| '\" |
| '\" Copyright (c) 1996-1997 Sun Microsystems, Inc. |
| '\" |
| '\" See the file "license.terms" for information on usage and redistribution |
| '\" of this file, and for a DISCLAIMER OF ALL WARRANTIES. |
| '\" |
| '\" RCS: @(#) $Id: Object.3,v 1.6 2002/10/22 12:16:53 dkf Exp $ |
| '\" |
| '\" The definitions below are for supplemental macros used in Tcl/Tk |
| '\" manual entries. |
| '\" |
| '\" .AP type name in/out ?indent? |
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| '\" .AS ?type? ?name? |
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| '\" |
| '\" .OP cmdName dbName dbClass |
| '\" Start of description of a specific option. cmdName gives the |
| '\" option's name as specified in the class command, dbName gives |
| '\" the option's name in the option database, and dbClass gives |
| '\" the option's class in the option database. |
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| '\" Print arg1 underlined, then print arg2 normally. |
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| '\" RCS: @(#) $Id: man.macros,v 1.4 2000/08/25 06:18:32 ericm Exp $ |
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| '\" # Set up traps and other miscellaneous stuff for Tcl/Tk man pages. |
| .if t .wh -1.3i ^B |
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| .ie !"\\$4"" .TP \\$4 |
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| .el \h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\h'|0u'\c |
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| .SH "STANDARD OPTIONS" |
| .LP |
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| .ft B |
| .. |
| '\" # SE - end of list of standard options |
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| .LP |
| See the \\fBoptions\\fR manual entry for details on the standard options. |
| .. |
| '\" # OP - start of full description for a single option |
| .de OP |
| .LP |
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| Command-Line Name: \\fB\\$1\\fR |
| Database Name: \\fB\\$2\\fR |
| Database Class: \\fB\\$3\\fR |
| .fi |
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| \\$1\l'|0\(ul'\\$2 |
| .. |
| .TH Tcl_Obj 3 8.1 Tcl "Tcl Library Procedures" |
| .BS |
| .SH NAME |
| Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl_InvalidateStringRep \- manipulate Tcl objects |
| .SH SYNOPSIS |
| .nf |
| \fB#include <tcl.h>\fR |
| .sp |
| Tcl_Obj * |
| \fBTcl_NewObj\fR() |
| .sp |
| Tcl_Obj * |
| \fBTcl_DuplicateObj\fR(\fIobjPtr\fR) |
| .sp |
| \fBTcl_IncrRefCount\fR(\fIobjPtr\fR) |
| .sp |
| \fBTcl_DecrRefCount\fR(\fIobjPtr\fR) |
| .sp |
| int |
| \fBTcl_IsShared\fR(\fIobjPtr\fR) |
| .sp |
| \fBTcl_InvalidateStringRep\fR(\fIobjPtr\fR) |
| .SH ARGUMENTS |
| .AS Tcl_Obj *objPtr in |
| .AP Tcl_Obj *objPtr in |
| Points to an object; |
| must have been the result of a previous call to \fBTcl_NewObj\fR. |
| .BE |
| |
| .SH INTRODUCTION |
| .PP |
| This man page presents an overview of Tcl objects and how they are used. |
| It also describes generic procedures for managing Tcl objects. |
| These procedures are used to create and copy objects, |
| and increment and decrement the count of references (pointers) to objects. |
| The procedures are used in conjunction with ones |
| that operate on specific types of objects such as |
| \fBTcl_GetIntFromObj\fR and \fBTcl_ListObjAppendElement\fR. |
| The individual procedures are described along with the data structures |
| they manipulate. |
| .PP |
| Tcl's \fIdual-ported\fR objects provide a general-purpose mechanism |
| for storing and exchanging Tcl values. |
| They largely replace the use of strings in Tcl. |
| For example, they are used to store variable values, |
| command arguments, command results, and scripts. |
| Tcl objects behave like strings but also hold an internal representation |
| that can be manipulated more efficiently. |
| For example, a Tcl list is now represented as an object |
| that holds the list's string representation |
| as well as an array of pointers to the objects for each list element. |
| Dual-ported objects avoid most runtime type conversions. |
| They also improve the speed of many operations |
| since an appropriate representation is immediately available. |
| The compiler itself uses Tcl objects to |
