Minimize <iosfwd>

["Zack Weinberg" <zackw at panix dot com>]

The attached patch cuts <iosfwd> down to the absolute minumum - it
declares nothing but what the standard says it declares plus a couple
things that cannot be avoided.  (A few other headers may also be
smaller.)  On a real application, preprocessed text shrinks by 50 to
300 lines per source file (this app has a lot of small source files,
so the total reduction is 47,000 lines!)  I also see the .s files
shrink, but I'm not sure I have a sound baseline for comparison there.

This patch may not be good as is; I'm making conditional use of a
feature of glibc whereby one may instruct <wchar.h> to declare
mbstate_t (actually __mbstate_t) and nothing else, and I'm not sure
this interacts correctly with the various hacks for getting the C
headers into namespace std.  I'd like to preserve this feature as I
suspect it's responsible for a lot of the real-world gain from the
change, but it may need modification.

I believe this change to have no ABI consequences, but there is a lot
of noise in my abi_check output (for some reason libstdc++ refuses to
use --enable-c-locale=gnu on my test system) so I am not certain.

It should be mentioned that I think it is possible, even after Paolo
Carlini's substantial improvements last month, to do similar things to
almost all the standard C++ headers.  I may do more as I have time and
motivation.

zw

 * include/bits/fposfwd.h: New file.
 * include/Makefile.am (bits_headers): Add fposfwd.h.
 * include/Makefile.in: Regenerate (by hand) (automake must die)
 * include/bits/localefwd.h: Use _GLIBCXX_MBSTATE_T.
 * include/bits/postypes.h: Include bits/fposfwd.h instead of
<cwchar>.  Do not declare
   fpos, streampos, or wstreampos here.
 * include/std/iosfwd: Do not include bits/c++locale.h,
bits/stringfwd.h, bits/postypes.h,
   or bits/functexcept.h.  Do include bits/fposfwd.h.  Declare
allocator, char_traits, and
    the standard char_traits specializations by hand.

==================================================================
--- include/Makefile.am	(revision 123704)
+++ include/Makefile.am	(local)
@@ -80,6 +80,7 @@ bits_headers = \
 	${bits_srcdir}/concept_check.h \
 	${bits_srcdir}/cpp_type_traits.h \
 	${bits_srcdir}/deque.tcc \
+	${bits_srcdir}/fposfwd.h \
 	${bits_srcdir}/fstream.tcc \
 	${bits_srcdir}/functexcept.h \
 	${bits_srcdir}/gslice.h \
==================================================================
--- include/Makefile.in	(revision 123704)
+++ include/Makefile.in	(local)
@@ -314,6 +314,7 @@ bits_headers = \
 	${bits_srcdir}/concept_check.h \
 	${bits_srcdir}/cpp_type_traits.h \
 	${bits_srcdir}/deque.tcc \
+	${bits_srcdir}/fposfwd.h \
 	${bits_srcdir}/fstream.tcc \
 	${bits_srcdir}/functexcept.h \
 	${bits_srcdir}/gslice.h \
==================================================================
--- include/bits/localefwd.h	(revision 123704)
+++ include/bits/localefwd.h	(local)
@@ -121,9 +121,9 @@ _GLIBCXX_BEGIN_NAMESPACE(std)
   class codecvt_base;
   template<typename _InternT, typename _ExternT, typename _StateT>
     class codecvt;
-  template<> class codecvt<char, char, mbstate_t>;
+  template<> class codecvt<char, char, _GLIBCXX_MBSTATE_T>;
 #ifdef _GLIBCXX_USE_WCHAR_T
-  template<> class codecvt<wchar_t, char, mbstate_t>;
+  template<> class codecvt<wchar_t, char, _GLIBCXX_MBSTATE_T>;
 #endif
   template<typename _InternT, typename _ExternT, typename _StateT>
     class codecvt_byname;
==================================================================
--- include/bits/postypes.h	(revision 123704)
+++ include/bits/postypes.h	(local)
@@ -43,7 +43,7 @@
 
 #pragma GCC system_header
 
-#include <cwchar> // For mbstate_t
+#include <bits/fposfwd.h>
 
 #ifdef _GLIBCXX_HAVE_STDINT_H
 #include <stdint.h> // For int64_t
@@ -77,9 +77,6 @@ _GLIBCXX_BEGIN_NAMESPACE(std)
   /// Integral type for I/O operation counts and buffer sizes.
   typedef ptrdiff_t	streamsize; // Signed integral type
 
