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Tcl_ObjType

Section: Tcl Library Procedures (3)
Updated: 8.0
Index Return to Main Contents

NAME

Tcl_RegisterObjType, Tcl_GetObjType, Tcl_AppendAllObjTypes, Tcl_ConvertToType - manipulate Tcl value types

SYNOPSIS

#include <tcl.h>

Tcl_RegisterObjType(typePtr)

const Tcl_ObjType *
Tcl_GetObjType(typeName)

int
Tcl_AppendAllObjTypes(interp, objPtr)

int
Tcl_ConvertToType(interp, objPtr, typePtr)

ARGUMENTS

const Tcl_ObjType *typePtr (in) Points to the structure containing information about the Tcl value type. This storage must live forever, typically by being statically allocated.

const char *typeName (in) The name of a Tcl value 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 value onto which it appends the name of each value type as a list element. For Tcl_ConvertToType, this points to a value that must have been the result of a previous call to Tcl_NewObj.

DESCRIPTION

The procedures in this man page manage Tcl value types (sometimes referred to as object types or Tcl_ObjTypes for historical reasons). They are used to register new value types, look up types, and force conversions from one type to another.

Tcl_RegisterObjType registers a new Tcl value type in the table of all value types that Tcl_GetObjType can look up by name. There are other value types supported by Tcl as well, which Tcl chooses not to register. Extensions can likewise choose to register the value 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 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 registered Tcl_ObjType with name typeName. It returns NULL if no type with that name is registered.

Tcl_AppendAllObjTypes appends the name of each registered value type as a list element onto the Tcl value referenced by objPtr. The return value is TCL_OK unless there was an error converting objPtr to a list value; in that case TCL_ERROR is returned.

Tcl_ConvertToType converts a value 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 as determined by calling the typePt->setFromAnyProc routine. 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 value 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 typePt->setFromAnyProc routine will set objPt->typePtr to the argument value typePtr, but that is no longer guaranteed. The setFromAnyProc is free to set the internal representation for objPtr to make use of another related Tcl_ObjType, if it sees fit.

THE TCL_OBJTYPE STRUCTURE

Extension writers can define new value types by defining four procedures and initializing a Tcl_ObjType structure to describe the type. Extension writers may also pass a pointer to their Tcl_ObjType structure 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 {
    const 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 value code:

THE SETFROMANYPROC FIELD

The setFromAnyProc member contains the address of a function called to create a valid internal representation from a value's string representation.


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 value for interp unless interp is NULL. If setFromAnyProc is successful, 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 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 buil-in Tcl list type gets an u-t-date string representation for objPtr by calling Tcl_GetStringFromObj. 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 succeeds, 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 a value'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 no-NULL before returning. We require the string representation'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 representations to be treated as conventional null characte-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 buil-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 member.

The updateStringProc member may be set to NULL, if the routines making use of the internal representation are written so that the string representation 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 value 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 value type determines what copying its internal representation means.

For example, the dupIntRepProc for the Tcl integer type simply copies an integer. The buil-in list type's dupIntRepProc uses a far more sophisticated scheme to continue sharing storage as much as it reasonably can.

THE FREEINTREPPROC FIELD

The freeIntRepProc member contains the address of a function that is called when a value is freed.


typedef void Tcl_FreeInternalRepProc(
        Tcl_Obj *objPtr);

The freeIntRepProc function can deallocate the storage for the value's internal representation and do other typ-specific processing necessary when a value is freed.

For example, the list type's freeIntRepProc respects the storage sharing scheme established by the dupIntRepProc so that it only frees storage when the last value 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 value, since Tcl makes its own internal uses of that field during value deletion. The defined tasks for the freeIntRepProc have no need to consult the bytes member.