| cache the instruction bytecodes resulting from compiling scripts. |
| .PP |
| The two representations are a cache of each other and are computed lazily. |
| That is, each representation is only computed when necessary, |
| it is computed from the other representation, |
| and, once computed, it is saved. |
| In addition, a change in one representation invalidates the other one. |
| As an example, a Tcl program doing integer calculations can |
| operate directly on a variable's internal machine integer |
| representation without having to constantly convert |
| between integers and strings. |
| Only when it needs a string representing the variable's value, |
| say to print it, |
| will the program regenerate the string representation from the integer. |
| Although objects contain an internal representation, |
| their semantics are defined in terms of strings: |
| an up-to-date string can always be obtained, |
| and any change to the object will be reflected in that string |
| when the object's string representation is fetched. |
| Because of this representation invalidation and regeneration, |
| it is dangerous for extension writers to access |
| \fBTcl_Obj\fR fields directly. |
| It is better to access Tcl_Obj information using |
| procedures like \fBTcl_GetStringFromObj\fR and \fBTcl_GetString\fR. |
| .PP |
| Objects are allocated on the heap |
| and are referenced using a pointer to their \fBTcl_Obj\fR structure. |
| Objects are shared as much as possible. |
| This significantly reduces storage requirements |
| because some objects such as long lists are very large. |
| Also, most Tcl values are only read and never modified. |
| This is especially true for procedure arguments, |
| which can be shared between the caller and the called procedure. |
| Assignment and argument binding is done by |
| simply assigning a pointer to the value. |
| Reference counting is used to determine when it is safe to |
| reclaim an object's storage. |
| .PP |
| Tcl objects are typed. |
| An object's internal representation is controlled by its type. |
| Seven types are predefined in the Tcl core |
| including integer, double, list, and bytecode. |
| Extension writers can extend the set of types |
| by using the procedure \fBTcl_RegisterObjType\fR . |
| |
| .SH "THE TCL_OBJ STRUCTURE" |
| .PP |
| Each Tcl object is represented by a \fBTcl_Obj\fR structure |
| which is defined as follows. |
| .CS |
| typedef struct Tcl_Obj { |
| int \fIrefCount\fR; |
| char *\fIbytes\fR; |
| int \fIlength\fR; |
| Tcl_ObjType *\fItypePtr\fR; |
| union { |
| long \fIlongValue\fR; |
| double \fIdoubleValue\fR; |
| VOID *\fIotherValuePtr\fR; |
| struct { |
| VOID *\fIptr1\fR; |
| VOID *\fIptr2\fR; |
| } \fItwoPtrValue\fR; |
| } \fIinternalRep\fR; |
| } Tcl_Obj; |
| .CE |
| The \fIbytes\fR and the \fIlength\fR members together hold |
| .VS 8.1 |
| an object's UTF-8 string representation, |
| which is a \fIcounted string\fR not containing null bytes (UTF-8 null |
| characters should be encoded as a two byte sequence: 192, 128.) |
| \fIbytes\fR points to the first byte of the string representation. |
| The \fIlength\fR member gives the number of bytes. |
| The byte array must always have a null byte after the last data byte, |
| at offset \fIlength\fR; |
| this allows string representations |
| to be treated as conventional null-terminated C strings. |
| .VE 8.1 |
| C programs use \fBTcl_GetStringFromObj\fR and \fBTcl_GetString\fR to get |
| an object's string representation. |
| If \fIbytes\fR is NULL, |
| the string representation is invalid. |
| .PP |
| An object's type manages its internal representation. |
| The member \fItypePtr\fR points to the Tcl_ObjType structure |
| that describes the type. |
| If \fItypePtr\fR is NULL, |
| the internal representation is invalid. |
| .PP |
| The \fIinternalRep\fR union member holds |
| an object's internal representation. |
| This is either a (long) integer, a double-precision floating point number, |
| a pointer to a value containing additional information |
| needed by the object's type to represent the object, |
| or two arbitrary pointers. |
| .PP |
| The \fIrefCount\fR member is used to tell when it is safe to free |
| an object's storage. |
| It holds the count of active references to the object. |
