Ubuntu Manpage: Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl

NAME

       Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl_InvalidateStringRep -
       manipulate Tcl objects

SYNOPSIS

       #include <tcl.h>

       Tcl_Obj *
       Tcl_NewObj()

       Tcl_Obj *
       Tcl_DuplicateObj(objPtr)

       Tcl_IncrRefCount(objPtr)

       Tcl_DecrRefCount(objPtr)

       int
       Tcl_IsShared(objPtr)

       Tcl_InvalidateStringRep(objPtr)

ARGUMENTS

       Tcl_Obj *objPtr (in)          Points to an object; must have been  the  result  of  a  previous  call  to
                                     Tcl_NewObj.
________________________________________________________________________________________________________________

INTRODUCTION

This man page presents an overview of Tcl objects and how they are used. It also describes generic
procedures for managing Tcl objects. These procedures are used to create and copy objects, and increment
and decrement the count of references (pointers) to objects. The procedures are used in conjunction with
ones that operate on specific types of objects such as Tcl_GetIntFromObj and Tcl_ListObjAppendElement.
The individual procedures are described along with the data structures they manipulate.

Tcl's dual-ported objects provide a general-purpose mechanism for storing and exchanging Tcl values.
They largely replace the use of strings in Tcl. For example, they are used to store variable values,
command arguments, command results, and scripts. Tcl objects behave like strings but also hold an
internal representation that can be manipulated more efficiently. For example, a Tcl list is now
represented as an object that holds the list's string representation as well as an array of pointers to
the objects for each list element. Dual-ported objects avoid most runtime type conversions. They also
improve the speed of many operations since an appropriate representation is immediately available. The
compiler itself uses Tcl objects to cache the instruction bytecodes resulting from compiling scripts.

The two representations are a cache of each other and are computed lazily. That is, each representation
is only computed when necessary, it is computed from the other representation, and, once computed, it is
saved. In addition, a change in one representation invalidates the other one. As an example, a Tcl
program doing integer calculations can operate directly on a variable's internal machine integer
representation without having to constantly convert between integers and strings. Only when it needs a
string representing the variable's value, say to print it, will the program regenerate the string
representation from the integer. Although objects contain an internal representation, their semantics
are defined in terms of strings: an up-to-date string can always be obtained, and any change to the
object will be reflected in that string when the object's string representation is fetched. Because of
this representation invalidation and regeneration, it is dangerous for extension writers to access
Tcl_Obj fields directly. It is better to access Tcl_Obj information using procedures like
Tcl_GetStringFromObj and Tcl_GetString.

Objects are allocated on the heap and are referenced using a pointer to their Tcl_Obj structure. Objects
are shared as much as possible. This significantly reduces storage requirements because some objects
such as long lists are very large. Also, most Tcl values are only read and never modified. This is
especially true for procedure arguments, which can be shared between the caller and the called procedure.
Assignment and argument binding is done by simply assigning a pointer to the value. Reference counting
is used to determine when it is safe to reclaim an object's storage.

Tcl objects are typed. An object's internal representation is controlled by its type. Several types are
predefined in the Tcl core including integer, double, list, and bytecode. Extension writers can extend
the set of types by defining their own Tcl_ObjType structs.

THE TCL_OBJ STRUCTURE

       Each Tcl object is represented by a Tcl_Obj structure which is defined as follows.
              typedef struct Tcl_Obj {
                      int refCount;
                      char *bytes;
                      int length;
                      Tcl_ObjType *typePtr;
                      union {
                              long longValue;
                              double doubleValue;
                              void *otherValuePtr;
                              Tcl_WideInt wideValue;
                              struct {
                                      void *ptr1;
                                      void *ptr2;
                              } twoPtrValue;
                              struct {
                                      void *ptr;
                                      unsigned long value;
                              } ptrAndLongRep;
                      } internalRep;
              } Tcl_Obj;
       The bytes and the length members together hold an  object's  UTF-8  string  representation,  which  is  a
       counted string not containing null bytes (UTF-8 null characters should be encoded as a two byte sequence:
       192, 128.)  bytes points to the first byte of the string representation.  The  length  member  gives  the
       number of bytes.  The byte array must always have a null byte after the last data byte, at offset length;
       this allows string representations to be treated as conventional null-terminated C strings.   C  programs
       use  Tcl_GetStringFromObj  and Tcl_GetString to get an object's string representation.  If bytes is NULL,
       the string representation is invalid.

       An object's type manages its internal representation.  The  member  typePtr  points  to  the  Tcl_ObjType
       structure that describes the type.  If typePtr is NULL, the internal representation is invalid.

       The internalRep union member holds an object's internal representation.  This is either a (long) integer,
       a double-precision floating-point number, a pointer to a value containing additional  information  needed
       by  the  object's  type to represent the object, a Tcl_WideInt integer, two arbitrary pointers, or a pair
       made up of an unsigned long integer and a pointer.

       The refCount member is used to tell when it is safe to free an object's storage.  It holds the  count  of
       active  references  to  the  object.   Maintaining the correct reference count is a key responsibility of
       extension writers.  Reference counting is discussed below in the section STORAGE MANAGEMENT OF OBJECTS.

       Although extension writers can directly access the members of a Tcl_Obj structure, it is much  better  to
       use  the  appropriate  procedures and macros.  For example, extension writers should never read or update
       refCount directly; they should use macros such as Tcl_IncrRefCount and Tcl_IsShared instead.

