Tcl_ObjType(3)

NAME
Tcl_RegisterObjType, Tcl_GetObjType, Tcl_AppendAllObjTypes, Tcl_ConvertToType, Tcl_FreeInternalRep, Tcl_InitStringRep, Tcl_HasStringRep, Tcl_StoreInternalRep, Tcl_FetchInternalRep - 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)

void
Tcl_FreeInternalRep(objPtr)

char *
Tcl_InitStringRep(objPtr, bytes, numBytes)

int
Tcl_HasStringRep(objPtr)

void
Tcl_StoreInternalRep(objPtr, typePtr, irPtr)

Tcl_ObjInternalRep *
Tcl_FetchInternalRep(objPtr, typePtr)

ARGUMENTS
Points to the structure containing information about the Tcl value type. This storage must live forever, typically by being statically allocated. The name of a Tcl value type that Tcl_GetObjType should look up. Interpreter to use for error reporting. 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. String representation. Length of the string representation in bytes. Internal object representation. Requested internal representation type.

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.

Tcl_FreeInternalRep performs the function of the existing internal macro TclInitStringRep, but is extended to return a pointer to the string rep, and to accept NULL as a value for bytes. When bytes is NULL and objPtr has no string rep, an uninitialzed buffer of numBytes bytes is created for filling by the caller. When bytes is NULL and objPtr has a string rep, the string rep will be truncated to a length of numBytes bytes. When numBytes is greater than zero, and the returned pointer is NULL, that indicates a failure to allocate memory for the string representation. The caller may then choose whether to raise an error or panic.

Tcl_InitStringRep performs the function of the existing internal macro TclInitStringRep, but is extended to return a pointer to the string rep, and to accept NULL as a value for bytes. When bytes is NULL and objPtr has no string rep, an uninitialzed buffer of numBytes bytes is created for filling by the caller. When bytes is NULL and objPtr has a string rep, the string rep will be truncated to a length of numBytes bytes. When numBytes is greater than zero, and the returned pointer is NULL, that indicates a failure to allocate memory for the string representation. The caller may then choose whether to raise an error or panic.

Tcl_HasStringRep returns a boolean indicating whether or not a string rep is currently stored in objPtr. This is used when the caller wants to act on objPtr differently depending on whether or not it is a pure value. Typically this only makes sense in an extension if it is already known that objPtr possesses an internal type that is managed by the extension.

Tcl_StoreInternalRep stores in objPtr a copy of the internal representation pointed to by irPtr and sets its type to typePtr. When irPtr is NULL, this leaves objPtr without a representation for type typePtr.

Tcl_FetchInternalRep returns a pointer to the internal representation stored in objPtr that matches the requested type typePtr. If no such internal representation is in objPtr, return NULL.

This returns a public type typedef union Tcl_ObjInternalRep {...} Tcl_ObjInternalRep where the contents are exactly the existing contents of the union in the internalRep field of the Tcl_Obj struct. This definition permits us to pass internal representations and pointers to them as arguments and results in public routines.

THE TCL_OBJTYPE STRUCTURE

Extension writers can define new value types by defining four to twelve 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 {
    const char *name;
    Tcl_FreeInternalRepProc *freeIntRepProc;
    Tcl_DupInternalRepProc *dupIntRepProc;
    Tcl_UpdateStringProc *updateStringProc;
    Tcl_SetFromAnyProc *setFromAnyProc;
    size_t version;
    /* List emulation functions - ObjType Version 1 & 2 */
    Tcl_ObjTypeLengthProc *lengthProc;
    /* List emulation functions - ObjType Version 2 */
    Tcl_ObjTypeIndexProc *indexProc;
    Tcl_ObjTypeSliceProc *sliceProc;
    Tcl_ObjTypeReverseProc *reverseProc;
    Tcl_ObjTypeGetElements *getElementsProc;
    Tcl_ObjTypeSetElement *setElementProc;
    Tcl_ObjTypeReplaceProc *replaceProc;
    Tcl_ObjTypeInOperatorProc *inOperProc; } 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. 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.

Note that if a subsidiary value has its reference count reduced to zero during the running of a freeIntRepProc, that value may be not freed immediately, in order to limit stack usage. However, the value will be freed before the outermost current Tcl_DecrRefCount returns.

  THE VERSION FIELD

The version member provides for future extensibility of the structure and should be set to TCL_OBJTYPE_V0 for compatibility of ObjType definitions prior to version 9.0. Specifics about versions will be described further in the sections below.

ABSTRACT LIST TYPES

Additional fields in the Tcl_ObjType descriptor allow for control over how custom data values can be manipulated using Tcl's List commands without converting the value to a List type. This requires the custom type to provide functions that will perform the given operation on the custom data representation. Not all functions are required. In the absence of a particular function (set to NULL), the fallback is to allow the internal List operation to perform the operation, most likely causing the value type to be converted to a traditional list.

