcl procedures to avoid having to copy objects. |
| 525 | It assigns the call's argument objects to the procedure's |
| 526 | formal parameter variables. |
| 527 | In doing so, it calls \fBTcl_IncrRefCount\fR to increment |
| 528 | the reference count of each argument since there is now a new |
| 529 | reference to it from the formal parameter. |
| 530 | When the called procedure returns, |
| 531 | the interpreter calls \fBTcl_DecrRefCount\fR to decrement |
| 532 | each argument's reference count. |
| 533 | When an object's reference count drops less than or equal to zero, |
| 534 | \fBTcl_DecrRefCount\fR reclaims its storage. |
| 535 | Most command procedures do not have to be concerned about |
| 536 | reference counting since they use an object's value immediately |
| 537 | and don't retain a pointer to the object after they return. |
| 538 | However, if they do retain a pointer to an object in a data structure, |
| 539 | they must be careful to increment its reference count |
| 540 | since the retained pointer is a new reference. |
| 541 | .PP |
| 542 | Command procedures that directly modify objects |
| 543 | such as those for \fBlappend\fR and \fBlinsert\fR must be careful to |
| 544 | copy a shared object before changing it. |
| 545 | They must first check whether the object is shared |
| 546 | by calling \fBTcl_IsShared\fR. |
| 547 | If the object is shared they must copy the object |
| 548 | by using \fBTcl_DuplicateObj\fR; |
| 549 | this returns a new duplicate of the original object |
| 550 | that has \fIrefCount\fR 0. |
| 551 | If the object is not shared, |
| 552 | the command procedure "owns" the object and can safely modify it directly. |
| 553 | For example, the following code appears in the command procedure |
| 554 | that implements \fBlinsert\fR. |
| 555 | This procedure modifies the list object passed to it in \fIobjv[1]\fR |
| 556 | by inserting \fIobjc-3\fR new elements before \fIindex\fR. |
| 557 | .CS |
| 558 | listPtr = objv[1]; |
| 559 | if (Tcl_IsShared(listPtr)) { |
| 560 | listPtr = Tcl_DuplicateObj(listPtr); |
| 561 | } |
| 562 | result = Tcl_ListObjReplace(interp, listPtr, index, 0, (objc-3), &(objv[3])); |
| 563 | .CE |
| 564 | As another example, \fBincr\fR's command procedure |
| 565 | must check whether the variable's object is shared before |
| 566 | incrementing the integer in its internal representation. |
| 567 | If it is shared, it needs to duplicate the object |
| 568 | in order to avoid accidently changing values in other data structures. |
| 569 | |
| 570 | .SH "SEE ALSO" |
| 571 | Tcl_ConvertToType, Tcl_GetIntFromObj, Tcl_ListObjAppendElement, Tcl_ListObjIndex, Tcl_ListObjReplace, Tcl_RegisterObjType |
| 572 | |
| 573 | .SH KEYWORDS |
| 574 | internal representation, object, object creation, object type, reference counting, string representation, type conversion |