xml/manual/backwards_compatibility.xml

<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>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>Known Issues include many of the limitations of its immediate ancestor.</para>

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

<section xml:id="backwards.first.ios_base"><info><title>No <code>ios_base</code></title></info>


<para> At least some older implementations don't have <code>std::ios_base</code>, so you should use <code>std::ios::badbit</code>, <code>std::ios::failbit</code> and <code>std::ios::eofbit</code> and <code>std::ios::goodbit</code>.
</para>
</section>

<section xml:id="backwards.first.cout_cin"><info><title>No <code>cout</code> in <filename class="headerfile">&lt;ostream.h&gt;</filename>, no <code>cin</code> in <filename class="headerfile">&lt;istream.h&gt;</filename></title></info>


<para>
	In earlier versions of the standard,
	<filename class="headerfile">&lt;fstream.h&gt;</filename>,
	<filename class="headerfile">&lt;ostream.h&gt;</filename>
	and <filename class="headerfile">&lt;istream.h&gt;</filename>
	used to define
	<code>cout</code>, <code>cin</code> and so on. ISO C++ specifies that one needs to include
	<filename class="headerfile">&lt;iostream&gt;</filename>
	explicitly to get the required definitions.
 </para>
<para> Some include adjustment may be required.</para>

<para>This 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="http://gcc.gnu.org/extensions.html">GCC extensions
page</link> describes where to find the last libg++ source. The code is
considered replaced and rewritten.
</para>
</section>
</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 this library are based on SGI/HP STL release 3.11.
</para>

<para>
  This project is no longer maintained or supported, and the sources
  archived.  The code is considered replaced and rewritten.
</para>

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

<section xml:id="backwards.second.std"><info><title>Namespace <code>std::</code> not supported</title></info>


  <para>
    Some care is required to support C++ compiler and or library
    implementation that do not have the standard library in
    <code>namespace std</code>.
  </para>

  <para>
    The following sections list some possible solutions to support compilers
    that cannot ignore <code>std::</code>-qualified names.
  </para>

  <para>
    First, see if the compiler has a flag for this. Namespace
    back-portability-issues are generally not a problem for g++
    compilers that do not have libstdc++ in <code>std::</code>, as the
    compilers use <option>-fno-honor-std</option> (ignore
    <code>std::</code>, <code>:: = std::</code>) by default. That is,
    the responsibility for enabling or disabling <code>std::</code> is
    on the user; the maintainer does not have to care about it. This
    probably applies to some other compilers as well.
  </para>

  <para>
    Second, experiment with a variety of pre-processor tricks.
  </para>

  <para>
    By defining <code>std</code> as a macro, fully-qualified namespace
    calls become global. Volia.
  </para>

<programlisting>
#ifdef WICKEDLY_OLD_COMPILER
# define std
#endif
</programlisting>

  <para>
    Thanks to Juergen Heinzl who posted this solution on gnu.gcc.help.
  </para>

  <para>
    Another pre-processor based approach is to define a macro
    <code>NAMESPACE_STD</code>, which is defined to either
    <quote> </quote> or <quote>std</quote> based on a compile-type
    test. On GNU systems, this can be done with autotools by means of
    an autoconf test (see below) for <code>HAVE_NAMESPACE_STD</code>,
    then using that to set a value for the <code>NAMESPACE_STD</code>
    macro.  At that point, one is able to use
    <code>NAMESPACE_STD::string</code>, which will evaluate to
    <code>std::string</code> or <code>::string</code> (i.e., in the
    global namespace on systems that do not put <code>string</code> in
    <code>std::</code>).
  </para>

