libstdc++-v3/doc/xml/manual/backwards_compatibility.xml - gcc - Git at Google

 <section xmlns="http://docbook.org/ns/docbook" version="5.0"
 	 xml:id="manual.appendix.porting.backwards" xreflabel="backwards">
 <?dbhtml filename="backwards.html"?>

 <info><title>Backwards Compatibility</title>
   <keywordset>
     <keyword>ISO C++</keyword>
     <keyword>backwards</keyword>
   </keywordset>
 </info>


 <section xml:id="backwards.first"><info><title>First</title></info>


 <para>The first generation GNU C++ library was called libg++.  It was a
 separate GNU project, although reliably paired with GCC. Rumors imply
 that it had a working relationship with at least two kinds of
 dinosaur.
 </para>

 <para>Some background: libg++ was designed and created when there was no
 ISO standard to provide guidance.  Classes like linked lists are now
 provided for by <classname>std::list&lt;T&gt;</classname> and do not need to be
 created by <function>genclass</function>.  (For that matter, templates exist
 now and are well-supported, whereas genclass (mostly) predates them.)
 </para>

 <para>There are other classes in libg++ that are not specified in the
 ISO Standard (e.g., statistical analysis).  While there are a lot of
 really useful things that are used by a lot of people, the Standards
 Committee couldn't include everything, and so a lot of those
 <quote>obvious</quote> classes didn't get included.
 </para>

 <para>That project is no longer maintained or supported, and the sources
 archived. For the desperate, the
 <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://ftp.gnu.org/old-gnu/libg++/">ftp.gnu.org</link>
 server still has the libg++ source.
 </para>
 </section>

 <section xml:id="backwards.second"><info><title>Second</title></info>


 <para>
   The second generation GNU C++ library was called libstdc++, or
   libstdc++-v2. It spans the time between libg++ and pre-ISO C++
   standardization and is usually associated with the following GCC
   releases: egcs 1.x, gcc 2.95, and gcc 2.96.
 </para>

 <para>
   The STL portions of that library are based on SGI/HP STL release 3.11.
 </para>

 <para>
   That project is no longer maintained or supported, and the sources
   archived.  The code was replaced and rewritten for libstdc++-v3.
 </para>

 </section>

 <section xml:id="backwards.third"><info><title>Third</title></info>


 <para> The third generation GNU C++ library is called libstdc++, or
 libstdc++-v3.
 </para>

       <para>The subset commonly known as the Standard Template Library
 	 (clauses 23 through 25 in C++98, mostly) is adapted from the final release
 	 of the SGI STL (version 3.3), with extensive changes.
       </para>

       <para>A more formal description of the V3 goals can be found in the
 	 official <link linkend="contrib.design_notes">design document</link>.
       </para>

 <para>Portability notes and known implementation limitations are as follows.</para>

 <section xml:id="backwards.third.headers"><info><title>Pre-ISO headers removed</title></info>


 <para> The pre-ISO C++ headers
       (<filename class="headerfile">&lt;iostream.h&gt;</filename>,
       <filename class="headerfile">&lt;defalloc.h&gt;</filename> etc.) are
       not supported.
 </para>

    <para>For those of you new to ISO C++ (welcome, time travelers!), the
       ancient pre-ISO headers have new names.
       The C++ FAQ has a good explanation in <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://isocpp.org/wiki/faq/coding-standards#std-headers">What's
       the difference between &lt;xxx&gt; and &lt;xxx.h&gt; headers?</link>.
    </para>

 <para>Porting between pre-ISO headers and ISO headers is simple: headers
 like <filename class="headerfile">&lt;vector.h&gt;</filename> can be replaced with <filename class="headerfile">&lt;vector&gt;</filename> and a using
 directive <code>using namespace std;</code> can be put at the global
 scope. This should be enough to get this code compiling, assuming the
 other usage is correct.
 </para>
 </section>

 <section xml:id="backwards.third.hash"><info><title>Extension headers hash_map, hash_set moved to ext or backwards</title></info>


       <para>At this time most of the features of the SGI STL extension have been
 	 replaced by standardized libraries.
 	 In particular, the <classname>unordered_map</classname> and
 	 <classname>unordered_set</classname> containers of TR1 and C++ 2011
 	 are suitable replacements for the non-standard
 	 <classname>hash_map</classname> and <classname>hash_set</classname>
 	 containers in the SGI STL.
       </para>
 <para> Header files <filename class="headerfile">&lt;hash_map&gt;</filename> and <filename class="headerfile">&lt;hash_set&gt;</filename> moved
 to <filename class="headerfile">&lt;ext/hash_map&gt;</filename> and  <filename class="headerfile">&lt;ext/hash_set&gt;</filename>,
 respectively. At the same time, all types in these files are enclosed
 in <code>namespace __gnu_cxx</code>. Later versions deprecate
 these files, and suggest using TR1's  <filename class="headerfile">&lt;unordered_map&gt;</filename>
 and  <filename class="headerfile">&lt;unordered_set&gt;</filename> instead.
 </para>

       <para>The extensions are no longer in the global or <code>std</code>
 	 namespaces, instead they are declared in the <code>__gnu_cxx</code>
 	 namespace. For maximum portability, consider defining a namespace
 	 alias to use to talk about extensions, e.g.:
       </para>
       <programlisting>
       #ifdef __GNUC__
       #if __GNUC__ &lt; 3
 	#include &lt;hash_map.h&gt;
 	namespace extension { using ::hash_map; }; // inherit globals
       #else
 	#include &lt;backward/hash_map&gt;
 	#if __GNUC__ == 3 &amp;&amp; __GNUC_MINOR__ == 0
 	  namespace extension = std;               // GCC 3.0
 	#else
 	  namespace extension = ::__gnu_cxx;       // GCC 3.1 and later
 	#endif
       #endif
       #else      // ...  there are other compilers, right?
 	namespace extension = std;
       #endif

       extension::hash_map&lt;int,int&gt; my_map;
       </programlisting>
       <para>This is a bit cleaner than defining typedefs for all the
 	 instantiations you might need.
       </para>


 <para>The following autoconf tests check for working HP/SGI hash containers.
 </para>

