path: root/doc/ObjectType.3

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'\" RCS: @(#) $Id: ObjectType.3,v 1.8 2004/06/18 15:17:58 dkf Exp $
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.so man.macros
.TH Tcl_ObjType 3 8.0 Tcl "Tcl Library Procedures"
.BS
.SH NAME
Tcl_RegisterObjType, Tcl_GetObjType, Tcl_AppendAllObjTypes, Tcl_ConvertToType  \- manipulate Tcl object types
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
\fBTcl_RegisterObjType\fR(\fItypePtr\fR)
.sp
Tcl_ObjType *
\fBTcl_GetObjType\fR(\fItypeName\fR)
.sp
int
\fBTcl_AppendAllObjTypes\fR(\fIinterp, objPtr\fR)
.sp
int
\fBTcl_ConvertToType\fR(\fIinterp, objPtr, typePtr\fR)
.SH ARGUMENTS
.AS Tcl_ObjType *typeName in
.AP Tcl_ObjType *typePtr in
Points to the structure containing information about the Tcl object type.
This storage must live forever,
typically by being statically allocated.
.AP "CONST char" *typeName in
The name of a Tcl object type that \fBTcl_GetObjType\fR should look up.
.AP Tcl_Interp *interp in
Interpreter to use for error reporting.
.AP Tcl_Obj *objPtr in
For \fBTcl_AppendAllObjTypes\fR, this points to the object onto which
it appends the name of each object type as a list element.
For \fBTcl_ConvertToType\fR, this points to an object that
must have been the result of a previous call to \fBTcl_NewObj\fR.
.BE

.SH DESCRIPTION
.PP
The procedures in this man page manage Tcl object types.
The are used to register new object types,
look up types,
and force conversions from one type to another.
.PP
\fBTcl_RegisterObjType\fR registers a new Tcl object type
in the table of all object types supported by Tcl.
The argument \fItypePtr\fR points to a Tcl_ObjType structure that
describes the new type by giving its name
and by supplying pointers to four procedures
that implement the type.
If the type table already contains a type
with the same name as in \fItypePtr\fR,
it is replaced with the new type.
The Tcl_ObjType structure is described
in the section \fBTHE TCL_OBJTYPE STRUCTURE\fR below.
.PP
\fBTcl_GetObjType\fR returns a pointer to the Tcl_ObjType
with name \fItypeName\fR.
It returns NULL if no type with that name is registered.
.PP
\fBTcl_AppendAllObjTypes\fR appends the name of each object type
as a list element onto the Tcl object referenced by \fIobjPtr\fR.
The return value is \fBTCL_OK\fR unless there was an error
converting \fIobjPtr\fR to a list object;
in that case \fBTCL_ERROR\fR is returned.
.PP
\fBTcl_ConvertToType\fR converts an object from one type to another
if possible.
It creates a new internal representation for \fIobjPtr\fR
appropriate for the target type \fItypePtr\fR
and sets its \fItypePtr\fR member to that type.
Any internal representation for \fIobjPtr\fR's old type is freed.
If an error occurs during conversion, it returns \fBTCL_ERROR\fR
and leaves an error message in the result object for \fIinterp\fR
unless \fIinterp\fR is NULL.
Otherwise, it returns \fBTCL_OK\fR.
Passing a NULL \fIinterp\fR allows this procedure to be used
as a test whether the conversion can be done (and in fact was done).

