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 typePtr->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 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 representation for objPtr to make use of another related Tcl_ObjType, if it sees fit.
typedef struct Tcl_ObjType {
const char *name;
Tcl_FreeInternalRepProc *freeIntRepProc;
Tcl_DupInternalRepProc *dupIntRepProc;
Tcl_UpdateStringProc *updateStringProc;
Tcl_SetFromAnyProc *setFromAnyProc;
} Tcl_ObjType;
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 built-in Tcl list type gets an up-to-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.
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, and to have no null bytes before that; this allows string representations 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 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.
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 built-in list type's dupIntRepProc uses a far more sophisticated scheme to continue sharing storage as much as it reasonably can.
typedef void Tcl_FreeInternalRepProc(
Tcl_Obj *objPtr);
The freeIntRepProc function can deallocate the storage for the value's internal representation and do other type-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.
Note that if a subsidiary value has its reference count reduced to zero during the running of a freeIntRepProc, that value may be not freed immediately, in order to limit stack usage. However, the value will be freed before the outermost current Tcl_DecrRefCount returns.
The objPtr argument to Tcl_ConvertToType can have any non-zero reference count; this function will not modify the reference count, but may write to the interpreter result on error so values that originate from there should have an additional reference made before calling this.
None of the callback functions in the Tcl_ObjType structure should modify the reference count of their arguments, but if the values contain subsidiary values (e.g., the elements of a list or the keys of a dictionary) then those subsidiary values may have their reference counts modified.