 <programlisting>
 # AC_HEADER_EXT_HASH_MAP
 AC_DEFUN([AC_HEADER_EXT_HASH_MAP], [
   AC_CACHE_CHECK(for ext/hash_map,
   ac_cv_cxx_ext_hash_map,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -Werror"
   AC_TRY_COMPILE([#include &lt;ext/hash_map&gt;], [using __gnu_cxx::hash_map;],
   ac_cv_cxx_ext_hash_map=yes, ac_cv_cxx_ext_hash_map=no)
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_ext_hash_map" = yes; then
     AC_DEFINE(HAVE_EXT_HASH_MAP,,[Define if ext/hash_map is present. ])
   fi
 ])
 </programlisting>

 <programlisting>
 # AC_HEADER_EXT_HASH_SET
 AC_DEFUN([AC_HEADER_EXT_HASH_SET], [
   AC_CACHE_CHECK(for ext/hash_set,
   ac_cv_cxx_ext_hash_set,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -Werror"
   AC_TRY_COMPILE([#include &lt;ext/hash_set&gt;], [using __gnu_cxx::hash_set;],
   ac_cv_cxx_ext_hash_set=yes, ac_cv_cxx_ext_hash_set=no)
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_ext_hash_set" = yes; then
     AC_DEFINE(HAVE_EXT_HASH_SET,,[Define if ext/hash_set is present. ])
   fi
 ])
 </programlisting>
 </section>

 <section xml:id="backwards.third.nocreate_noreplace"><info><title>No <code>ios::nocreate/ios::noreplace</code>.
 </title></info>


 <para>Historically these flags were used with iostreams to control whether
 new files are created or not when opening a file stream, similar to the
 <code>O_CREAT</code> and <code>O_EXCL</code> flags for the
 <function>open(2)</function> system call. Because iostream modes correspond
 to <function>fopen(3)</function> modes these flags are not supported.
 For input streams a new file will not be created anyway, so
 <code>ios::nocreate</code> is not needed.
 For output streams, a new file will be created if it does not exist, which is
 consistent with the behaviour of <function>fopen</function>.
 </para>

 <para>When one of these flags is needed a possible alternative is to attempt
 to open the file using <type>std::ifstream</type> first to determine whether
 the file already exists or not. This may not be reliable however, because
 whether the file exists or not could change between opening the
 <type>std::istream</type> and re-opening with an output stream. If you need
 to check for existence and open a file as a single operation then you will
 need to use OS-specific facilities outside the C++ standard library, such
 as <function>open(2)</function>.
 </para>
 </section>

 <section xml:id="backwards.third.streamattach"><info><title>
 No <code>stream::attach(int fd)</code>
 </title></info>


 <para>
       Phil Edwards writes: It was considered and rejected for the ISO
       standard.  Not all environments use file descriptors.  Of those
       that do, not all of them use integers to represent them.
     </para>

 <para>
       For a portable solution (among systems which use
       file descriptors), you need to implement a subclass of
       <code>std::streambuf</code> (or
       <code>std::basic_streambuf&lt;..&gt;</code>) which opens a file
       given a descriptor, and then pass an instance of this to the
       stream-constructor.
     </para>

 <para>
       An extension is available that implements this.
       <filename class="headerfile">&lt;ext/stdio_filebuf.h&gt;</filename>
       contains a derived class called
       <classname>__gnu_cxx::stdio_filebuf</classname>.
       This class can be constructed from a C <code>FILE*</code> or a file
       descriptor, and provides the <code>fd()</code> function.
     </para>

 <para>
  For another example of this, refer to
       <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.josuttis.com/cppcode/fdstream.html">fdstream example</link>
       by Nicolai Josuttis.
 </para>
 </section>

 <section xml:id="backwards.third.support_cxx98"><info><title>
 Support for C++98 dialect.
 </title></info>


 <para>Check for complete library coverage of the C++1998/2003 standard.
 </para>

 <programlisting>
 # AC_HEADER_STDCXX_98
 AC_DEFUN([AC_HEADER_STDCXX_98], [
   AC_CACHE_CHECK(for ISO C++ 98 include files,
   ac_cv_cxx_stdcxx_98,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   AC_TRY_COMPILE([
     #include &lt;cassert&gt;
     #include &lt;cctype&gt;
     #include &lt;cerrno&gt;
     #include &lt;cfloat&gt;
     #include &lt;ciso646&gt;
     #include &lt;climits&gt;
     #include &lt;clocale&gt;
     #include &lt;cmath&gt;
     #include &lt;csetjmp&gt;
     #include &lt;csignal&gt;
     #include &lt;cstdarg&gt;
     #include &lt;cstddef&gt;
     #include &lt;cstdio&gt;
     #include &lt;cstdlib&gt;
     #include &lt;cstring&gt;
     #include &lt;ctime&gt;

     #include &lt;algorithm&gt;
     #include &lt;bitset&gt;
     #include &lt;complex&gt;
     #include &lt;deque&gt;
     #include &lt;exception&gt;
     #include &lt;fstream&gt;
     #include &lt;functional&gt;
     #include &lt;iomanip&gt;
     #include &lt;ios&gt;
     #include &lt;iosfwd&gt;
     #include &lt;iostream&gt;
     #include &lt;istream&gt;
     #include &lt;iterator&gt;
     #include &lt;limits&gt;
     #include &lt;list&gt;
     #include &lt;locale&gt;
     #include &lt;map&gt;
     #include &lt;memory&gt;
     #include &lt;new&gt;
     #include &lt;numeric&gt;
     #include &lt;ostream&gt;
     #include &lt;queue&gt;
     #include &lt;set&gt;
     #include &lt;sstream&gt;
     #include &lt;stack&gt;
     #include &lt;stdexcept&gt;
     #include &lt;streambuf&gt;
     #include &lt;string&gt;
     #include &lt;typeinfo&gt;
     #include &lt;utility&gt;
     #include &lt;valarray&gt;
     #include &lt;vector&gt;
   ],,
   ac_cv_cxx_stdcxx_98=yes, ac_cv_cxx_stdcxx_98=no)
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_stdcxx_98" = yes; then
     AC_DEFINE(STDCXX_98_HEADERS,,[Define if ISO C++ 1998 header files are present. ])
   fi
 ])
 </programlisting>
 </section>

 <section xml:id="backwards.third.support_tr1"><info><title>
 Support for C++TR1 dialect.
 </title></info>