| Maintaining the correct reference count is a key responsibility |
| of extension writers. |
| Reference counting is discussed below |
| in the section \fBSTORAGE MANAGEMENT OF OBJECTS\fR. |
| .PP |
| Although extension writers can directly access |
| the members of a Tcl_Obj structure, |
| it is much better to use the appropriate procedures and macros. |
| For example, extension writers should never |
| read or update \fIrefCount\fR directly; |
| they should use macros such as |
| \fBTcl_IncrRefCount\fR and \fBTcl_IsShared\fR instead. |
| .PP |
| A key property of Tcl objects is that they hold two representations. |
| An object typically starts out containing only a string representation: |
| it is untyped and has a NULL \fItypePtr\fR. |
| An object containing an empty string or a copy of a specified string |
| is created using \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR respectively. |
| An object's string value is gotten with |
| \fBTcl_GetStringFromObj\fR or \fBTcl_GetString\fR |
| and changed with \fBTcl_SetStringObj\fR. |
| If the object is later passed to a procedure like \fBTcl_GetIntFromObj\fR |
| that requires a specific internal representation, |
| the procedure will create one and set the object's \fItypePtr\fR. |
| The internal representation is computed from the string representation. |
| An object's two representations are duals of each other: |
| changes made to one are reflected in the other. |
| For example, \fBTcl_ListObjReplace\fR will modify an object's |
| internal representation and the next call to \fBTcl_GetStringFromObj\fR |
| or \fBTcl_GetString\fR will reflect that change. |
| .PP |
| Representations are recomputed lazily for efficiency. |
| A change to one representation made by a procedure |
| such as \fBTcl_ListObjReplace\fR is not reflected immediately |
| in the other representation. |
| Instead, the other representation is marked invalid |
| so that it is only regenerated if it is needed later. |
| Most C programmers never have to be concerned with how this is done |
| and simply use procedures such as \fBTcl_GetBooleanFromObj\fR or |
| \fBTcl_ListObjIndex\fR. |
| Programmers that implement their own object types |
| must check for invalid representations |
| and mark representations invalid when necessary. |
| The procedure \fBTcl_InvalidateStringRep\fR is used |
| to mark an object's string representation invalid and to |
| free any storage associated with the old string representation. |
| .PP |
| Objects usually remain one type over their life, |
| but occasionally an object must be converted from one type to another. |
| For example, a C program might build up a string in an object |
| with repeated calls to \fBTcl_AppendToObj\fR, |
| and then call \fBTcl_ListObjIndex\fR to extract a list element from |
| the object. |
| The same object holding the same string value |
| can have several different internal representations |
| at different times. |
| Extension writers can also force an object to be converted from one type |
| to another using the \fBTcl_ConvertToType\fR procedure. |
| Only programmers that create new object types need to be concerned |
| about how this is done. |
| A procedure defined as part of the object type's implementation |
| creates a new internal representation for an object |
| and changes its \fItypePtr\fR. |
| See the man page for \fBTcl_RegisterObjType\fR |
| to see how to create a new object type. |
| |
| .SH "EXAMPLE OF THE LIFETIME OF AN OBJECT" |
| .PP |
| As an example of the lifetime of an object, |
| consider the following sequence of commands: |
| .CS |
| \fBset x 123\fR |
| .CE |
| This assigns to \fIx\fR an untyped object whose |
| \fIbytes\fR member points to \fB123\fR and \fIlength\fR member contains 3. |
| The object's \fItypePtr\fR member is NULL. |
| .CS |
| \fBputs "x is $x"\fR |
| .CE |
| \fIx\fR's string representation is valid (since \fIbytes\fR is non-NULL) |
| and is fetched for the command. |
| .CS |
| \fBincr x\fR |
| .CE |
| The \fBincr\fR command first gets an integer from \fIx\fR's object |
| by calling \fBTcl_GetIntFromObj\fR. |
| This procedure checks whether the object is already an integer object. |
| Since it is not, it converts the object |
