libstdc++-v3/doc/html/manual/backwards.html - gcc - Git at Google


            
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            <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Backwards Compatibility</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, backwards" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="appendix_porting.html" title="Appendix B.  Porting and Maintenance" /><link rel="prev" href="api.html" title="API Evolution and Deprecation History" /><link rel="next" href="appendix_free.html" title="Appendix C.  Free Software Needs Free Documentation" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Backwards Compatibility</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="api.html">Prev</a> </td><th width="60%" align="center">Appendix B.

            
              Porting and Maintenance


            </th><td width="20%" align="right"> <a accesskey="n" href="appendix_free.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.appendix.porting.backwards"></a>Backwards Compatibility</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="backwards.first"></a>First</h3></div></div></div><p>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.

            
            </p><p>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 <code class="classname">std::list&lt;T&gt;</code> and do not need to be

            
            created by <code class="function">genclass</code>.  (For that matter, templates exist

            
            now and are well-supported, whereas genclass (mostly) predates them.)

            
            </p><p>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

            
            <span class="quote">“<span class="quote">obvious</span>”</span> classes didn't get included.

            
            </p><p>That project is no longer maintained or supported, and the sources

            
            archived. For the desperate, the

            
            <a class="link" href="https://ftp.gnu.org/old-gnu/libg++/" target="_top">ftp.gnu.org</a>

            
            server still has the libg++ source.

            
            </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="backwards.second"></a>Second</h3></div></div></div><p>

            
              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.

            
            </p><p>

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

            
            </p><p>

            
              That project is no longer maintained or supported, and the sources

            
              archived.  The code was replaced and rewritten for libstdc++-v3.

            
            </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="backwards.third"></a>Third</h3></div></div></div><p> The third generation GNU C++ library is called libstdc++, or

            
            libstdc++-v3.

            
            </p><p>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.

            
                  </p><p>A more formal description of the V3 goals can be found in the

            
            	 official <a class="link" href="source_design_notes.html" title="Design Notes">design document</a>.

            
                  </p><p>Portability notes and known implementation limitations are as follows.</p><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.headers"></a>Pre-ISO headers removed</h4></div></div></div><p> The pre-ISO C++ headers

            
                  (<code class="filename">&lt;iostream.h&gt;</code>,

            
                  <code class="filename">&lt;defalloc.h&gt;</code> etc.) are

            
                  not supported.

            
            </p><p>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 <a class="link" href="https://isocpp.org/wiki/faq/coding-standards#std-headers" target="_top">What's

            
                  the difference between &lt;xxx&gt; and &lt;xxx.h&gt; headers?</a>.

            
               </p><p>Porting between pre-ISO headers and ISO headers is simple: headers

            
            like <code class="filename">&lt;vector.h&gt;</code> can be replaced with <code class="filename">&lt;vector&gt;</code> and a using

            
            directive <code class="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.

            
            </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.hash"></a>Extension headers hash_map, hash_set moved to ext or backwards</h4></div></div></div><p>At this time most of the features of the SGI STL extension have been

            
            	 replaced by standardized libraries.

            
            	 In particular, the <code class="classname">unordered_map</code> and

            
            	 <code class="classname">unordered_set</code> containers of TR1 and C++ 2011

            
            	 are suitable replacements for the non-standard

            
            	 <code class="classname">hash_map</code> and <code class="classname">hash_set</code>

            
            	 containers in the SGI STL.

            
                  </p><p> Header files <code class="filename">&lt;hash_map&gt;</code> and <code class="filename">&lt;hash_set&gt;</code> moved

            
            to <code class="filename">&lt;ext/hash_map&gt;</code> and  <code class="filename">&lt;ext/hash_set&gt;</code>,

            
            respectively. At the same time, all types in these files are enclosed

            
            in <code class="code">namespace __gnu_cxx</code>. Later versions deprecate

            
            these files, and suggest using TR1's  <code class="filename">&lt;unordered_map&gt;</code>

            
            and  <code class="filename">&lt;unordered_set&gt;</code> instead.