 <para>Check for library coverage of the TR1 standard.
 </para>

 <programlisting>
 # AC_HEADER_STDCXX_TR1
 AC_DEFUN([AC_HEADER_STDCXX_TR1], [
   AC_CACHE_CHECK(for ISO C++ TR1 include files,
   ac_cv_cxx_stdcxx_tr1,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   AC_TRY_COMPILE([
   #include &lt;tr1/array&gt;
   #include &lt;tr1/ccomplex&gt;
   #include &lt;tr1/cctype&gt;
   #include &lt;tr1/cfenv&gt;
   #include &lt;tr1/cfloat&gt;
   #include &lt;tr1/cinttypes&gt;
   #include &lt;tr1/climits&gt;
   #include &lt;tr1/cmath&gt;
   #include &lt;tr1/complex&gt;
   #include &lt;tr1/cstdarg&gt;
   #include &lt;tr1/cstdbool&gt;
   #include &lt;tr1/cstdint&gt;
   #include &lt;tr1/cstdio&gt;
   #include &lt;tr1/cstdlib&gt;
   #include &lt;tr1/ctgmath&gt;
   #include &lt;tr1/ctime&gt;
   #include &lt;tr1/cwchar&gt;
   #include &lt;tr1/cwctype&gt;
   #include &lt;tr1/functional&gt;
   #include &lt;tr1/memory&gt;
   #include &lt;tr1/random&gt;
   #include &lt;tr1/regex&gt;
   #include &lt;tr1/tuple&gt;
   #include &lt;tr1/type_traits&gt;
   #include &lt;tr1/unordered_set&gt;
   #include &lt;tr1/unordered_map&gt;
   #include &lt;tr1/utility&gt;
   ],,
   ac_cv_cxx_stdcxx_tr1=yes, ac_cv_cxx_stdcxx_tr1=no)
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_stdcxx_tr1" = yes; then
     AC_DEFINE(STDCXX_TR1_HEADERS,,[Define if ISO C++ TR1 header files are present. ])
   fi
 ])
 </programlisting>

 <para>An alternative is to check just for specific TR1 includes, such as &lt;unordered_map&gt; and &lt;unordered_set&gt;.
 </para>

 <programlisting>
 # AC_HEADER_TR1_UNORDERED_MAP
 AC_DEFUN([AC_HEADER_TR1_UNORDERED_MAP], [
   AC_CACHE_CHECK(for tr1/unordered_map,
   ac_cv_cxx_tr1_unordered_map,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   AC_TRY_COMPILE([#include &lt;tr1/unordered_map&gt;], [using std::tr1::unordered_map;],
   ac_cv_cxx_tr1_unordered_map=yes, ac_cv_cxx_tr1_unordered_map=no)
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_tr1_unordered_map" = yes; then
     AC_DEFINE(HAVE_TR1_UNORDERED_MAP,,[Define if tr1/unordered_map is present. ])
   fi
 ])
 </programlisting>

 <programlisting>
 # AC_HEADER_TR1_UNORDERED_SET
 AC_DEFUN([AC_HEADER_TR1_UNORDERED_SET], [
   AC_CACHE_CHECK(for tr1/unordered_set,
   ac_cv_cxx_tr1_unordered_set,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   AC_TRY_COMPILE([#include &lt;tr1/unordered_set&gt;], [using std::tr1::unordered_set;],
   ac_cv_cxx_tr1_unordered_set=yes, ac_cv_cxx_tr1_unordered_set=no)
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_tr1_unordered_set" = yes; then
     AC_DEFINE(HAVE_TR1_UNORDERED_SET,,[Define if tr1/unordered_set is present. ])
   fi
 ])
 </programlisting>
 </section>


 <section xml:id="backwards.third.support_cxx11"><info><title>
 Support for C++11 dialect.
 </title></info>


 <para>Check for baseline language coverage in the compiler for the C++11 standard.
 </para>

 <programlisting>
 # AC_COMPILE_STDCXX_11
 AC_DEFUN([AC_COMPILE_STDCXX_11], [
   AC_CACHE_CHECK(if g++ supports C++11 features without additional flags,
   ac_cv_cxx_compile_cxx11_native,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   AC_TRY_COMPILE([
   template &lt;typename T&gt;
     struct check final
     {
       static constexpr T value{ __cplusplus };
     };

     typedef check&lt;check&lt;bool&gt;&gt; right_angle_brackets;

     int a;
     decltype(a) b;

     typedef check&lt;int&gt; check_type;
     check_type c{};
     check_type&amp;&amp; cr = static_cast&lt;check_type&amp;&amp;&gt;(c);

     static_assert(check_type::value == 201103L, "C++11 compiler");],,
   ac_cv_cxx_compile_cxx11_native=yes, ac_cv_cxx_compile_cxx11_native=no)
   AC_LANG_RESTORE
   ])

   AC_CACHE_CHECK(if g++ supports C++11 features with -std=c++11,
   ac_cv_cxx_compile_cxx11_cxx,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -std=c++11"
   AC_TRY_COMPILE([
   template &lt;typename T&gt;
     struct check final
     {
       static constexpr T value{ __cplusplus };
     };

     typedef check&lt;check&lt;bool&gt;&gt; right_angle_brackets;

     int a;
     decltype(a) b;

     typedef check&lt;int&gt; check_type;
     check_type c{};
     check_type&amp;&amp; cr = static_cast&lt;check_type&amp;&amp;&gt;(c);

     static_assert(check_type::value == 201103L, "C++11 compiler");],,
   ac_cv_cxx_compile_cxx11_cxx=yes, ac_cv_cxx_compile_cxx11_cxx=no)
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
   ])