       A key property of Tcl objects is that they hold two representations.   An  object  typically  starts  out
       containing  only  a string representation: it is untyped and has a NULL typePtr.  An object containing an
       empty string  or  a  copy  of  a  specified  string  is  created  using  Tcl_NewObj  or  Tcl_NewStringObj
       respectively.   An object's string value is gotten with Tcl_GetStringFromObj or Tcl_GetString and changed
       with Tcl_SetStringObj.  If the object is later passed to a procedure like Tcl_GetIntFromObj that requires
       a  specific  internal  representation,  the  procedure will create one and set the object's typePtr.  The
       internal representation is computed from the string representation.  An object's two representations  are
       duals  of  each  other:  changes made to one are reflected in the other.  For example, Tcl_ListObjReplace
       will  modify  an  object's  internal  representation  and  the  next  call  to  Tcl_GetStringFromObj   or
       Tcl_GetString will reflect that change.

       Representations are recomputed lazily for efficiency.  A change to one representation made by a procedure
       such as Tcl_ListObjReplace is not reflected immediately in the other representation.  Instead, the  other
       representation  is  marked  invalid  so  that  it  is  only  regenerated  if  it is needed later.  Most C
       programmers never have to be concerned  with  how  this  is  done  and  simply  use  procedures  such  as
       Tcl_GetBooleanFromObj  or Tcl_ListObjIndex.  Programmers that implement their own object types must check
       for  invalid  representations  and  mark  representations  invalid   when   necessary.    The   procedure
       Tcl_InvalidateStringRep is used to mark an object's string representation invalid and to free any storage
       associated with the old string representation.

       Objects usually remain one type over their life, but occasionally an object must be  converted  from  one
       type  to  another.   For example, a C program might build up a string in an object with repeated calls to
       Tcl_AppendToObj, and then call Tcl_ListObjIndex to extract a list element  from  the  object.   The  same
       object  holding  the  same  string value can have several different internal representations at different
       times.  Extension writers can also force an object to be converted from one type  to  another  using  the
       Tcl_ConvertToType  procedure.   Only  programmers that create new object types need to be concerned about
       how this is done.  A procedure defined as part of the object type's implementation creates a new internal
       representation  for  an  object and changes its typePtr.  See the man page for Tcl_RegisterObjType to see
       how to create a new object type.

EXAMPLE OF THE LIFETIME OF AN OBJECT

       As an example of the lifetime of an object, consider the following sequence of commands:
              set x 123
       This assigns to x an untyped object whose bytes member points to 123 and length member contains  3.   The
       object's typePtr member is NULL.
              puts "x is $x"
       x's string representation is valid (since bytes is non-NULL) and is fetched for the command.
              incr x
       The  incr  command  first  gets  an integer from x's object by calling Tcl_GetIntFromObj.  This procedure
       checks whether the object is already an integer object.  Since it is  not,  it  converts  the  object  by
       setting  the object's internalRep.longValue member to the integer 123 and setting the object's typePtr to
       point to the integer Tcl_ObjType structure.  Both representations are now  valid.   incr  increments  the
       object's  integer  internal  representation  then  invalidates  its  string  representation  (by  calling
       Tcl_InvalidateStringRep)  since  the  string  representation  no  longer  corresponds  to  the   internal
       representation.
              puts "x is now $x"
       The  string  representation  of x's object is needed and is recomputed.  The string representation is now
       124 and both representations are again valid.

STORAGE MANAGEMENT OF OBJECTS

Tcl objects are allocated on the heap and are shared as much as possible to reduce storage requirements.
Reference counting is used to determine when an object is no longer needed and can safely be freed. An
object just created by Tcl_NewObj or Tcl_NewStringObj has refCount 0. The macro Tcl_IncrRefCount
increments the reference count when a new reference to the object is created. The macro Tcl_DecrRefCount
decrements the count when a reference is no longer needed and, if the object's reference count drops to
zero, frees its storage. An object shared by different code or data structures has refCount greater than
1. Incrementing an object's reference count ensures that it will not be freed too early or have its
value change accidentally.

As an example, the bytecode interpreter shares argument objects between calling and called Tcl procedures
to avoid having to copy objects. It assigns the call's argument objects to the procedure's formal
parameter variables. In doing so, it calls Tcl_IncrRefCount to increment the reference count of each
argument since there is now a new reference to it from the formal parameter. When the called procedure
returns, the interpreter calls Tcl_DecrRefCount to decrement each argument's reference count. When an
object's reference count drops less than or equal to zero, Tcl_DecrRefCount reclaims its storage. Most
command procedures do not have to be concerned about reference counting since they use an object's value
immediately and do not retain a pointer to the object after they return. However, if they do retain a
pointer to an object in a data structure, they must be careful to increment its reference count since the
retained pointer is a new reference.

Command procedures that directly modify objects such as those for lappend and linsert must be careful to
copy a shared object before changing it. They must first check whether the object is shared by calling
Tcl_IsShared. If the object is shared they must copy the object by using Tcl_DuplicateObj; this returns
a new duplicate of the original object that has refCount 0. If the object is not shared, the command
procedure “owns” the object and can safely modify it directly. For example, the following code appears
in the command procedure that implements linsert. This procedure modifies the list object passed to it
in objv[1] by inserting objc-3 new elements before index.

listPtr = objv[1];
if (Tcl_IsShared(listPtr)) {
listPtr = Tcl_DuplicateObj(listPtr);
}
result = Tcl_ListObjReplace(interp, listPtr, index, 0,
(objc-3), &(objv[3]));

As another example, incr's command procedure must check whether the variable's object is shared before
incrementing the integer in its internal representation. If it is shared, it needs to duplicate the
object in order to avoid accidentally changing values in other data structures.

KEYWORDS

       internal representation, object, object creation, object type, reference counting, string representation,
       type conversion