[thirdparty/gcc.git] / gcc / doc / gcc / extensions-to-the-c++-language / backwards-compatibility.rst

..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. index:: Backwards Compatibility, ARM [Annotated C++ Reference Manual]

.. _backwards-compatibility:

Backwards Compatibility
***********************

Now that there is a definitive ISO standard C++, G++ has a specification
to adhere to.  The C++ language evolved over time, and features that
used to be acceptable in previous drafts of the standard, such as the ARM
[Annotated C++ Reference Manual], are no longer accepted.  In order to allow
compilation of C++ written to such drafts, G++ contains some backwards
compatibilities.  *All such backwards compatibility features are
liable to disappear in future versions of G++.* They should be considered
deprecated.   See :ref:`deprecated-features`.

``Implicit C language``
  Old C system header files did not contain an ``extern "C" {...}``
  scope to set the language.  On such systems, all system header files are
  implicitly scoped inside a C language scope.  Such headers must
  correctly prototype function argument types, there is no leeway for
  ``()`` to indicate an unspecified set of arguments.
Commit	Line	Data
c63539ff ML	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:: Backwards Compatibility, ARM [Annotated C++ Reference Manual]
	7
	8	.. _backwards-compatibility:
	9
	10	Backwards Compatibility
	11	***********************
	12
	13	Now that there is a definitive ISO standard C++, G++ has a specification
	14	to adhere to. The C++ language evolved over time, and features that
	15	used to be acceptable in previous drafts of the standard, such as the ARM
	16	[Annotated C++ Reference Manual], are no longer accepted. In order to allow
	17	compilation of C++ written to such drafts, G++ contains some backwards
	18	compatibilities. *All such backwards compatibility features are
	19	liable to disappear in future versions of G++.* They should be considered
	20	deprecated. See :ref:`deprecated-features`.
	21
	22	``Implicit C language``
	23	Old C system header files did not contain an ``extern "C" {...}``
	24	scope to set the language. On such systems, all system header files are
	25	implicitly scoped inside a C language scope. Such headers must
	26	correctly prototype function argument types, there is no leeway for
3ed1b4ce	27	``()`` to indicate an unspecified set of arguments.