   AC_CACHE_CHECK(if g++ supports C++11 features with -std=gnu++11,
   ac_cv_cxx_compile_cxx11_gxx,
   [AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -std=gnu++11"
   AC_TRY_COMPILE([
   template &lt;typename T&gt;
     struct check final
     {
       static constexpr T value{ __cplusplus };
     };

     typedef check&lt;check&lt;bool&gt;&gt; right_angle_brackets;

     int a;
     decltype(a) b;

     typedef check&lt;int&gt; check_type;
     check_type c{};
     check_type&amp;&amp; cr = static_cast&lt;check_type&amp;&amp;&gt;(c);

     static_assert(check_type::value == 201103L, "C++11 compiler");],,
   ac_cv_cxx_compile_cxx11_gxx=yes, ac_cv_cxx_compile_cxx11_gxx=no)
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
   ])

   if test "$ac_cv_cxx_compile_cxx11_native" = yes ||
      test "$ac_cv_cxx_compile_cxx11_cxx" = yes ||
      test "$ac_cv_cxx_compile_cxx11_gxx" = yes; then
     AC_DEFINE(HAVE_STDCXX_11,,[Define if g++ supports C++11 features. ])
   fi
 ])
 </programlisting>


 <para>Check for library coverage of the C++2011 standard.
   (Some library headers are commented out in this check, they are
   not currently provided by libstdc++).
 </para>

 <programlisting>
 # AC_HEADER_STDCXX_11
 AC_DEFUN([AC_HEADER_STDCXX_11], [
   AC_CACHE_CHECK(for ISO C++11 include files,
   ac_cv_cxx_stdcxx_11,
   [AC_REQUIRE([AC_COMPILE_STDCXX_11])
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -std=gnu++11"

   AC_TRY_COMPILE([
     #include &lt;cassert&gt;
     #include &lt;ccomplex&gt;
     #include &lt;cctype&gt;
     #include &lt;cerrno&gt;
     #include &lt;cfenv&gt;
     #include &lt;cfloat&gt;
     #include &lt;cinttypes&gt;
     #include &lt;ciso646&gt;
     #include &lt;climits&gt;
     #include &lt;clocale&gt;
     #include &lt;cmath&gt;
     #include &lt;csetjmp&gt;
     #include &lt;csignal&gt;
     #include &lt;cstdalign&gt;
     #include &lt;cstdarg&gt;
     #include &lt;cstdbool&gt;
     #include &lt;cstddef&gt;
     #include &lt;cstdint&gt;
     #include &lt;cstdio&gt;
     #include &lt;cstdlib&gt;
     #include &lt;cstring&gt;
     #include &lt;ctgmath&gt;
     #include &lt;ctime&gt;
     // #include &lt;cuchar&gt;
     #include &lt;cwchar&gt;
     #include &lt;cwctype&gt;

     #include &lt;algorithm&gt;
     #include &lt;array&gt;
     #include &lt;atomic&gt;
     #include &lt;bitset&gt;
     #include &lt;chrono&gt;
     // #include &lt;codecvt&gt;
     #include &lt;complex&gt;
     #include &lt;condition_variable&gt;
     #include &lt;deque&gt;
     #include &lt;exception&gt;
     #include &lt;forward_list&gt;
     #include &lt;fstream&gt;
     #include &lt;functional&gt;
     #include &lt;future&gt;
     #include &lt;initializer_list&gt;
     #include &lt;iomanip&gt;
     #include &lt;ios&gt;
     #include &lt;iosfwd&gt;
     #include &lt;iostream&gt;
     #include &lt;istream&gt;
     #include &lt;iterator&gt;
     #include &lt;limits&gt;
     #include &lt;list&gt;
     #include &lt;locale&gt;
     #include &lt;map&gt;
     #include &lt;memory&gt;
     #include &lt;mutex&gt;
     #include &lt;new&gt;
     #include &lt;numeric&gt;
     #include &lt;ostream&gt;
     #include &lt;queue&gt;
     #include &lt;random&gt;
     #include &lt;ratio&gt;
     #include &lt;regex&gt;
     #include &lt;scoped_allocator&gt;
     #include &lt;set&gt;
     #include &lt;sstream&gt;
     #include &lt;stack&gt;
     #include &lt;stdexcept&gt;
     #include &lt;streambuf&gt;
     #include &lt;string&gt;
     #include &lt;system_error&gt;
     #include &lt;thread&gt;
     #include &lt;tuple&gt;
     #include &lt;typeindex&gt;
     #include &lt;typeinfo&gt;
     #include &lt;type_traits&gt;
     #include &lt;unordered_map&gt;
     #include &lt;unordered_set&gt;
     #include &lt;utility&gt;
     #include &lt;valarray&gt;
     #include &lt;vector&gt;
   ],,
   ac_cv_cxx_stdcxx_11=yes, ac_cv_cxx_stdcxx_11=no)
   AC_LANG_RESTORE
   CXXFLAGS="$ac_save_CXXFLAGS"
   ])
   if test "$ac_cv_cxx_stdcxx_11" = yes; then
     AC_DEFINE(STDCXX_11_HEADERS,,[Define if ISO C++11 header files are present. ])
   fi
 ])
 </programlisting>

 <para>As is the case for TR1 support, these autoconf macros can be made for a finer-grained, per-header-file check. For
 <filename class="headerfile">&lt;unordered_map&gt;</filename>
 </para>

 <programlisting>
 # AC_HEADER_UNORDERED_MAP
 AC_DEFUN([AC_HEADER_UNORDERED_MAP], [
   AC_CACHE_CHECK(for unordered_map,
   ac_cv_cxx_unordered_map,
   [AC_REQUIRE([AC_COMPILE_STDCXX_11])
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -std=gnu++11"
   AC_TRY_COMPILE([#include &lt;unordered_map&gt;], [using std::unordered_map;],
   ac_cv_cxx_unordered_map=yes, ac_cv_cxx_unordered_map=no)
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_unordered_map" = yes; then
     AC_DEFINE(HAVE_UNORDERED_MAP,,[Define if unordered_map is present. ])
   fi
 ])
 </programlisting>

 <programlisting>
 # AC_HEADER_UNORDERED_SET
 AC_DEFUN([AC_HEADER_UNORDERED_SET], [
   AC_CACHE_CHECK(for unordered_set,
   ac_cv_cxx_unordered_set,
   [AC_REQUIRE([AC_COMPILE_STDCXX_11])
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
   CXXFLAGS="$CXXFLAGS -std=gnu++11"
   AC_TRY_COMPILE([#include &lt;unordered_set&gt;], [using std::unordered_set;],
   ac_cv_cxx_unordered_set=yes, ac_cv_cxx_unordered_set=no)
   CXXFLAGS="$ac_save_CXXFLAGS"
   AC_LANG_RESTORE
   ])
   if test "$ac_cv_cxx_unordered_set" = yes; then
     AC_DEFINE(HAVE_UNORDERED_SET,,[Define if unordered_set is present. ])
   fi
 ])
 </programlisting>