            
            </p><p>The extensions are no longer in the global or <code class="code">std</code>

            
            	 namespaces, instead they are declared in the <code class="code">__gnu_cxx</code>

            
            	 namespace. For maximum portability, consider defining a namespace

            
            	 alias to use to talk about extensions, e.g.:

            
                  </p><pre class="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;

            
            </pre><p>This is a bit cleaner than defining typedefs for all the

            
            	 instantiations you might need.

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

            
            </p><pre class="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

            
            ])

            
            </pre><pre class="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

            
            ])

            
            </pre></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.nocreate_noreplace"></a>No <code class="code">ios::nocreate/ios::noreplace</code>.

            
            </h4></div></div></div><p>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 class="code">O_CREAT</code> and <code class="code">O_EXCL</code> flags for the

            
            <code class="function">open(2)</code> system call. Because iostream modes correspond

            
            to <code class="function">fopen(3)</code> modes these flags are not supported.

            
            For input streams a new file will not be created anyway, so

            
            <code class="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 <code class="function">fopen</code>.

            
            </p><p>When one of these flags is needed a possible alternative is to attempt

            
            to open the file using <span class="type">std::ifstream</span> 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

            
            <span class="type">std::istream</span> 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 <code class="function">open(2)</code>.

            
            </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.streamattach"></a>

            
            No <code class="code">stream::attach(int fd)</code>

            
            </h4></div></div></div><p>

            
                  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.

            
                </p><p>

            
                  For a portable solution (among systems which use

            
                  file descriptors), you need to implement a subclass of

            
                  <code class="code">std::streambuf</code> (or

            
                  <code class="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.

            
                </p><p>

            
                  An extension is available that implements this.

            
                  <code class="filename">&lt;ext/stdio_filebuf.h&gt;</code>

            
                  contains a derived class called

            
                  <code class="classname">__gnu_cxx::stdio_filebuf</code>.

            
                  This class can be constructed from a C <code class="code">FILE*</code> or a file

            
                  descriptor, and provides the <code class="code">fd()</code> function.

            
                </p><p>

            
             For another example of this, refer to

            
                  <a class="link" href="http://www.josuttis.com/cppcode/fdstream.html" target="_top">fdstream example</a>

            
                  by Nicolai Josuttis.

            
            </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.support_cxx98"></a>

            
            Support for C++98 dialect.

            
            </h4></div></div></div><p>Check for complete library coverage of the C++1998/2003 standard.

            
            </p><pre class="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

            
            ])

            
            </pre></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.support_tr1"></a>

            
            Support for C++TR1 dialect.

            
            </h4></div></div></div><p>Check for library coverage of the TR1 standard.

            
            </p><pre class="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

            
            ])

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

            
            </p><pre class="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

            
            ])

            
            </pre><pre class="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

            
            ])

            
            </pre></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.support_cxx11"></a>

            
            Support for C++11 dialect.

            
            </h4></div></div></div><p>Check for baseline language coverage in the compiler for the C++11 standard.

            
            </p><pre class="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

            
            ])

            
            </pre><p>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++).

            
            </p><pre class="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

            
            ])

            
            </pre><p>As is the case for TR1 support, these autoconf macros can be made for a finer-grained, per-header-file check. For

            
            <code class="filename">&lt;unordered_map&gt;</code>

            
            </p><pre class="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

            
            ])

            
            </pre><pre class="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

            
            ])

            
            </pre><p>

            
              Some C++11 features first appeared in GCC 4.3 and could be enabled by

            
              <code class="option">-std=c++0x</code> and <code class="option">-std=gnu++0x</code> 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 <code class="option">-std=c++0x</code> and

            
              <code class="option">-std=gnu++0x</code>.

            
            </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="backwards.third.iterator_type"></a>

            
              <code class="code">Container::iterator_type</code> is not necessarily <code class="code">Container::value_type*</code>

            
            </h4></div></div></div><p>

            
              This is a change in behavior from older versions. Now, most

            
              <span class="type">iterator_type</span> typedefs in container classes are POD

            
              objects, not <span class="type">value_type</span> pointers.

            
            </p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="api.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="appendix_porting.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="appendix_free.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">API Evolution and Deprecation History </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Appendix C.

            
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