-  template<typename _StateT>
-    class fpos;
-
   /**
    *  @brief  Class representing stream positions.
    *
@@ -204,14 +201,6 @@ _GLIBCXX_BEGIN_NAMESPACE(std)
     operator!=(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs)
     { return streamoff(__lhs) != streamoff(__rhs); }
 
-  // Clauses 21.1.3.1 and 21.1.3.2 describe streampos and wstreampos
-  // as implementation defined types, but clause 27.2 requires that
-  // they must both be typedefs for fpos<mbstate_t>
-  /// File position for char streams.
-  typedef fpos<mbstate_t> streampos;
-  /// File position for wchar_t streams.
-  typedef fpos<mbstate_t> wstreampos;
-
 _GLIBCXX_END_NAMESPACE
 
 #endif
==================================================================
--- include/bits/fposfwd.h	(revision 123704)
+++ include/bits/fposfwd.h	(local)
@@ -0,0 +1,86 @@
+// Position types -*- C++ -*-
+
+// Copyright (C) 2007
+// Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 2, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING.  If not, write to the Free
+// Software Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
+// USA.
+
+// As a special exception, you may use this file as part of a free software
+// library without restriction.  Specifically, if other files instantiate
+// templates or use macros or inline functions from this file, or you compile
+// this file and link it with other files to produce an executable, this
+// file does not by itself cause the resulting executable to be covered by
+// the GNU General Public License.  This exception does not however
+// invalidate any other reasons why the executable file might be covered by
+// the GNU General Public License.
+
+/** @file fposfwd.h
+ *  This is an internal header file, included by other library headers.
+ *  You should not attempt to use it directly.
+ */
+
+//
+// ISO C++ 14882: 27.4.3 - Template class fpos
+//
+
+#ifndef _GLIBCXX_FPOSFWD_H
+#define _GLIBCXX_FPOSFWD_H 1
+
+#pragma GCC system_header
+
+#include <bits/c++config.h>
+
+// We may be able to use a special feature of glibc's <wchar.h> to
+// get a definition of mbstate_t and nothing else.  If used, it exposes
+// __mbstate_t rather than mbstate_t (the latter is a typedef to the
+// former when the full header is included, so we have no linkage issues).
+// This header provides the macro _GLIBCXX_MBSTATE_T which expands to
+// mbstate_t or __mbstate_t as appropriate.
+//
+// N.B. The special feature does not work if used via <cwchar>, nor does
+// it work if we attempt to force the <wchar.h> definitions into a
+// namespace.
+
+#if __GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ > 1)
+
+  #define __need_mbstate_t
+  #include <wchar.h>
+  #define _GLIBCXX_MBSTATE_T ::__mbstate_t
+
+#else
+
+  #include <cwchar>
+  #define _GLIBCXX_MBSTATE_T mbstate_t
+
+#endif
+
+_GLIBCXX_BEGIN_NAMESPACE(std)
+
+  template<typename _StateT>
+    class fpos;
+
+  // Clauses 21.1.3.1 and 21.1.3.2 describe streampos and wstreampos
+  // as implementation defined types, but clause 27.2 requires that
+  // they must both be typedefs for fpos<mbstate_t>
+  /// File position for char streams.
+  typedef fpos<_GLIBCXX_MBSTATE_T> streampos;
+  /// File position for wchar_t streams.
+  typedef fpos<_GLIBCXX_MBSTATE_T> wstreampos;
+
+_GLIBCXX_END_NAMESPACE
+
+#endif

Property changes on: include/bits/fposfwd.h
___________________________________________________________________
Name: svn:mime-type
 +text/cpp

==================================================================
--- include/std/iosfwd	(revision 123704)
+++ include/std/iosfwd	(local)
@@ -43,13 +43,23 @@
 #pragma GCC system_header
 
 #include <bits/c++config.h>
-#include <bits/c++locale.h> 
-#include <bits/stringfwd.h> 	// For string forward declarations.
-#include <bits/postypes.h>
-#include <bits/functexcept.h>
+#include <bits/fposfwd.h>
 
 _GLIBCXX_BEGIN_NAMESPACE(std)
 
+  // allocator, char_traits, and the two specializations of char_traits
+  // are also declared in <bits/stringfwd.h>.
+  template<typename _Alloc>
+    class allocator;
+
+  template<class _CharT>
+    struct char_traits;
+
+  template<> struct char_traits<char>;
+#ifdef _GLIBCXX_USE_WCHAR_T
+  template<> struct char_traits<wchar_t>;
+#endif
+
   template<typename _CharT, typename _Traits = char_traits<_CharT> >
     class basic_ios;