 <para>
   Some C++11 features first appeared in GCC 4.3 and could be enabled by
   <option>-std=c++0x</option> and <option>-std=gnu++0x</option> for GCC
   releases which pre-date the 2011 standard. Those C++11 features and GCC's
   support for them were still changing until the 2011 standard was finished,
   but the autoconf checks above could be extended to test for incomplete
   C++11 support with <option>-std=c++0x</option> and
   <option>-std=gnu++0x</option>.
 </para>

 </section>

 <section xml:id="backwards.third.iterator_type"><info><title>
   <code>Container::iterator_type</code> is not necessarily <code>Container::value_type*</code>
 </title></info>


 <para>
   This is a change in behavior from older versions. Now, most
   <type>iterator_type</type> typedefs in container classes are POD
   objects, not <type>value_type</type> pointers.
 </para>
 </section>

 </section>

 </section>
	<section xmlns="http://docbook.org/ns/docbook" version="5.0"
	xml:id="manual.appendix.porting.backwards" xreflabel="backwards">
	<?dbhtml filename="backwards.html"?>

	<info><title>Backwards Compatibility</title>
	<keywordset>
	<keyword>ISO C++</keyword>
	<keyword>backwards</keyword>
	</keywordset>
	</info>



	<section xml:id="backwards.first"><info><title>First</title></info>


	<para>The first generation GNU C++ library was called libg++. It was a
	separate GNU project, although reliably paired with GCC. Rumors imply
	that it had a working relationship with at least two kinds of
	dinosaur.
	</para>

	<para>Some background: libg++ was designed and created when there was no
	ISO standard to provide guidance. Classes like linked lists are now
	provided for by <classname>std::list<T></classname> and do not need to be
	created by <function>genclass</function>. (For that matter, templates exist
	now and are well-supported, whereas genclass (mostly) predates them.)
	</para>

	<para>There are other classes in libg++ that are not specified in the
	ISO Standard (e.g., statistical analysis). While there are a lot of
	really useful things that are used by a lot of people, the Standards
	Committee couldn't include everything, and so a lot of those
	<quote>obvious</quote> classes didn't get included.
	</para>

	<para>That project is no longer maintained or supported, and the sources
	archived. For the desperate, the
	<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://ftp.gnu.org/old-gnu/libg++/">ftp.gnu.org</link>
	server still has the libg++ source.
	</para>
	</section>

	<section xml:id="backwards.second"><info><title>Second</title></info>


	<para>
	The second generation GNU C++ library was called libstdc++, or
	libstdc++-v2. It spans the time between libg++ and pre-ISO C++
	standardization and is usually associated with the following GCC
	releases: egcs 1.x, gcc 2.95, and gcc 2.96.
	</para>

	<para>
	The STL portions of that library are based on SGI/HP STL release 3.11.
	</para>

	<para>
	That project is no longer maintained or supported, and the sources
	archived. The code was replaced and rewritten for libstdc++-v3.
	</para>

	</section>

	<section xml:id="backwards.third"><info><title>Third</title></info>


	<para> The third generation GNU C++ library is called libstdc++, or
	libstdc++-v3.
	</para>

	<para>The subset commonly known as the Standard Template Library
	(clauses 23 through 25 in C++98, mostly) is adapted from the final release
	of the SGI STL (version 3.3), with extensive changes.
	</para>

	<para>A more formal description of the V3 goals can be found in the
	official <link linkend="contrib.design_notes">design document</link>.
	</para>

	<para>Portability notes and known implementation limitations are as follows.</para>

	<section xml:id="backwards.third.headers"><info><title>Pre-ISO headers removed</title></info>


	<para> The pre-ISO C++ headers
	(<filename class="headerfile"><iostream.h></filename>,
	<filename class="headerfile"><defalloc.h></filename> etc.) are
	not supported.
	</para>

	<para>For those of you new to ISO C++ (welcome, time travelers!), the
	ancient pre-ISO headers have new names.
	The C++ FAQ has a good explanation in <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://isocpp.org/wiki/faq/coding-standards#std-headers">What's
	the difference between <xxx> and <xxx.h> headers?</link>.
	</para>

	<para>Porting between pre-ISO headers and ISO headers is simple: headers
	like <filename class="headerfile"><vector.h></filename> can be replaced with <filename class="headerfile"><vector></filename> and a using
	directive <code>using namespace std;</code> can be put at the global
	scope. This should be enough to get this code compiling, assuming the
	other usage is correct.
	</para>
	</section>

	<section xml:id="backwards.third.hash"><info><title>Extension headers hash_map, hash_set moved to ext or backwards</title></info>


	<para>At this time most of the features of the SGI STL extension have been
	replaced by standardized libraries.
	In particular, the <classname>unordered_map</classname> and
	<classname>unordered_set</classname> containers of TR1 and C++ 2011
	are suitable replacements for the non-standard
	<classname>hash_map</classname> and <classname>hash_set</classname>
	containers in the SGI STL.
	</para>
	<para> Header files <filename class="headerfile"><hash_map></filename> and <filename class="headerfile"><hash_set></filename> moved
	to <filename class="headerfile"><ext/hash_map></filename> and <filename class="headerfile"><ext/hash_set></filename>,
	respectively. At the same time, all types in these files are enclosed
	in <code>namespace __gnu_cxx</code>. Later versions deprecate
	these files, and suggest using TR1's <filename class="headerfile"><unordered_map></filename>
	and <filename class="headerfile"><unordered_set></filename> instead.
	</para>

	<para>The extensions are no longer in the global or <code>std</code>
	namespaces, instead they are declared in the <code>__gnu_cxx</code>
	namespace. For maximum portability, consider defining a namespace
	alias to use to talk about extensions, e.g.:
	</para>
	<programlisting>
	#ifdef __GNUC__
	#if __GNUC__ < 3
	#include <hash_map.h>
	namespace extension { using ::hash_map; }; // inherit globals
	#else
	#include <backward/hash_map>
	#if __GNUC__ == 3 && __GNUC_MINOR__ == 0
	namespace extension = std; // GCC 3.0
	#else
	namespace extension = ::__gnu_cxx; // GCC 3.1 and later
	#endif
	#endif
	#else // ... there are other compilers, right?
	namespace extension = std;
	#endif

	extension::hash_map<int,int> my_map;
	</programlisting>
	<para>This is a bit cleaner than defining typedefs for all the
	instantiations you might need.
	</para>


