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Diffstat (limited to 'doc/Object.3')
| -rw-r--r-- | doc/Object.3 | 231 |
1 files changed, 121 insertions, 110 deletions
diff --git a/doc/Object.3 b/doc/Object.3 index 4817b9b..3d52f61 100644 --- a/doc/Object.3 +++ b/doc/Object.3 @@ -8,7 +8,7 @@ .TH Tcl_Obj 3 8.5 Tcl "Tcl Library Procedures" .BS .SH NAME -Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl_InvalidateStringRep \- manipulate Tcl objects +Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl_InvalidateStringRep \- manipulate Tcl values .SH SYNOPSIS .nf \fB#include <tcl.h>\fR @@ -30,36 +30,36 @@ int .SH ARGUMENTS .AS Tcl_Obj *objPtr .AP Tcl_Obj *objPtr in -Points to an object; +Points to a value; 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. +This man page presents an overview of Tcl values (called \fBTcl_Obj\fRs for +historical reasons) and how they are used. +It also describes generic procedures for managing Tcl values. +These procedures are used to create and copy values, +and increment and decrement the count of references (pointers) to values. The procedures are used in conjunction with ones -that operate on specific types of objects such as +that operate on specific types of values 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 +Tcl's \fIdual-ported\fR values 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 +Tcl values 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 +For example, a Tcl list is now represented as a value 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. +as well as an array of pointers to the values for each list element. +Dual-ported values 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 +The compiler itself uses Tcl values to cache the instruction bytecodes resulting from compiling scripts. .PP The two representations are a cache of each other and are computed lazily. @@ -74,64 +74,66 @@ 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, +Although values 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. +and any change to the value will be reflected in that string +when the value'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 +Values are allocated on the heap and are referenced using a pointer to their \fBTcl_Obj\fR structure. -Objects are shared as much as possible. +Values are shared as much as possible. This significantly reduces storage requirements -because some objects such as long lists are very large. +because some values 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. +reclaim a value's storage. .PP -Tcl objects are typed. -An object's internal representation is controlled by its type. +Tcl values are typed. +A value's internal representation is controlled by its type. Several types are predefined in the Tcl core including integer, double, list, and bytecode. Extension writers can extend the set of types by defining their own \fBTcl_ObjType\fR structs. .SH "THE TCL_OBJ STRUCTURE" .PP -Each Tcl object is represented by a \fBTcl_Obj\fR structure +Each Tcl value is represented by a \fBTcl_Obj\fR structure which is defined as follows. +.PP .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; - Tcl_WideInt \fIwideValue\fR; - struct { - void *\fIptr1\fR; - void *\fIptr2\fR; - } \fItwoPtrValue\fR; - struct { - void *\fIptr\fR; - unsigned long \fIvalue\fR; - } \fIptrAndLongRep\fR; - } \fIinternalRep\fR; -} Tcl_Obj; + int \fIrefCount\fR; + char *\fIbytes\fR; + int \fIlength\fR; + const Tcl_ObjType *\fItypePtr\fR; + union { + long \fIlongValue\fR; + double \fIdoubleValue\fR; + void *\fIotherValuePtr\fR; + Tcl_WideInt \fIwideValue\fR; + struct { + void *\fIptr1\fR; + void *\fIptr2\fR; + } \fItwoPtrValue\fR; + struct { + void *\fIptr\fR; + unsigned long \fIvalue\fR; + } \fIptrAndLongRep\fR; + } \fIinternalRep\fR; +} \fBTcl_Obj\fR; .CE +.PP The \fIbytes\fR and the \fIlength\fR members together hold -an object's UTF-8 string representation, +a value'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. @@ -141,31 +143,31 @@ at offset \fIlength\fR; this allows string representations to be treated as conventional null-terminated C strings. C programs use \fBTcl_GetStringFromObj\fR and \fBTcl_GetString\fR to get -an object's string representation. +a value's string representation. If \fIbytes\fR is NULL, the string representation is invalid. .PP -An object's type manages its internal representation. +A value'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. +a value'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, a Tcl_WideInt +needed by the value's type to represent the value, a Tcl_WideInt integer, two arbitrary pointers, or a pair made up of an unsigned long integer and a pointer. .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. +a value's storage. +It holds the count of active references to the value. 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. +in the section \fBSTORAGE MANAGEMENT OF VALUES\fR. .PP Although extension writers can directly access the members of a Tcl_Obj structure, @@ -175,21 +177,21 @@ 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: +A key property of Tcl values is that they hold two representations. +A value 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 +A value 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 +A value'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 +If the value 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 procedure will create one and set the value's \fItypePtr\fR. The internal representation is computed from the string representation. -An object's two representations are duals of each other: +A value'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 +For example, \fBTcl_ListObjReplace\fR will modify a value's internal representation and the next call to \fBTcl_GetStringFromObj\fR or \fBTcl_GetString\fR will reflect that change. .PP @@ -202,91 +204,99 @@ 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 +Programmers