Tcl Library Procedures - Tcl_ObjType manual page

### 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 must live forever, typically by being statically allocated.

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 containes 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.

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. 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

    Copyright © 1996-1997 Sun Microsystems, Inc.
    Copyright © 1995-1997 Roger E. Critchlow Jr.