	<para>The following autoconf tests check for working HP/SGI hash containers.
	</para>

	<programlisting>
	# AC_HEADER_EXT_HASH_MAP
	AC_DEFUN([AC_HEADER_EXT_HASH_MAP], [
	AC_CACHE_CHECK(for ext/hash_map,
	ac_cv_cxx_ext_hash_map,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	ac_save_CXXFLAGS="$CXXFLAGS"
	CXXFLAGS="$CXXFLAGS -Werror"
	AC_TRY_COMPILE([#include <ext/hash_map>], [using __gnu_cxx::hash_map;],
	ac_cv_cxx_ext_hash_map=yes, ac_cv_cxx_ext_hash_map=no)
	CXXFLAGS="$ac_save_CXXFLAGS"
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_ext_hash_map" = yes; then
	AC_DEFINE(HAVE_EXT_HASH_MAP,,[Define if ext/hash_map is present. ])
	fi
	])
	</programlisting>

	<programlisting>
	# AC_HEADER_EXT_HASH_SET
	AC_DEFUN([AC_HEADER_EXT_HASH_SET], [
	AC_CACHE_CHECK(for ext/hash_set,
	ac_cv_cxx_ext_hash_set,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	ac_save_CXXFLAGS="$CXXFLAGS"
	CXXFLAGS="$CXXFLAGS -Werror"
	AC_TRY_COMPILE([#include <ext/hash_set>], [using __gnu_cxx::hash_set;],
	ac_cv_cxx_ext_hash_set=yes, ac_cv_cxx_ext_hash_set=no)
	CXXFLAGS="$ac_save_CXXFLAGS"
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_ext_hash_set" = yes; then
	AC_DEFINE(HAVE_EXT_HASH_SET,,[Define if ext/hash_set is present. ])
	fi
	])
	</programlisting>
	</section>

	<section xml:id="backwards.third.nocreate_noreplace"><info><title>No <code>ios::nocreate/ios::noreplace</code>.
	</title></info>


	<para>Historically these flags were used with iostreams to control whether
	new files are created or not when opening a file stream, similar to the
	<code>O_CREAT</code> and <code>O_EXCL</code> flags for the
	<function>open(2)</function> system call. Because iostream modes correspond
	to <function>fopen(3)</function> modes these flags are not supported.
	For input streams a new file will not be created anyway, so
	<code>ios::nocreate</code> is not needed.
	For output streams, a new file will be created if it does not exist, which is
	consistent with the behaviour of <function>fopen</function>.
	</para>

	<para>When one of these flags is needed a possible alternative is to attempt
	to open the file using <type>std::ifstream</type> first to determine whether
	the file already exists or not. This may not be reliable however, because
	whether the file exists or not could change between opening the
	<type>std::istream</type> and re-opening with an output stream. If you need
	to check for existence and open a file as a single operation then you will
	need to use OS-specific facilities outside the C++ standard library, such
	as <function>open(2)</function>.
	</para>
	</section>

	<section xml:id="backwards.third.streamattach"><info><title>
	No <code>stream::attach(int fd)</code>
	</title></info>


	<para>
	Phil Edwards writes: It was considered and rejected for the ISO
	standard. Not all environments use file descriptors. Of those
	that do, not all of them use integers to represent them.
	</para>

	<para>
	For a portable solution (among systems which use
	file descriptors), you need to implement a subclass of
	<code>std::streambuf</code> (or
	<code>std::basic_streambuf<..></code>) which opens a file
	given a descriptor, and then pass an instance of this to the
	stream-constructor.
	</para>

	<para>
	An extension is available that implements this.
	<filename class="headerfile"><ext/stdio_filebuf.h></filename>
	contains a derived class called
	<classname>__gnu_cxx::stdio_filebuf</classname>.
	This class can be constructed from a C <code>FILE*</code> or a file
	descriptor, and provides the <code>fd()</code> function.
	</para>

	<para>
	For another example of this, refer to
	<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.josuttis.com/cppcode/fdstream.html">fdstream example</link>
	by Nicolai Josuttis.
	</para>
	</section>

	<section xml:id="backwards.third.support_cxx98"><info><title>
	Support for C++98 dialect.
	</title></info>


	<para>Check for complete library coverage of the C++1998/2003 standard.
	</para>

	<programlisting>
	# AC_HEADER_STDCXX_98
	AC_DEFUN([AC_HEADER_STDCXX_98], [
	AC_CACHE_CHECK(for ISO C++ 98 include files,
	ac_cv_cxx_stdcxx_98,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	AC_TRY_COMPILE([
	#include <cassert>
	#include <cctype>
	#include <cerrno>
	#include <cfloat>
	#include <ciso646>
	#include <climits>
	#include <clocale>
	#include <cmath>
	#include <csetjmp>
	#include <csignal>
	#include <cstdarg>
	#include <cstddef>
	#include <cstdio>
	#include <cstdlib>
	#include <cstring>
	#include <ctime>

	#include <algorithm>
	#include <bitset>
	#include <complex>
	#include <deque>
	#include <exception>
	#include <fstream>
	#include <functional>
	#include <iomanip>
	#include <ios>
	#include <iosfwd>
	#include <iostream>
	#include <istream>
	#include <iterator>
	#include <limits>
	#include <list>
	#include <locale>
	#include <map>
	#include <memory>
	#include <new>
	#include <numeric>
	#include <ostream>
	#include <queue>
	#include <set>
	#include <sstream>
	#include <stack>
	#include <stdexcept>
	#include <streambuf>
	#include <string>
	#include <typeinfo>
	#include <utility>
	#include <valarray>
	#include <vector>
	],,
	ac_cv_cxx_stdcxx_98=yes, ac_cv_cxx_stdcxx_98=no)
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_stdcxx_98" = yes; then
	AC_DEFINE(STDCXX_98_HEADERS,,[Define if ISO C++ 1998 header files are present. ])
	fi
	])
	</programlisting>
	</section>

