Tcl_ObjType(3) Tcl Library Procedures Tcl_ObjType(3)
NAME
Tcl_RegisterObjType, Tcl_GetObjType, Tcl_AppendAllObjTypes, Tcl_Con-
vertToType - manipulate Tcl object types
SYNOPSIS
#include <tcl.h>
Tcl_RegisterObjType(typePtr)
Tcl_ObjType *
Tcl_GetObjType(typeName)
int
Tcl_AppendAllObjTypes(interp, objPtr)
int
Tcl_ConvertToType(interp, objPtr, typePtr)
ARGUMENTS
Points to the structure containing information about the Tcl object
type. This storage must live forever, typically by being statically
allocated. The name of a Tcl object type that Tcl_GetObjType should
look up. Interpreter to use for error reporting. For Tcl_AppendAllOb-
jTypes, this points to the object onto which it appends the name of
each object type as a list element. For Tcl_ConvertToType, this points
to an object that must have been the result of a previous call to
Tcl_NewObj.
DESCRIPTION
The procedures in this man page manage Tcl object types. They are used
to register new object types, look up types, and force conversions from
one type to another.
Tcl_RegisterObjType registers a new Tcl object type in the table of all
object types that Tcl_GetObjType can look up by name. There are other
object types supported by Tcl as well, which Tcl chooses not to regis-
ter. Extensions can likewise choose to register the object types they
create or not. The argument typePtr points to a Tcl_ObjType structure
that describes the new type by giving its name and by supplying point-
ers to four procedures that implement the type. If the type table
already contains a type with the same name as in typePtr, it is
replaced with the new type. The Tcl_ObjType structure is described in
the section THE TCL_OBJTYPE STRUCTURE below.
Tcl_GetObjType returns a pointer to the registered Tcl_ObjType with
name typeName. It returns NULL if no type with that name is regis-
tered.
Tcl_AppendAllObjTypes appends the name of each registered object type
as a list element onto the Tcl object referenced by objPtr. The return
value is TCL_OK unless there was an error converting objPtr to a list
object; in that case TCL_ERROR is returned.
Tcl_ConvertToType converts an object from one type to another if possi-
ble. It creates a new internal representation for objPtr appropriate
for the target type typePtr and sets its typePtr member as determined
by calling the typePtr->setFromAnyProc routine. Any internal represen-
tation for objPtr's old type is freed. If an error occurs during con-
version, it returns TCL_ERROR and leaves an error message in the result
object for interp unless interp is NULL. Otherwise, it returns TCL_OK.
Passing a NULL interp allows this procedure to be used as a test
whether the conversion can be done (and in fact was done).
In many cases, the typePtr->setFromAnyProc routine will set
objPtr->typePtr to the argument value typePtr, but that is no longer
guaranteed. The setFromAnyProc is free to set the internal representa-
tion for objPtr to make use of another related Tcl_ObjType, if it sees
fit.
THE TCL_OBJTYPE STRUCTURE
Extension writers can define new object types by defining four proce-
dures and initializing a Tcl_ObjType structure to describe the type.
Extension writers may also pass a pointer to their Tcl_ObjType struc-
tire to Tcl_RegisterObjType if they wish to permit other extensions to
look up their Tcl_ObjType by name with the Tcl_GetObjType routine. The
Tcl_ObjType structure is defined as follows:
typedef struct Tcl_ObjType {
char *name;
Tcl_FreeInternalRepProc *freeIntRepProc;
Tcl_DupInternalRepProc *dupIntRepProc;
Tcl_UpdateStringProc *updateStringProc;
Tcl_SetFromAnyProc *setFromAnyProc; } Tcl_ObjType;
THE NAME FIELD
The name member describes the name of the type, e.g. int. When a type
is registered, this is the name used by callers of Tcl_GetObjType to
lookup the type. For unregistered types, the name field is primarily
of value for debugging. The remaining four members are pointers to
procedures called by the generic Tcl object code:
THE SETFROMANYPROC FIELD
The setFromAnyProc member contains the address of a function called to
create a valid internal representation from an object's string repre-
sentation.
typedef int (Tcl_SetFromAnyProc) (Tcl_Interp *interp,
Tcl_Obj *objPtr);
If an internal representation cannot be created from the string, it
returns TCL_ERROR and puts a message describing the error in the result
object for interp unless interp is NULL. If setFromAnyProc is success-
ful, it stores the new internal representation, sets objPtr's typePtr
member to point to the Tcl_ObjType struct corresponding to the new
internal representation, and returns TCL_OK. Before setting the new
internal representation, the setFromAnyProc must free any internal rep-
resentation of objPtr's old type; it does this by calling the old
type's freeIntRepProc if it is not NULL.
