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Tcl_Obj(3) Tcl Library Procedures Tcl_Obj(3)
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NAME
Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl_Inval-
idateStringRep - manipulate Tcl objects
SYNOPSIS
#include <tcl.h>
Tcl_Obj *
Tcl_NewObj()
Tcl_Obj *
Tcl_DuplicateObj(objPtr)
Tcl_IncrRefCount(objPtr)
Tcl_DecrRefCount(objPtr)
int
Tcl_IsShared(objPtr)
Tcl_InvalidateStringRep(objPtr)
ARGUMENTS
Tcl_Obj *objPtr (in) Points to an object; must have been the result of a previous
call to Tcl_NewObj.
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INTRODUCTION
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 cre-
ate 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 Tcl_GetIntFromObj and Tcl_ListObjAppendElement. The individual proce-
dures are described along with the data structures they manipulate.
Tcl's dual-ported 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.
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 representa-
tion, and, once computed, it is saved. In addition, a change in one representation inval-
idates 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 con-
stantly 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 representa-
tion from the integer. Although objects contain an internal representation, their seman-
tics 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 representa-
tion is fetched. Because of this representation invalidation and regeneration, it is dan-
gerous for extension writers to access Tcl_Obj fields directly. It is better to access
Tcl_Obj information using procedures like Tcl_GetStringFromObj and Tcl_GetString.
Objects are allocated on the heap and are referenced using a pointer to their Tcl_Obj
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 val-
ues 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.
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 Tcl_RegisterObjType .
THE TCL_OBJ STRUCTURE
Each Tcl object is represented by a Tcl_Obj structure which is defined as follows.
typedef struct Tcl_Obj {
int refCount;
char *bytes;
int length;
Tcl_ObjType *typePtr;
union {
long longValue;
double doubleValue;
VOID *otherValuePtr;
struct {
VOID *ptr1;
VOID *ptr2;
} twoPtrValue;
} internalRep;
} Tcl_Obj;
The bytes and the length members together hold an object's UTF-8 string representation, |
which is a counted string not containing null bytes (UTF-8 null characters should be |
encoded as a two byte sequence: 192, 128.) bytes points to the first byte of the string |
representation. The length member gives the number of bytes. The byte array must always |
have a null byte after the last data byte, at offset length; this allows string represen- |
tations to be treated as conventional null-terminated C strings. C programs use Tcl_Get-
StringFromObj and Tcl_GetString to get an object's string representation. If bytes is
NULL, the string representation is invalid.
An object's type manages its internal representation. The member typePtr points to the
Tcl_ObjType structure that describes the type. If typePtr is NULL, the internal represen-
tation is invalid.
The internalRep 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 arbi-
trary pointers.
The refCount 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 STORAGE MANAGEMENT OF OBJECTS.
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 refCount directly; they should use macros such as Tcl_IncrRef-
Count and Tcl_IsShared instead.
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 typePtr.
An object containing an empty string or a copy of a specified string is created using
Tcl_NewObj or Tcl_NewStringObj respectively. An object's string value is gotten with
Tcl_GetStringFromObj or Tcl_GetString and changed with Tcl_SetStringObj. If the object is
later passed to a procedure like Tcl_GetIntFromObj that requires a specific internal rep-
resentation, the procedure will create one and set the object's typePtr. The internal
representation is computed from the string representation. An object's two representa-
tions are duals of each other: changes made to one are reflected in the other. For exam-
ple, Tcl_ListObjReplace will modify an object's internal representation and the next call
to Tcl_GetStringFromObj or Tcl_GetString will reflect that change.
Representations are recomputed lazily for efficiency. A change to one representation made
by a procedure such as Tcl_ListObjReplace is not reflected immediately in the other repre-
sentation. Instead, the other representation is marked invalid so that it is only regen-
erated if it is needed later. Most C programmers never have to be concerned with how this
is done and simply use procedures such as Tcl_GetBooleanFromObj or Tcl_ListObjIndex. Pro-
grammers that implement their own object types must check for invalid representations and
mark representations invalid when necessary. The procedure Tcl_InvalidateStringRep is
used to mark an object's string representation invalid and to free any storage associated
with the old string representation.
Objects usually remain one type over their life, but occasionally an object must be con-
verted from one type to another. For example, a C program might build up a string in an
object with repeated calls to Tcl_AppendToObj, and then call Tcl_ListObjIndex to extract a
list element from the object. The same object holding the same string value can have sev-
eral different internal representations at different times. Extension writers can also
force an object to be converted from one type to another using the Tcl_ConvertToType pro-
cedure. 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 typePtr. See the man page for
Tcl_RegisterObjType to see how to create a new object type.
EXAMPLE OF THE LIFETIME OF AN OBJECT
As an example of the lifetime of an object, consider the following sequence of commands:
set x 123
This assigns to x an untyped object whose bytes member points to 123 and length member
contains 3. The object's typePtr member is NULL.
puts "x is $x"
x's string representation is valid (since bytes is non-NULL) and is fetched for the com-
mand.
incr x
The incr command first gets an integer from x's object by calling Tcl_GetIntFromObj. This
procedure checks whether the object is already an integer object. Since it is not, it
converts the object by setting the object's internalRep.longValue member to the integer
123 and setting the object's typePtr to point to the integer Tcl_ObjType structure. Both
representations are now valid. incr increments the object's integer internal representa-
tion then invalidates its string representation (by calling Tcl_InvalidateStringRep) since
the string representation no longer corresponds to the internal representation.
puts "x is now $x"
The string representation of x's object is needed and is recomputed. The string represen-
tation is now 124. and both representations are again valid.
STORAGE MANAGEMENT OF OBJECTS
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 Tcl_NewObj or Tcl_NewStringObj has
refCount 0. The macro Tcl_IncrRefCount increments the reference count when a new refer-
ence to the object is created. The macro Tcl_DecrRefCount 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 refCount greater
than 1. Incrementing an object's reference count ensures that it won't be freed too early
or have its value change accidently.
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 Tcl_IncrRefCount 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
Tcl_DecrRefCount to decrement each argument's reference count. When an object's reference
count drops less than or equal to zero, Tcl_DecrRefCount reclaims its storage. Most com-
mand 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 care-
ful to increment its reference count since the retained pointer is a new reference.
Command procedures that directly modify objects such as those for lappend and linsert must
be careful to copy a shared object before changing it. They must first check whether the
object is shared by calling Tcl_IsShared. If the object is shared they must copy the
object by using Tcl_DuplicateObj; this returns a new duplicate of the original object that
has refCount 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 linsert. This procedure modifies the list object passed to it
in objv[1] by inserting objc-3 new elements before index.
listPtr = objv[1];
if (Tcl_IsShared(listPtr)) {
listPtr = Tcl_DuplicateObj(listPtr);
}
result = Tcl_ListObjReplace(interp, listPtr, index, 0, (objc-3), &(objv[3]));
As another example, incr'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.
SEE ALSO
Tcl_ConvertToType, Tcl_GetIntFromObj, Tcl_ListObjAppendElement, Tcl_ListObjIndex, Tcl_Lis-
tObjReplace, Tcl_RegisterObjType
KEYWORDS
internal representation, object, object creation, object type, reference counting, string
representation, type conversion
Tcl 8.1 Tcl_Obj(3) |
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