	<section xml:id="backwards.third.support_tr1"><info><title>
	Support for C++TR1 dialect.
	</title></info>


	<para>Check for library coverage of the TR1 standard.
	</para>

	<programlisting>
	# AC_HEADER_STDCXX_TR1
	AC_DEFUN([AC_HEADER_STDCXX_TR1], [
	AC_CACHE_CHECK(for ISO C++ TR1 include files,
	ac_cv_cxx_stdcxx_tr1,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	AC_TRY_COMPILE([
	#include <tr1/array>
	#include <tr1/ccomplex>
	#include <tr1/cctype>
	#include <tr1/cfenv>
	#include <tr1/cfloat>
	#include <tr1/cinttypes>
	#include <tr1/climits>
	#include <tr1/cmath>
	#include <tr1/complex>
	#include <tr1/cstdarg>
	#include <tr1/cstdbool>
	#include <tr1/cstdint>
	#include <tr1/cstdio>
	#include <tr1/cstdlib>
	#include <tr1/ctgmath>
	#include <tr1/ctime>
	#include <tr1/cwchar>
	#include <tr1/cwctype>
	#include <tr1/functional>
	#include <tr1/memory>
	#include <tr1/random>
	#include <tr1/regex>
	#include <tr1/tuple>
	#include <tr1/type_traits>
	#include <tr1/unordered_set>
	#include <tr1/unordered_map>
	#include <tr1/utility>
	],,
	ac_cv_cxx_stdcxx_tr1=yes, ac_cv_cxx_stdcxx_tr1=no)
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_stdcxx_tr1" = yes; then
	AC_DEFINE(STDCXX_TR1_HEADERS,,[Define if ISO C++ TR1 header files are present. ])
	fi
	])
	</programlisting>

	<para>An alternative is to check just for specific TR1 includes, such as <unordered_map> and <unordered_set>.
	</para>

	<programlisting>
	# AC_HEADER_TR1_UNORDERED_MAP
	AC_DEFUN([AC_HEADER_TR1_UNORDERED_MAP], [
	AC_CACHE_CHECK(for tr1/unordered_map,
	ac_cv_cxx_tr1_unordered_map,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	AC_TRY_COMPILE([#include <tr1/unordered_map>], [using std::tr1::unordered_map;],
	ac_cv_cxx_tr1_unordered_map=yes, ac_cv_cxx_tr1_unordered_map=no)
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_tr1_unordered_map" = yes; then
	AC_DEFINE(HAVE_TR1_UNORDERED_MAP,,[Define if tr1/unordered_map is present. ])
	fi
	])
	</programlisting>

	<programlisting>
	# AC_HEADER_TR1_UNORDERED_SET
	AC_DEFUN([AC_HEADER_TR1_UNORDERED_SET], [
	AC_CACHE_CHECK(for tr1/unordered_set,
	ac_cv_cxx_tr1_unordered_set,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	AC_TRY_COMPILE([#include <tr1/unordered_set>], [using std::tr1::unordered_set;],
	ac_cv_cxx_tr1_unordered_set=yes, ac_cv_cxx_tr1_unordered_set=no)
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_tr1_unordered_set" = yes; then
	AC_DEFINE(HAVE_TR1_UNORDERED_SET,,[Define if tr1/unordered_set is present. ])
	fi
	])
	</programlisting>
	</section>


	<section xml:id="backwards.third.support_cxx11"><info><title>
	Support for C++11 dialect.
	</title></info>


	<para>Check for baseline language coverage in the compiler for the C++11 standard.
	</para>

	<programlisting>
	# AC_COMPILE_STDCXX_11
	AC_DEFUN([AC_COMPILE_STDCXX_11], [
	AC_CACHE_CHECK(if g++ supports C++11 features without additional flags,
	ac_cv_cxx_compile_cxx11_native,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	AC_TRY_COMPILE([
	template <typename T>
	struct check final
	{
	static constexpr T value{ __cplusplus };
	};

	typedef check<check<bool>> right_angle_brackets;

	int a;
	decltype(a) b;

	typedef check<int> check_type;
	check_type c{};
	check_type&& cr = static_cast<check_type&&>(c);

	static_assert(check_type::value == 201103L, "C++11 compiler");],,
	ac_cv_cxx_compile_cxx11_native=yes, ac_cv_cxx_compile_cxx11_native=no)
	AC_LANG_RESTORE
	])

	AC_CACHE_CHECK(if g++ supports C++11 features with -std=c++11,
	ac_cv_cxx_compile_cxx11_cxx,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	ac_save_CXXFLAGS="$CXXFLAGS"
	CXXFLAGS="$CXXFLAGS -std=c++11"
	AC_TRY_COMPILE([
	template <typename T>
	struct check final
	{
	static constexpr T value{ __cplusplus };
	};

	typedef check<check<bool>> right_angle_brackets;

	int a;
	decltype(a) b;

	typedef check<int> check_type;
	check_type c{};
	check_type&& cr = static_cast<check_type&&>(c);

	static_assert(check_type::value == 201103L, "C++11 compiler");],,
	ac_cv_cxx_compile_cxx11_cxx=yes, ac_cv_cxx_compile_cxx11_cxx=no)
	CXXFLAGS="$ac_save_CXXFLAGS"
	AC_LANG_RESTORE
	])

	AC_CACHE_CHECK(if g++ supports C++11 features with -std=gnu++11,
	ac_cv_cxx_compile_cxx11_gxx,
	[AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	ac_save_CXXFLAGS="$CXXFLAGS"
	CXXFLAGS="$CXXFLAGS -std=gnu++11"
	AC_TRY_COMPILE([
	template <typename T>
	struct check final
	{
	static constexpr T value{ __cplusplus };
	};

	typedef check<check<bool>> right_angle_brackets;

	int a;
	decltype(a) b;

	typedef check<int> check_type;
	check_type c{};
	check_type&& cr = static_cast<check_type&&>(c);

	static_assert(check_type::value == 201103L, "C++11 compiler");],,
	ac_cv_cxx_compile_cxx11_gxx=yes, ac_cv_cxx_compile_cxx11_gxx=no)
	CXXFLAGS="$ac_save_CXXFLAGS"
	AC_LANG_RESTORE
	])

	if test "$ac_cv_cxx_compile_cxx11_native" = yes \|\|
	test "$ac_cv_cxx_compile_cxx11_cxx" = yes \|\|
	test "$ac_cv_cxx_compile_cxx11_gxx" = yes; then
	AC_DEFINE(HAVE_STDCXX_11,,[Define if g++ supports C++11 features. ])
	fi
	])
	</programlisting>


