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Tcl_ObjType(3)			      Tcl Library Procedures			   Tcl_ObjType(3)

_________________________________________________________________________________________________

NAME
       Tcl_RegisterObjType,  Tcl_GetObjType,  Tcl_AppendAllObjTypes, Tcl_ConvertToType	- manipu-
       late Tcl object types

SYNOPSIS
       #include <tcl.h>

       Tcl_RegisterObjType(typePtr)

       Tcl_ObjType *
       Tcl_GetObjType(typeName)

       int
       Tcl_AppendAllObjTypes(interp, objPtr)

       int
       Tcl_ConvertToType(interp, objPtr, typePtr)

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

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

DESCRIPTION
       The  procedures	in  this man page manage Tcl object types.  They are used to register new
       object types, look up types, and force conversions from one type to another.

       Tcl_RegisterObjType registers a new Tcl object type in the table of all object types  that
       Tcl_GetObjType  can  look  up  by  name.  There are other object types supported by Tcl as
       well, which Tcl chooses not to register.  Extensions can likewise choose to  register  the
       object  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  proce-
       dures  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 object type as  a  list  element
       onto  the Tcl object referenced by objPtr.  The return value is TCL_OK unless there was an
       error converting objPtr to a list object; in that case TCL_ERROR is returned.

       Tcl_ConvertToType converts an object 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 conver-
       sion, it returns TCL_ERROR and leaves an error message in the  result  object  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  argu- |
       ment  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 object 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 exten-
       sions to look up their Tcl_ObjType by name with the Tcl_GetObjType routine.  The  Tcl_Obj-
       Type structure is defined as follows:

	      typedef struct Tcl_ObjType {
		  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 point-
       ers to procedures called by the generic Tcl object code:

   THE SETFROMANYPROC FIELD
       The setFromAnyProc member contains the address of a function  called  to  create  a  valid
       internal representation from an object'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 object 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 repre-
       sentation,  and	returns TCL_OK.  Before setting the new internal representation, the set-
       FromAnyProc 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 an object'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	represen-
       tation,	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 termi-
       nating 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 represen-
       tation 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.   Fail-
       ure  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 object 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
       object 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  shar-
       ing storage as much as it reasonably can.

   THE FREEINTREPPROC FIELD
       The freeIntRepProc member contains the address of a function that is called when an object
       is freed.

	      typedef void (Tcl_FreeInternalRepProc) (Tcl_Obj *objPtr);

       The freeIntRepProc function can deallocate the storage for the object's internal represen-
       tation and do other type-specific processing necessary when an object is freed.

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

SEE ALSO
       Tcl_NewObj, Tcl_DecrRefCount, Tcl_IncrRefCount

KEYWORDS
       internal representation, object, object type, string representation, type conversion

Tcl					       8.0				   Tcl_ObjType(3)
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