gcc/doc/gcc/extensions-to-the-c-language-family/support-for-offsetof.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: __builtin_offsetof
   7
   8 .. _offsetof:
   9
  10 Support for offsetof
  11 ********************
  12
  13 GCC implements for both C and C++ a syntactic extension to implement
  14 the ``offsetof`` macro.
  15
  16 .. code-block:: c++
  17
  18   primary:
  19           "__builtin_offsetof" "(" typename "," offsetof_member_designator ")"
  20
  21   offsetof_member_designator:
  22             identifier
  23           | offsetof_member_designator "." identifier
  24           | offsetof_member_designator "[" expr "]"
  25
  26 This extension is sufficient such that
  27
  28 .. code-block:: c++
  29
  30   #define offsetof(type, member)  __builtin_offsetof (type, member)
  31
  32 is a suitable definition of the ``offsetof`` macro.  In C++, :samp:`{type}`
  33 may be dependent.  In either case, :samp:`{member}` may consist of a single
  34 identifier, or a sequence of member accesses and array references.