<\/a> <\/h2>\nExtension 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<\/b> if they wish to permit other extensions to look up their Tcl_ObjType by name with the Tcl_GetObjType<\/b> routine. The Tcl_ObjType<\/b> structure is defined as follows:<\/p>\n
typedef struct Tcl_ObjType {
const char *name<\/i>;
Tcl_FreeInternalRepProc *freeIntRepProc<\/i>;
Tcl_DupInternalRepProc *dupIntRepProc<\/i>;
Tcl_UpdateStringProc *updateStringProc<\/i>;
Tcl_SetFromAnyProc *setFromAnyProc<\/i>;
} Tcl_ObjType<\/b>;<\/p>\n
THE NAME FIELD<\/b>
The name<\/i> member describes the name of the type, e.g. int<\/b>. When a type is registered, this is the name used by callers of Tcl_GetObjType<\/b> to lookup the type. For unregistered types, the name<\/i> field is primarily of value for debugging. The remaining four members are pointers to procedures called by the generic Tcl value code:<\/p>\n
THE SETFROMANYPROC FIELD<\/b>
The setFromAnyProc<\/i> member contains the address of a function called to create a valid internal representation from a value\u2019s string representation.<\/p>\n
typedef int Tcl_SetFromAnyProc<\/b>(
Tcl_Interp *interp<\/i>,
Tcl_Obj *objPtr<\/i>);<\/p>\n
If an internal representation cannot be created from the string, it returns TCL_ERROR<\/b> and puts a message describing the error in the result value for interp<\/i> unless interp<\/i> is NULL. If setFromAnyProc<\/i> is successful, it stores the new internal representation, sets objPtr<\/i>\u2019s typePtr<\/i> member to point to the Tcl_ObjType<\/b> struct corresponding to the new internal representation, and returns TCL_OK<\/b>. Before setting the new internal representation, the setFromAnyProc<\/i> must free any internal representation of objPtr<\/i>\u2019s old type; it does this by calling the old type\u2019s freeIntRepProc<\/i> if it is not NULL.<\/p>\n
As an example, the setFromAnyProc<\/i> for the built-in Tcl list type gets an up-to-date string representation for objPtr<\/i> by calling Tcl_GetStringFromObj<\/b>. 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<\/i>\u2019s internal representation and sets objPtr<\/i>\u2019s typePtr<\/i> member to point to the list type\u2019s Tcl_ObjType structure.<\/p>\n
Do not release objPtr<\/i>\u2019s old internal representation unless you replace it with a new one or reset the typePtr<\/i> member to NULL.<\/p>\n
The setFromAnyProc<\/i> 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<\/b> will lead to a panic, so to avoid this possibility, the type should not<\/i> be registered.<\/p>\n
THE UPDATESTRINGPROC FIELD<\/b>
The updateStringProc<\/i> member contains the address of a function called to create a valid string representation from a value\u2019s internal representation.<\/p>\n
typedef void Tcl_UpdateStringProc<\/b>(
Tcl_Obj *objPtr<\/i>);<\/p>\n
objPtr<\/i>\u2019s bytes<\/i> member is always NULL when it is called. It must always set bytes<\/i> non-NULL before returning. We require the string representation\u2019s byte array to have a null after the last byte, at offset length<\/i>, 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\u2019s 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<\/b> or ckalloc<\/b>. Note that updateStringProc<\/i>s must allocate enough storage for the string\u2019s bytes and the terminating null byte.<\/p>\n
The updateStringProc<\/i> for Tcl\u2019s 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<\/i> member.<\/p>\n
The updateStringProc<\/i> 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<\/b> or other similar routines ask for the string representation.<\/p>\n
THE DUPINTREPPROC FIELD<\/b>
The dupIntRepProc<\/i> member contains the address of a function called to copy an internal representation from one value to another.<\/p>\n
typedef void Tcl_DupInternalRepProc<\/b>(
Tcl_Obj *srcPtr<\/i>,
Tcl_Obj *dupPtr<\/i>);<\/p>\n
dupPtr<\/i>\u2019s internal representation is made a copy of srcPtr<\/i>\u2019s internal representation. Before the call, srcPtr<\/i>\u2019s internal representation is valid and dupPtr<\/i>\u2019s is not. srcPtr<\/i>\u2019s value type determines what copying its internal representation means.<\/p>\n
For example, the dupIntRepProc<\/i> for the Tcl integer type simply copies an integer. The built-in list type\u2019s dupIntRepProc<\/i> uses a far more sophisticated scheme to continue sharing storage as much as it reasonably can.<\/p>\n
THE FREEINTREPPROC FIELD<\/b>
The freeIntRepProc<\/i> member contains the address of a function that is called when a value is freed.<\/p>\n
typedef void Tcl_FreeInternalRepProc<\/b>(
Tcl_Obj *objPtr<\/i>);<\/p>\n
The freeIntRepProc<\/i> function can deallocate the storage for the value\u2019s internal representation and do other type-specific processing necessary when a value is freed.<\/p>\n
For example, the list type\u2019s freeIntRepProc<\/i> respects the storage sharing scheme established by the dupIntRepProc<\/i> so that it only frees storage when the last value sharing it is being freed.<\/p>\n
The freeIntRepProc<\/i> member can be set to NULL to indicate that the internal representation does not require freeing. The freeIntRepProc<\/i> implementation must not access the bytes<\/i> member of the value, since Tcl makes its own internal uses of that field during value deletion. The defined tasks for the freeIntRepProc<\/i> have no need to consult the bytes<\/i> member.<\/p>\n