<programlisting>
dnl @synopsis AC_CXX_NAMESPACE_STD
dnl
dnl If the compiler supports namespace std, define
dnl HAVE_NAMESPACE_STD.
dnl
dnl @category Cxx
dnl @author Todd Veldhuizen
dnl @author Luc Maisonobe &lt;luc@spaceroots.org&gt;
dnl @version 2004-02-04
dnl @license AllPermissive
AC_DEFUN([AC_CXX_NAMESPACE_STD], [
  AC_CACHE_CHECK(if g++ supports namespace std,
  ac_cv_cxx_have_std_namespace,
  [AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  AC_TRY_COMPILE([#include &lt;iostream&gt;
		  std::istream&amp; is = std::cin;],,
  ac_cv_cxx_have_std_namespace=yes, ac_cv_cxx_have_std_namespace=no)
  AC_LANG_RESTORE
  ])
  if test "$ac_cv_cxx_have_std_namespace" = yes; then
    AC_DEFINE(HAVE_NAMESPACE_STD,,[Define if g++ supports namespace std. ])
  fi
])
</programlisting>
</section>

<section xml:id="backwards.second.iterators"><info><title>Illegal iterator usage</title></info>

<para>
  The following illustrate implementation-allowed illegal iterator
  use, and then correct use.
</para>

<itemizedlist>
  <listitem>
    <para>
      you cannot do <code>ostream::operator&lt;&lt;(iterator)</code>
      to print the address of the iterator =&gt; use
      <code>operator&lt;&lt; &amp;*iterator</code> instead
    </para>
  </listitem>
  <listitem>
    <para>
      you cannot clear an iterator's reference (<code>iterator =
      0</code>) =&gt; use <code>iterator = iterator_type();</code>
    </para>
  </listitem>
  <listitem>
    <para>
      <code>if (iterator)</code> won't work any more =&gt; use
      <code>if (iterator != iterator_type())</code>
    </para>
  </listitem>
</itemizedlist>
</section>

<section xml:id="backwards.second.isspace"><info><title><code>isspace</code> from <filename class="headerfile">&lt;cctype&gt;</filename> is a macro
  </title></info>


  <para>
    Glibc 2.0.x and 2.1.x define <filename class="headerfile">&lt;ctype.h&gt;</filename> functionality as macros
    (isspace, isalpha etc.).
  </para>

  <para>
    This implementations of libstdc++, however, keep these functions
    as macros, and so it is not back-portable to use fully qualified
    names. For example:
  </para>

<programlisting>
#include &lt;cctype&gt;
int main() { std::isspace('X'); }
</programlisting>

<para>
  Results in something like this:
</para>

<programlisting>
std:: (__ctype_b[(int) ( ( 'X' ) )] &amp; (unsigned short int) _ISspace ) ;
</programlisting>

<para>
  A solution is to modify a header-file so that the compiler tells
  <filename class="headerfile">&lt;ctype.h&gt;</filename> to define functions
  instead of macros:
</para>

<programlisting>
// This keeps isalnum, et al from being propagated as macros.
#if __linux__
# define __NO_CTYPE 1
#endif
</programlisting>

<para>
  Then, include <filename class="headerfile">&lt;ctype.h&gt;</filename>
</para>

<para>
  Another problem arises if you put a <code>using namespace
  std;</code> declaration at the top, and include
  <filename class="headerfile">&lt;ctype.h&gt;</filename>. This will
  result in ambiguities between the definitions in the global namespace
  (<filename class="headerfile">&lt;ctype.h&gt;</filename>) and the
  definitions in namespace <code>std::</code>
  (<code>&lt;cctype&gt;</code>).
</para>
</section>

<section xml:id="backwards.second.at"><info><title>No <code>vector::at</code>, <code>deque::at</code>, <code>string::at</code></title></info>


<para>
  One solution is to add an autoconf-test for this:
</para>

<programlisting>
AC_MSG_CHECKING(for container::at)
AC_TRY_COMPILE(
[
#include &lt;vector&gt;
#include &lt;deque&gt;
#include &lt;string&gt;

using namespace std;
],
[
deque&lt;int&gt; test_deque(3);
test_deque.at(2);
vector&lt;int&gt; test_vector(2);
test_vector.at(1);
string test_string(<quote>test_string</quote>);
test_string.at(3);
],
[AC_MSG_RESULT(yes)
AC_DEFINE(HAVE_CONTAINER_AT)],
[AC_MSG_RESULT(no)])
</programlisting>

<para>
  If you are using other (non-GNU) compilers it might be a good idea
  to check for <code>string::at</code> separately.
</para>