that implement their own value 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 +to mark a value'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 +Values usually remain one type over their life, +but occasionally a value must be converted from one type to another. +For example, a C program might build up a string in a value 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 +the value. +The same value 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 +Extension writers can also force a value 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 +Only programmers that create new value 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 +A procedure defined as part of the value type's implementation +creates a new internal representation for a value 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" +to see how to create a new value type. +.SH "EXAMPLE OF THE LIFETIME OF A VALUE" .PP -As an example of the lifetime of an object, +As an example of the lifetime of a value, consider the following sequence of commands: +.PP .CS \fBset x 123\fR .CE -This assigns to \fIx\fR an untyped object whose +.PP +This assigns to \fIx\fR an untyped value whose \fIbytes\fR member points to \fB123\fR and \fIlength\fR member contains 3. -The object's \fItypePtr\fR member is NULL. +The value's \fItypePtr\fR member is NULL. +.PP .CS \fBputs "x is $x"\fR .CE +.PP \fIx\fR's string representation is valid (since \fIbytes\fR is non-NULL) and is fetched for the command. +.PP .CS \fBincr x\fR .CE -The \fBincr\fR command first gets an integer from \fIx\fR's object +.PP +The \fBincr\fR command first gets an integer from \fIx\fR's value 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 +This procedure checks whether the value is already an integer value. +Since it is not, it converts the value +by setting the value's \fIinternalRep.longValue\fR member to the integer \fB123\fR -and setting the object's \fItypePtr\fR +and setting the value'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 +\fBincr\fR increments the value'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. +.PP .CS \fBputs "x is now $x"\fR .CE -The string representation of \fIx\fR's object is needed +.PP +The string representation of \fIx\fR's value is needed and is recomputed. The string representation is now \fB124\fR and both representations are again valid. -.SH "STORAGE MANAGEMENT OF OBJECTS" +.SH "STORAGE MANAGEMENT OF VALUES" .PP -Tcl objects are allocated on the heap and are shared as much as possible +Tcl values 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 +Reference counting is used to determine when a value is no longer needed and can safely be freed. -An object just created by \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR +A value 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. +when a new reference to the value 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 +if the value's reference count drops to zero, frees its storage. +A value shared by different code or data structures has \fIrefCount\fR greater than 1. -Incrementing an object's reference count ensures that +Incrementing a value's reference count ensures that it will not be freed too early or have its value change accidentally. .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 +As an example, the bytecode interpreter shares argument values +between calling and called Tcl procedures to avoid having to copy values. +It assigns the call's argument values 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 @@ -294,48 +304,49 @@ 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, +When a value'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 do not retain a pointer to the object after they return. -However, if they do retain a pointer to an object in a data structure, +reference counting since they use a value's value immediately +and do not retain a pointer to the value after they return. +However, if they do retain a pointer to a value 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 +Command procedures that directly modify values 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 +copy a shared value before changing it. +They must first check whether the value is shared by calling \fBTcl_IsShared\fR. -If the object is shared they must copy the object +If the value is shared they must copy the value by using \fBTcl_DuplicateObj\fR; -this returns a new duplicate of the original object +this returns a new duplicate of the original value that has \fIrefCount\fR 0. -If the object is not shared, +If the value is not shared, the command procedure .QW "owns" -the object and can safely modify it directly. +the value 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 +This procedure modifies the list value passed to it in \fIobjv[1]\fR by inserting \fIobjc-3\fR new elements before \fIindex\fR. .PP .CS listPtr = objv[1]; -if (Tcl_IsShared(listPtr)) { - listPtr = Tcl_DuplicateObj(listPtr); +if (\fBTcl_IsShared\fR(listPtr)) { + listPtr = \fBTcl_DuplicateObj\fR(listPtr); } result = Tcl_ListObjReplace(interp, listPtr, index, 0, (objc-3), &(objv[3])); .CE .PP As another example, \fBincr\fR's command procedure -must check whether the variable's object is shared before +must check whether the variable's value is shared before incrementing the integer in its internal representation. -If it is shared, it needs to duplicate the object +If it is shared, it needs to duplicate the value in order to avoid accidentally changing values in other data structures. .SH "SEE ALSO" Tcl_ConvertToType(3), Tcl_GetIntFromObj(3), Tcl_ListObjAppendElement(3), Tcl_ListObjIndex(3), Tcl_ListObjReplace(3), Tcl_RegisterObjType(3) .SH KEYWORDS -internal representation, object, object creation, object type, reference counting, string representation, type conversion +internal representation, value, value creation, value type, +reference counting, string representation, type conversion |