| by setting the object's \fIinternalRep.longValue\fR member |
| to the integer \fB123\fR |
| and setting the object's \fItypePtr\fR |
| to point to the integer Tcl_ObjType structure. |
| Both representations are now valid. |
| \fBincr\fR increments the object's integer internal representation |
| then invalidates its string representation |
| (by calling \fBTcl_InvalidateStringRep\fR) |
| since the string representation |
| no longer corresponds to the internal representation. |
| .CS |
| \fBputs "x is now $x"\fR |
| .CE |
| The string representation of \fIx\fR's object is needed |
| and is recomputed. |
| The string representation is now \fB124\fR. |
| and both representations are again valid. |
| |
| .SH "STORAGE MANAGEMENT OF OBJECTS" |
| .PP |
| Tcl objects are allocated on the heap and are shared as much as possible |
| to reduce storage requirements. |
| Reference counting is used to determine when an object is |
| no longer needed and can safely be freed. |
| An object just created by \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR |
| has \fIrefCount\fR 0. |
| The macro \fBTcl_IncrRefCount\fR increments the reference count |
| when a new reference to the object is created. |
| The macro \fBTcl_DecrRefCount\fR decrements the count |
| when a reference is no longer needed and, |
| if the object's reference count drops to zero, frees its storage. |
| An object shared by different code or data structures has |
| \fIrefCount\fR greater than 1. |
| Incrementing an object's reference count ensures that |
| it won't be freed too early or have its value change accidently. |
| .PP |
| As an example, the bytecode interpreter shares argument objects |
| between calling and called Tcl procedures to avoid having to copy objects. |
| It assigns the call's argument objects to the procedure's |
| formal parameter variables. |
| In doing so, it calls \fBTcl_IncrRefCount\fR to increment |
| the reference count of each argument since there is now a new |
| reference to it from the formal parameter. |
| When the called procedure returns, |
| the interpreter calls \fBTcl_DecrRefCount\fR to decrement |
| each argument's reference count. |
| When an object's reference count drops less than or equal to zero, |
| \fBTcl_DecrRefCount\fR reclaims its storage. |
| Most command procedures do not have to be concerned about |
| reference counting since they use an object's value immediately |
| and don't retain a pointer to the object after they return. |
| However, if they do retain a pointer to an object in a data structure, |
| they must be careful to increment its reference count |
| since the retained pointer is a new reference. |
| .PP |
| Command procedures that directly modify objects |
| such as those for \fBlappend\fR and \fBlinsert\fR must be careful to |
| copy a shared object before changing it. |
| They must first check whether the object is shared |
| by calling \fBTcl_IsShared\fR. |
| If the object is shared they must copy the object |
| by using \fBTcl_DuplicateObj\fR; |
| this returns a new duplicate of the original object |
| that has \fIrefCount\fR 0. |
| If the object is not shared, |
| the command procedure "owns" the object and can safely modify it directly. |
| For example, the following code appears in the command procedure |
| that implements \fBlinsert\fR. |
| This procedure modifies the list object passed to it in \fIobjv[1]\fR |
| by inserting \fIobjc-3\fR new elements before \fIindex\fR. |
| .CS |
| listPtr = objv[1]; |
| if (Tcl_IsShared(listPtr)) { |
| listPtr = Tcl_DuplicateObj(listPtr); |
| } |
| result = Tcl_ListObjReplace(interp, listPtr, index, 0, (objc-3), &(objv[3])); |
| .CE |
| As another example, \fBincr\fR's command procedure |
| must check whether the variable's object is shared before |
| incrementing the integer in its internal representation. |
| If it is shared, it needs to duplicate the object |
| in order to avoid accidently changing values in other data structures. |
| |
| .SH "SEE ALSO" |
| Tcl_ConvertToType, Tcl_GetIntFromObj, Tcl_ListObjAppendElement, Tcl_ListObjIndex, Tcl_ListObjReplace, Tcl_RegisterObjType |
| |
| .SH KEYWORDS |
| internal representation, object, object creation, object type, reference counting, string representation, type conversion |