</section>

<section xml:id="backwards.second.eof"><info><title>No <code>std::char_traits&lt;char&gt;::eof</code></title></info>


<para>
  Use some kind of autoconf test, plus this:
</para>

<programlisting>
#ifdef HAVE_CHAR_TRAITS
#define CPP_EOF std::char_traits&lt;char&gt;::eof()
#else
#define CPP_EOF EOF
#endif
</programlisting>

</section>

<section xml:id="backwards.second.stringclear"><info><title>No <code>string::clear</code></title></info>


<para>
  There are two functions for deleting the contents of a string:
  <code>clear</code> and <code>erase</code> (the latter returns the
  string).
</para>

<programlisting>
void
clear() { _M_mutate(0, this-&gt;size(), 0); }
</programlisting>

<programlisting>
basic_string&amp;
erase(size_type __pos = 0, size_type __n = npos)
{
  return this-&gt;replace(_M_check(__pos), _M_fold(__pos, __n),
			  _M_data(), _M_data());
}
</programlisting>

<para>
  Unfortunately, <code>clear</code> is not implemented in this
  version, so you should use <code>erase</code> (which is probably
  faster than <code>operator=(charT*)</code>).
</para>
</section>

<section xml:id="backwards.second.ostreamform_istreamscan"><info><title>
  Removal of <code>ostream::form</code> and <code>istream::scan</code>
  extensions
</title></info>


<para>
  These are no longer supported. Please use stringstreams instead.
</para>
</section>

<section xml:id="backwards.second.stringstreams"><info><title>No <code>basic_stringbuf</code>, <code>basic_stringstream</code></title></info>


<para>
  Although the ISO standard <code>i/ostringstream</code>-classes are
  provided, (<filename class="headerfile">&lt;sstream&gt;</filename>), for
  compatibility with older implementations the pre-ISO
  <code>i/ostrstream</code> (<filename class="headerfile">&lt;strstream&gt;</filename>) interface is also provided,
  with these caveats:
</para>

<itemizedlist>
  <listitem>
    <para>
      <code>strstream</code> is considered to be deprecated
    </para>
  </listitem>
  <listitem>
    <para>
      <code>strstream</code> is limited to <code>char</code>
    </para>
  </listitem>
  <listitem>
    <para>
      with <code>ostringstream</code> you don't have to take care of
      terminating the string or freeing its memory
    </para>
  </listitem>
  <listitem>
    <para>
      <code>istringstream</code> can be re-filled (clear();
      str(input);)
    </para>
  </listitem>
</itemizedlist>

<para>
  You can then use output-stringstreams like this:
</para>

<programlisting>
#ifdef HAVE_SSTREAM
# include &lt;sstream&gt;
#else
# include &lt;strstream&gt;
#endif

#ifdef HAVE_SSTREAM
  std::ostringstream oss;
#else
  std::ostrstream oss;
#endif

oss &lt;&lt; "Name=" &lt;&lt; m_name &lt;&lt; ", number=" &lt;&lt; m_number &lt;&lt; std::endl;
...
#ifndef HAVE_SSTREAM
  oss &lt;&lt; std::ends; // terminate the char*-string
#endif

// str() returns char* for ostrstream and a string for ostringstream
// this also causes ostrstream to think that the buffer's memory
// is yours
m_label.set_text(oss.str());
#ifndef HAVE_SSTREAM
  // let the ostrstream take care of freeing the memory
  oss.freeze(false);
#endif
</programlisting>

<para>
      Input-stringstreams can be used similarly:
</para>

<programlisting>
std::string input;
...
#ifdef HAVE_SSTREAM
std::istringstream iss(input);
#else
std::istrstream iss(input.c_str());
#endif

int i;
iss &gt;&gt; i;
</programlisting>

<para> One (the only?) restriction is that an istrstream cannot be re-filled:
</para>

<programlisting>
std::istringstream iss(numerator);
iss &gt;&gt; m_num;
// this is not possible with istrstream
iss.clear();
iss.str(denominator);
iss &gt;&gt; m_den;
</programlisting>

