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That project is no longer maintained or supported, and the sources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That project is no longer maintained or supported, and the sources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