Tribblix: manual page: Tcl

Tcl_ObjType(3) Tcl Library Procedures Tcl_ObjType(3)

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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.
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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 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.

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 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.

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 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.

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
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 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.

KEYWORDS

internal representation, value, value type, string representation,
type conversion

Tcl 8.0 Tcl_ObjType(3)