The Tcl_ObjType structure holds the type descriptor for a Tcl_Obj of a particular type. It contains five fields that characterize the common operations that Tcl can perform on all values; all other operations require that values be of one (or one of a few) specific type(s). Each instance of this structure is assumed to be a C constant: Tcl does not ever try to deallocate them.
It's definition is:
/*
* The following structure represents a type of object, which is a particular
* internal representation for an object plus a set of functions that provide
* standard operations on objects of that type.
*/
typedef struct Tcl_ObjType {
const char *name; /* Name of the type, e.g. "int". */
Tcl_FreeInternalRepProc *freeIntRepProc;
/* Called to free any storage for the type's
* internal rep. NULL if the internal rep does
* not need freeing. */
Tcl_DupInternalRepProc *dupIntRepProc;
/* Called to create a new object as a copy of
* an existing object. */
Tcl_UpdateStringProc *updateStringProc;
/* Called to update the string rep from the
* type's internal representation. */
Tcl_SetFromAnyProc *setFromAnyProc;
/* Called to convert the object's internal rep
* to this type. Frees the internal rep of the
* old type. Returns TCL_ERROR on failure. */
} Tcl_ObjType;