trusty (3) Tcl_ConvertToType.3tcl.gz
NAME
Tcl_RegisterObjType, Tcl_GetObjType, Tcl_AppendAllObjTypes, Tcl_ConvertToType - 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
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.
CONST char *typeName (in) The name of a Tcl object type that Tcl_GetObjType should look up.
Tcl_Interp *interp (in) Interpreter to use for error reporting.
Tcl_Obj *objPtr (in) For Tcl_AppendAllObjTypes, 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. The 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 supported by Tcl.
The argument typePtr 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 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 Tcl_ObjType with name typeName. It returns NULL if no type with
that name is registered.
Tcl_AppendAllObjTypes appends the name of each 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 possible. It creates a new internal
representation for objPtr appropriate for the target type typePtr and sets its typePtr member to that
type. Any internal representation for objPtr's old type is freed. If an error occurs during conversion,
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).
THE TCL_OBJTYPE STRUCTURE
Extension writers can define new object types by defining four procedures, initializing a Tcl_ObjType
structure to describe the type, and calling Tcl_RegisterObjType. 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 member describes the name of the type, e.g. int. Extension writers can look up an object type
using its name with the Tcl_GetObjType procedure. The remaining four members are pointers to procedures
called by the generic Tcl object code:
The setFromAnyProc member contains the address of a function called to create a valid internal
representation from an object's string representation.
typedef int (Tcl_SetFromAnyProc) (Tcl_Interp *interp, Tcl_Obj *objPtr);
If an internal representation can't 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
successful, it stores the new internal representation, sets objPtr's typePtr member to point to
setFromAnyProc's Tcl_ObjType, and returns TCL_OK. Before setting the new internal representation, the
setFromAnyProc must free any internal representation 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 builtin Tcl
integer type gets an up-to-date string representation for objPtr by calling Tcl_GetStringFromObj. It
parses the string to obtain an integer and, if this succeeds, stores the integer in objPtr's internal
representation and sets objPtr's typePtr member to point to the integer 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 updateStringProc member contains the address of a function called to create a valid string
representation from an object's internal representation.
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 representation's byte array to have a null after the last byte, at
offset length; 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 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 builtin list type, for example,
builds an array of strings for each element object and then calls Tcl_Merge to construct a string with
proper Tcl list structure. It stores this string as the list object's string representation.
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
copying its internal representation means. For example, the dupIntRepProc for the Tcl integer type
simply copies an integer. The builtin list type's dupIntRepProc 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).
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, Tcl list objects have an
internalRep.otherValuePtr that points to an array of pointers to each element in the list. The list
type's freeIntRepProc 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
freeIntRepProc member can be set to NULL to indicate that the internal representation does not require
freeing.
SEE ALSO
Tcl_NewObj, Tcl_DecrRefCount, Tcl_IncrRefCount
KEYWORDS
internal representation, object, object type, string representation, type conversion