<para>
If you don't care about speed, you can put these conversions in
      a template-function:
</para>
<programlisting>
template &lt;class X&gt;
void fromString(const string&amp; input, X&amp; any)
{
#ifdef HAVE_SSTREAM
std::istringstream iss(input);
#else
std::istrstream iss(input.c_str());
#endif
X temp;
iss &gt;&gt; temp;
if (iss.fail())
throw runtime_error(..)
any = temp;
}
</programlisting>

<para>
  Another example of using stringstreams is in <link linkend="strings.string.shrink">this howto</link>.
</para>

<para> There is additional information in the libstdc++-v2 info files, in
particular <quote>info iostream</quote>.
</para>
</section>

<section xml:id="backwards.second.wchar"><info><title>Little or no wide character support</title></info>

  <para>
    Classes <classname>wstring</classname> and
    <classname>char_traits&lt;wchar_t&gt;</classname> are
    not supported.
  </para>
</section>

<section xml:id="backwards.second.iostream_templates"><info><title>No templatized iostreams</title></info>

  <para>
    Classes <classname>wfilebuf</classname> and
    <classname>wstringstream</classname> are not supported.
  </para>
</section>

<section xml:id="backwards.second.thread_safety"><info><title>Thread safety issues</title></info>


  <para>
    Earlier GCC releases had a somewhat different approach to
    threading configuration and proper compilation.  Before GCC 3.0,
    configuration of the threading model was dictated by compiler
    command-line options and macros (both of which were somewhat
    thread-implementation and port-specific).  There were no
    guarantees related to being able to link code compiled with one
    set of options and macro setting with another set.
  </para>

  <para>
    For GCC 3.0, configuration of the threading model used with
    libraries and user-code is performed when GCC is configured and
    built using the --enable-threads and --disable-threads options.
    The ABI is stable for symbol name-mangling and limited functional
    compatibility exists between code compiled under different
    threading models.
  </para>

   <para>
     The libstdc++ library has been designed so that it can be used in
     multithreaded applications (with libstdc++-v2 this was only true
     of the STL parts.)  The first problem is finding a
     <emphasis>fast</emphasis> method of implementation portable to
     all platforms.  Due to historical reasons, some of the library is
     written against per-CPU-architecture spinlocks and other parts
     against the gthr.h abstraction layer which is provided by gcc.  A
     minor problem that pops up every so often is different
     interpretations of what "thread-safe" means for a
     library (not a general program).  We currently use the <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.sgi.com/tech/stl/thread_safety.html">same
     definition that SGI</link> uses for their STL subset.  However,
     the exception for read-only containers only applies to the STL
     components. This definition is widely-used and something similar
     will be used in the next version of the C++ standard library.
   </para>

   <para>
     Here is a small link farm to threads (no pun) in the mail
     archives that discuss the threading problem.  Each link is to the
     first relevant message in the thread; from there you can use
     "Thread Next" to move down the thread.  This farm is in
     latest-to-oldest order.
   </para>

      <itemizedlist>
	<listitem>
	  <para>
	    Our threading expert Loren gives a breakdown of <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2001-10/msg00024.html">the
	    six situations involving threads</link> for the 3.0
	    release series.
	  </para>
      </listitem>
	<listitem>
	  <para>
	    <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/ml/libstdc++/2001-05/msg00384.html">
	This message</link> inspired a recent updating of issues with
	threading and the SGI STL library.  It also contains some
	example POSIX-multithreaded STL code.
	  </para>
	</listitem>
      </itemizedlist>

   <para>
     (A large selection of links to older messages has been removed;
     many of the messages from 1999 were lost in a disk crash, and the
     few people with access to the backup tapes have been too swamped
     with work to restore them.  Many of the points have been
     superseded anyhow.)
   </para>
</section>

</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, 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 moved to backwards or 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
      available, unlike previous libstdc++ versions, but inclusion
      generates a warning that you are using deprecated headers.
</para>