	<para>Check for library coverage of the C++2011 standard.
	(Some library headers are commented out in this check, they are
	not currently provided by libstdc++).
	</para>

	<programlisting>
	# AC_HEADER_STDCXX_11
	AC_DEFUN([AC_HEADER_STDCXX_11], [
	AC_CACHE_CHECK(for ISO C++11 include files,
	ac_cv_cxx_stdcxx_11,
	[AC_REQUIRE([AC_COMPILE_STDCXX_11])
	AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	ac_save_CXXFLAGS="$CXXFLAGS"
	CXXFLAGS="$CXXFLAGS -std=gnu++11"

	AC_TRY_COMPILE([
	#include <cassert>
	#include <ccomplex>
	#include <cctype>
	#include <cerrno>
	#include <cfenv>
	#include <cfloat>
	#include <cinttypes>
	#include <ciso646>
	#include <climits>
	#include <clocale>
	#include <cmath>
	#include <csetjmp>
	#include <csignal>
	#include <cstdalign>
	#include <cstdarg>
	#include <cstdbool>
	#include <cstddef>
	#include <cstdint>
	#include <cstdio>
	#include <cstdlib>
	#include <cstring>
	#include <ctgmath>
	#include <ctime>
	// #include <cuchar>
	#include <cwchar>
	#include <cwctype>

	#include <algorithm>
	#include <array>
	#include <atomic>
	#include <bitset>
	#include <chrono>
	// #include <codecvt>
	#include <complex>
	#include <condition_variable>
	#include <deque>
	#include <exception>
	#include <forward_list>
	#include <fstream>
	#include <functional>
	#include <future>
	#include <initializer_list>
	#include <iomanip>
	#include <ios>
	#include <iosfwd>
	#include <iostream>
	#include <istream>
	#include <iterator>
	#include <limits>
	#include <list>
	#include <locale>
	#include <map>
	#include <memory>
	#include <mutex>
	#include <new>
	#include <numeric>
	#include <ostream>
	#include <queue>
	#include <random>
	#include <ratio>
	#include <regex>
	#include <scoped_allocator>
	#include <set>
	#include <sstream>
	#include <stack>
	#include <stdexcept>
	#include <streambuf>
	#include <string>
	#include <system_error>
	#include <thread>
	#include <tuple>
	#include <typeindex>
	#include <typeinfo>
	#include <type_traits>
	#include <unordered_map>
	#include <unordered_set>
	#include <utility>
	#include <valarray>
	#include <vector>
	],,
	ac_cv_cxx_stdcxx_11=yes, ac_cv_cxx_stdcxx_11=no)
	AC_LANG_RESTORE
	CXXFLAGS="$ac_save_CXXFLAGS"
	])
	if test "$ac_cv_cxx_stdcxx_11" = yes; then
	AC_DEFINE(STDCXX_11_HEADERS,,[Define if ISO C++11 header files are present. ])
	fi
	])
	</programlisting>

	<para>As is the case for TR1 support, these autoconf macros can be made for a finer-grained, per-header-file check. For
	<filename class="headerfile"><unordered_map></filename>
	</para>

	<programlisting>
	# AC_HEADER_UNORDERED_MAP
	AC_DEFUN([AC_HEADER_UNORDERED_MAP], [
	AC_CACHE_CHECK(for unordered_map,
	ac_cv_cxx_unordered_map,
	[AC_REQUIRE([AC_COMPILE_STDCXX_11])
	AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	ac_save_CXXFLAGS="$CXXFLAGS"
	CXXFLAGS="$CXXFLAGS -std=gnu++11"
	AC_TRY_COMPILE([#include <unordered_map>], [using std::unordered_map;],
	ac_cv_cxx_unordered_map=yes, ac_cv_cxx_unordered_map=no)
	CXXFLAGS="$ac_save_CXXFLAGS"
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_unordered_map" = yes; then
	AC_DEFINE(HAVE_UNORDERED_MAP,,[Define if unordered_map is present. ])
	fi
	])
	</programlisting>

	<programlisting>
	# AC_HEADER_UNORDERED_SET
	AC_DEFUN([AC_HEADER_UNORDERED_SET], [
	AC_CACHE_CHECK(for unordered_set,
	ac_cv_cxx_unordered_set,
	[AC_REQUIRE([AC_COMPILE_STDCXX_11])
	AC_LANG_SAVE
	AC_LANG_CPLUSPLUS
	ac_save_CXXFLAGS="$CXXFLAGS"
	CXXFLAGS="$CXXFLAGS -std=gnu++11"
	AC_TRY_COMPILE([#include <unordered_set>], [using std::unordered_set;],
	ac_cv_cxx_unordered_set=yes, ac_cv_cxx_unordered_set=no)
	CXXFLAGS="$ac_save_CXXFLAGS"
	AC_LANG_RESTORE
	])
	if test "$ac_cv_cxx_unordered_set" = yes; then
	AC_DEFINE(HAVE_UNORDERED_SET,,[Define if unordered_set is present. ])
	fi
	])
	</programlisting>

	<para>
	Some C++11 features first appeared in GCC 4.3 and could be enabled by
	<option>-std=c++0x</option> and <option>-std=gnu++0x</option> for GCC
	releases which pre-date the 2011 standard. Those C++11 features and GCC's
	support for them were still changing until the 2011 standard was finished,
	but the autoconf checks above could be extended to test for incomplete
	C++11 support with <option>-std=c++0x</option> and
	<option>-std=gnu++0x</option>.
	</para>

	</section>

	<section xml:id="backwards.third.iterator_type"><info><title>
	<code>Container::iterator_type</code> is not necessarily <code>Container::value_type*</code>
	</title></info>


	<para>
	This is a change in behavior from older versions. Now, most
	<type>iterator_type</type> typedefs in container classes are POD
	objects, not <type>value_type</type> pointers.
	</para>
	</section>

	</section>

	</section>