Tcl_ObjType(3tcl) Tcl Library Procedures Tcl_ObjType(3tcl)
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NAME
Tcl_RegisterObjType, Tcl_GetObjType, Tcl_AppendAllObjTypes, Tcl_Con-
vertToType - 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 for-
ever, 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 report-
ing.
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_Convert-
ToType, this points to a value that
must have been the result of a pre-
vious call to Tcl_NewObj.
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DESCRIPTION
The procedures in this man page manage Tcl value types (sometimes re-
ferred 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 regis-
ter. 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 point-
ers to four procedures that implement the type. If the type table al-
ready 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 sec-
tion 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 regis-
tered.
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 possi-
ble. 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 represen-
tation for objPtr's old type is freed. If an error occurs during con-
version, 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 ob-
jPtr->typePtr to the argument value typePtr, but that is no longer
guaranteed. The setFromAnyProc is free to set the internal representa-
tion 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 proce-
dures and initializing a Tcl_ObjType structure to describe the type.
Extension writers may also pass a pointer to their Tcl_ObjType struc-
ture 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 represen-
tation.
typedef int Tcl_SetFromAnyProc(
Tcl_Interp *interp,
Tcl_Obj *objPtr);
If an internal representation cannot be created from the string, it re-
turns TCL_ERROR and puts a message describing the error in the result
value for interp unless interp is NULL. If setFromAnyProc is success-
ful, it stores the new internal representation, sets objPtr's typePtr
member to point to the Tcl_ObjType struct corresponding to the new in-
ternal representation, and returns TCL_OK. Before setting the new in-
ternal representation, the setFromAnyProc must free any internal repre-
sentation 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_Get-
StringFromObj. 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 suc-
ceeds, 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 repre-
sentation.
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 representa-
tion'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 representa-
tions 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 al-
locates and copies the string representation to just enough space to
hold it. A pointer to the allocated space is stored in the bytes mem-
ber.
The updateStringProc member may be set to NULL, if the routines making
use of the internal representation are written so that the string rep-
resentation 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 built-in list type's dupIntRepProc uses a far more so-
phisticated 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 type-specific processing necessary
when a value is freed.
For example, the list type's freeIntRepProc respects the storage shar-
ing scheme established by the dupIntRepProc so that it only frees stor-
age when the last value sharing it is being freed.
The freeIntRepProc member can be set to NULL to indicate that the in-
ternal representation does not require freeing. The freeIntRepProc im-
plementation 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.
SEE ALSO
Tcl_NewObj(3tcl), Tcl_DecrRefCount(3tcl), Tcl_IncrRefCount(3tcl)
KEYWORDS
internal representation, value, value type, string representation, type
conversion
Tcl 8.0 Tcl_ObjType(3tcl)
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