    <para>This compatibility layer is constructed by including the
    standard C++ headers, and injecting any items in
    <code>std::</code> into the global namespace.
   </para>
   <para>For those of you new to ISO C++ (welcome, time travelers!), no,
      that isn't a typo. Yes, the headers really have new names.
      Marshall Cline's C++ FAQ Lite has a good explanation in <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.parashift.com/c++-faq-lite/std-headers.html">What's
      the difference between &lt;xxx&gt; and &lt;xxx.h&gt; headers?</link>.
   </para>

<para> Some include adjustment may be required. What follows is an
autoconf test that defines <code>PRE_STDCXX_HEADERS</code> when they
exist.</para>

<programlisting>
# AC_HEADER_PRE_STDCXX
AC_DEFUN([AC_HEADER_PRE_STDCXX], [
  AC_CACHE_CHECK(for pre-ISO C++ include files,
  ac_cv_cxx_pre_stdcxx,
  [AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  ac_save_CXXFLAGS="$CXXFLAGS"
  CXXFLAGS="$CXXFLAGS -Wno-deprecated"

  # Omit defalloc.h, as compilation with newer compilers is problematic.
  AC_TRY_COMPILE([
  #include &lt;new.h&gt;
  #include &lt;iterator.h&gt;
  #include &lt;alloc.h&gt;
  #include &lt;set.h&gt;
  #include &lt;hashtable.h&gt;
  #include &lt;hash_set.h&gt;
  #include &lt;fstream.h&gt;
  #include &lt;tempbuf.h&gt;
  #include &lt;istream.h&gt;
  #include &lt;bvector.h&gt;
  #include &lt;stack.h&gt;
  #include &lt;rope.h&gt;
  #include &lt;complex.h&gt;
  #include &lt;ostream.h&gt;
  #include &lt;heap.h&gt;
  #include &lt;iostream.h&gt;
  #include &lt;function.h&gt;
  #include &lt;multimap.h&gt;
  #include &lt;pair.h&gt;
  #include &lt;stream.h&gt;
  #include &lt;iomanip.h&gt;
  #include &lt;slist.h&gt;
  #include &lt;tree.h&gt;
  #include &lt;vector.h&gt;
  #include &lt;deque.h&gt;
  #include &lt;multiset.h&gt;
  #include &lt;list.h&gt;
  #include &lt;map.h&gt;
  #include &lt;algobase.h&gt;
  #include &lt;hash_map.h&gt;
  #include &lt;algo.h&gt;
  #include &lt;queue.h&gt;
  #include &lt;streambuf.h&gt;
  ],,
  ac_cv_cxx_pre_stdcxx=yes, ac_cv_cxx_pre_stdcxx=no)
  CXXFLAGS="$ac_save_CXXFLAGS"
  AC_LANG_RESTORE
  ])
  if test "$ac_cv_cxx_pre_stdcxx" = yes; then
    AC_DEFINE(PRE_STDCXX_HEADERS,,[Define if pre-ISO C++ header files are present. ])
  fi
])
</programlisting>

<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
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a00074.html"><code>__gnu_cxx::stdio_filebuf</code></link>.
      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>

<bibliography xml:id="backwards.biblio"><info><title>Bibliography</title></info>


  <biblioentry>
      <title>
	<link xmlns:xlink="http://www.w3.org/1999/xlink"
	      xlink:href="http://www.kegel.com/gcc/gcc4.html">
      Migrating to GCC 4.1
	</link>
      </title>

    <author><personname><firstname>Dan</firstname><surname>Kegel</surname></personname></author>
  </biblioentry>

  <biblioentry>
      <title>
	<link xmlns:xlink="http://www.w3.org/1999/xlink"
	      xlink:href="http://lists.debian.org/debian-gcc/2006/03/msg00405.html">
      Building the Whole Debian Archive with GCC 4.1: A Summary
	</link>
      </title>
    <author><personname><firstname>Martin</firstname><surname>Michlmayr</surname></personname></author>
  </biblioentry>

  <biblioentry>
      <title>
	<link xmlns:xlink="http://www.w3.org/1999/xlink"
	      xlink:href="http://annwm.lbl.gov/~leggett/Atlas/gcc-3.2.html">
      Migration guide for GCC-3.2
	</link>
      </title>

  </biblioentry>

</bibliography>

</section>