As an example, the setFromAnyProc for the built-in Tcl list type gets
an up-to-date string representation for objPtr by calling Tcl_Get-
StringFromObj. It parses the string to verify it is in a valid list
format and to obtain each element value in the list, and, if this suc-
ceeds, stores the list elements in objPtr's internal representation and
sets objPtr's typePtr member to point to the list type's Tcl_ObjType
structure.
Do not release objPtr's old internal representation unless you replace
it with a new one or reset the typePtr member to NULL.
The setFromAnyProc member may be set to NULL, if the routines making
use of the internal representation have no need to derive that internal
representation from an arbitrary string value. However, in this case,
passing a pointer to the type to Tcl_ConvertToType() will lead to a
panic, so to avoid this possibility, the type should not be registered.
THE UPDATESTRINGPROC FIELD
The updateStringProc member contains the address of a function called
to create a valid string representation from an object's internal rep-
resentation.
typedef void (Tcl_UpdateStringProc) (Tcl_Obj *objPtr);
objPtr's bytes member is always NULL when it is called. It must always
set bytes non-NULL before returning. We require the string representa-
tion's byte array to have a null after the last byte, at offset length,
and to have no null bytes before that; this allows string representa-
tions to be treated as conventional null character-terminated C
strings. These restrictions are easily met by using Tcl's internal UTF
encoding for the string representation, same as one would do for other
Tcl routines accepting string values as arguments. Storage for the
byte array must be allocated in the heap by Tcl_Alloc or ckalloc. Note
that updateStringProcs must allocate enough storage for the string's
bytes and the terminating null byte.
The updateStringProc for Tcl's built-in double type, for example, calls
Tcl_PrintDouble to write to a buffer of size TCL_DOUBLE_SPACE, then
allocates and copies the string representation to just enough space to
hold it. A pointer to the allocated space is stored in the bytes mem-
ber.
The updateStringProc member may be set to NULL, if the routines making
use of the internal representation are written so that the string rep-
resentation is never invalidated. Failure to meet this obligation will
lead to panics or crashes when Tcl_GetStringFromObj or other similar
routines ask for the string representation.
THE DUPINTREPPROC FIELD
The dupIntRepProc member contains the address of a function called to
copy an internal representation from one object to another.
typedef void (Tcl_DupInternalRepProc) (Tcl_Obj *srcPtr,
Tcl_Obj *dupPtr);
dupPtr's internal representation is made a copy of srcPtr's internal
representation. Before the call, srcPtr's internal representation is
valid and dupPtr's is not. srcPtr's object type determines what copy-
ing its internal representation means.
For example, the dupIntRepProc for the Tcl integer type simply copies
an integer. The built-in list type's dupIntRepProc uses a far more
sophisticated scheme to continue sharing storage as much as it reason-
ably can.
THE FREEINTREPPROC FIELD
The freeIntRepProc member contains the address of a function that is
called when an object is freed.
typedef void (Tcl_FreeInternalRepProc) (Tcl_Obj *objPtr);
The freeIntRepProc 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, the list type's freeIntRepProc respects the storage shar-
ing scheme established by the dupIntRepProc so that it only frees stor-
age when the last object sharing it is being freed.
The freeIntRepProc member can be set to NULL to indicate that the
internal representation does not require freeing. The freeIntRepProc
implementation must not access the bytes member of the object, since
Tcl makes its own internal uses of that field during object deletion.
The defined tasks for the freeIntRepProc have no need to consult the
bytes member.
SEE ALSO
Tcl_NewObj(3), Tcl_DecrRefCount(3), Tcl_IncrRefCount(3)
KEYWORDS
internal representation, object, object type, string representation,
type conversion
Tcl 8.0 Tcl_ObjType(3)
ObjectType 8.5.4 - Generated Tue Aug 19 20:57:24 CDT 2008