  SCALAR VALUE TYPES

For a custom value type that is scalar or atomic in nature, i.e., not a divisible collection, version TCL_OBJTYPE_V1 is recommended. In this case, List commands will treat the scalar value as if it where a list of length 1, and not convert the value to a List type.

  VERSION 2: ABSTRACT LISTS

Version 2, TCL_OBJTYPE_V2, allows full List support when the functions described below are provided. This allows for script level use of the List commands without causing the type of the Tcl_Obj value to be converted to a list.

  THE LENGTHPROC FIELD

The LengthProc function correlates with the Tcl_ListObjLength C API. The function returns the number of elements in the list. It is used in every List operation and is required for all Abstract List implementations. typedef Tcl_Size (Tcl_ObjTypeLengthProc) (Tcl_Obj *listPtr);

  THE INDEXPROC FIELD

The IndexProc function correlates with with the Tcl_ListObjIndex C API. The function returns a Tcl_Obj value for the element at the specified index. typedef int (Tcl_ObjTypeIndexProc) (
    Tcl_Interp *interp,
    Tcl_Obj *listPtr,
    Tcl_Size index,
    Tcl_Obj** elemObj);

  THE SLICEPROC FIELD

The SliceProc correlates with the lrange command, returning a new List or Abstract List for the portion of the original list specified. typedef int (Tcl_ObjTypeSliceProc) (
    Tcl_Interp *interp,
    Tcl_Obj *listPtr,
    Tcl_Size fromIdx,
    Tcl_Size toIdx,
    Tcl_Obj **newObjPtr);

  THE REVERSEPROC FIELD

The ReverseProc correlates with the lreverse command, returning a List or Abstract List that has the same elements as the input Abstract List, with the elements in the reverse order. typedef int (Tcl_ObjTypeReverseProc) (
    Tcl_Interp *interp,
    Tcl_Obj *listPtr,
    Tcl_Obj **newObjPtr);

  THE GETELEMENTS FIELD

The GetElements function returns a count and a pointer to an array of Tcl_Obj values for the entire Abstract List. This correlates to the Tcl_ListObjGetElements C API call. typedef int (Tcl_ObjTypeGetElements) (
    Tcl_Interp *interp,
    Tcl_Obj *listPtr,
    Tcl_Size *objcptr,
    Tcl_Obj ***objvptr);

  THE SETELEMENT FIELD

The SetElement function replaces the element within the specified list at the give index. This function correlates to the lset command. typedef Tcl_Obj *(Tcl_ObjTypeSetElement) (
    Tcl_Interp *interp,
    Tcl_Obj *listPtr,
    Tcl_Size indexCount,
    Tcl_Obj *const indexArray[],
    Tcl_Obj *valueObj);

  REPLACEPROC FIELD

The ReplaceProc returns a new list after modifying the list replacing the elements to be deleted, and adding the elements to be inserted. This function correlates to the Tcl_ListObjReplace C API. typedef int (Tcl_ObjTypeReplaceProc) (
    Tcl_Interp *interp,
    Tcl_Obj *listObj,
    Tcl_Size first,
    Tcl_Size numToDelete,
    Tcl_Size numToInsert,
    Tcl_Obj *const insertObjs[]);

  THE INOPERPROC FIELD

The InOperProc function determines whether the value is present in the given list, according to equivalent string comparison of elements. The boolResult is set to 1 (true) if the value is present, and 0 (false) if it is not present. This function implements the "in" and "ni" math operators for an abstract list. typedef int (Tcl_ObjTypeInOperatorProc) (
    Tcl_Interp *interp,
    Tcl_Obj *valueObj,
    Tcl_Obj *listObj,
    int *boolResult);

REFERENCE COUNT MANAGEMENT

The objPtr argument to Tcl_AppendAllObjTypes should be an unshared value; this function will not modify the reference count of that value, but will modify its contents. If objPtr is not (interpretable as) a list, this function will set the interpreter result and produce an error; using an unshared empty value is strongly recommended.

The objPtr argument to Tcl_ConvertToType can have any non-zero reference count; this function will not modify the reference count, but may write to the interpreter result on error so values that originate from there should have an additional reference made before calling this.

None of the callback functions in the Tcl_ObjType structure should modify the reference count of their arguments, but if the values contain subsidiary values (e.g., the elements of a list or the keys of a dictionary) then those subsidiary values may have their reference counts modified.

SEE ALSO
Tcl_NewObj(3), Tcl_DecrRefCount(3), Tcl_IncrRefCount(3), Tcl_BounceRefCount(3)

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