.SH "THE TCL_OBJTYPE STRUCTURE"
.PP
Extension writers can define new object types by defining four
procedures,
initializing a Tcl_ObjType structure to describe the type,
and calling \fBTcl_RegisterObjType\fR.
The \fBTcl_ObjType\fR structure is defined as follows:
.CS
typedef struct Tcl_ObjType {
	char *\fIname\fR;
	Tcl_FreeInternalRepProc *\fIfreeIntRepProc\fR;
	Tcl_DupInternalRepProc *\fIdupIntRepProc\fR;
	Tcl_UpdateStringProc *\fIupdateStringProc\fR;
	Tcl_SetFromAnyProc *\fIsetFromAnyProc\fR;
} Tcl_ObjType;
.CE
.PP
The \fIname\fR member describes the name of the type, e.g. \fBint\fR.
Extension writers can look up an object type using its name
with the \fBTcl_GetObjType\fR procedure.
The remaining four members are pointers to procedures
called by the generic Tcl object code:
.PP
The \fIsetFromAnyProc\fR member contains the address of a function
called to create a valid internal representation
from an object's string representation.
.CS
typedef int (Tcl_SetFromAnyProc) (Tcl_Interp *\fIinterp\fR, Tcl_Obj *\fIobjPtr\fR);
.CE
If an internal representation can't be created from the string,
it returns \fBTCL_ERROR\fR and puts a message
describing the error in the result object for \fIinterp\fR
unless \fIinterp\fR is NULL.
If \fIsetFromAnyProc\fR is successful,
it stores the new internal representation,
sets \fIobjPtr\fR's \fItypePtr\fR member to point to
\fIsetFromAnyProc\fR's \fBTcl_ObjType\fR, and returns \fBTCL_OK\fR.
Before setting the new internal representation,
the \fIsetFromAnyProc\fR must free any internal representation
of \fIobjPtr\fR's old type;
it does this by calling the old type's \fIfreeIntRepProc\fR
if it is not NULL.
As an example, the \fIsetFromAnyProc\fR for the builtin Tcl integer type
gets an up-to-date string representation for \fIobjPtr\fR
by calling \fBTcl_GetStringFromObj\fR.
It parses the string to obtain an integer and,
if this succeeds,
stores the integer in \fIobjPtr\fR's internal representation
and sets \fIobjPtr\fR's \fItypePtr\fR member to point to the integer type's
Tcl_ObjType structure.
Do not release \fIobjPtr\fR's old internal representation unless you
replace it with a new one or reset the \fItypePtr\fR member to NULL.
.PP
The \fIupdateStringProc\fR member contains the address of a function
called to create a valid string representation
from an object's internal representation.
.CS
typedef void (Tcl_UpdateStringProc) (Tcl_Obj *\fIobjPtr\fR);
.CE
\fIobjPtr\fR's \fIbytes\fR member is always NULL when it is called.
It must always set \fIbytes\fR non-NULL before returning.
We require the string representation's byte array
to have a null after the last byte, at offset \fIlength\fR;
this allows string representations that do not contain null bytes
to be treated as conventional null character-terminated C strings.
Storage for the byte array must be allocated in the heap by \fBTcl_Alloc\fR
or \fBckalloc\fR.  Note that \fIupdateStringProc\fRs must allocate
enough storage for the string's bytes and the terminating null byte.
The \fIupdateStringProc\fR for Tcl's builtin list type, for example,
builds an array of strings for each element object
and then calls \fBTcl_Merge\fR
to construct a string with proper Tcl list structure.
It stores this string as the list object's string representation.
.PP
The \fIdupIntRepProc\fR member contains the address of a function
called to copy an internal representation from one object to another.
.CS
typedef void (Tcl_DupInternalRepProc) (Tcl_Obj *\fIsrcPtr\fR, Tcl_Obj *\fIdupPtr\fR);
.CE
\fIdupPtr\fR's internal representation is made a copy of \fIsrcPtr\fR's
internal representation.
Before the call,
\fIsrcPtr\fR's internal representation is valid and \fIdupPtr\fR's is not.
\fIsrcPtr\fR's object type determines what
copying its internal representation means.
For example, the \fIdupIntRepProc\fR for the Tcl integer type
simply copies an integer.
The builtin list type's \fIdupIntRepProc\fR
allocates a new array that points at the original element objects;
the elements are shared between the two lists
(and their reference counts are incremented to reflect the new references).
.PP
The \fIfreeIntRepProc\fR member contains the address of a function
that is called when an object is freed.
.CS
typedef void (Tcl_FreeInternalRepProc) (Tcl_Obj *\fIobjPtr\fR);
.CE
The \fIfreeIntRepProc\fR function can deallocate the storage
for the object's internal representation
and do other type-specific processing necessary when an object is freed.
For example, Tcl list objects have an \fIinternalRep.otherValuePtr\fR
that points to an array of pointers to each element in the list.
The list type's \fIfreeIntRepProc\fR decrements
the reference count for each element object
(since the list will no longer refer to those objects),
then deallocates the storage for the array of pointers.
The \fIfreeIntRepProc\fR member can be set to NULL
to indicate that the internal representation does not require freeing.
The \fIfreeIntRepProc\fR implementation should not access the
\fIbytes\fR member of the object, as this may potentially have already
been deleted.

.SH "SEE ALSO"
Tcl_NewObj, Tcl_DecrRefCount, Tcl_IncrRefCount

.SH KEYWORDS
internal representation, object, object type, string representation, type conversion