# Backwards Compatibility

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.

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 `std::list<T>` and do not need to be created by `genclass`. (For that matter, templates exist now and are well-supported, whereas genclass (mostly) predates them.)

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 obvious classes didn't get included.

That project is no longer maintained or supported, and the sources archived. For the desperate, the ftp.gnu.org server still has the libg++ source.

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.

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

That project is no longer maintained or supported, and the sources archived.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

The third generation GNU C++ library is called libstdc++, or libstdc++-v3.

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.

A more formal description of the V3 goals can be found in the official design document.

Portability notes and known implementation limitations are as follows.

The pre-ISO C++ headers (`<iostream.h>`, `<defalloc.h>` etc.) are not supported.

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 What's the difference between <xxx> and <xxx.h> headers?.

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

At this time most of the features of the SGI STL extension have been replaced by standardized libraries. In particular, the `unordered_map` and `unordered_set` containers of TR1 and C++ 2011 are suitable replacements for the non-standard `hash_map` and `hash_set` containers in the SGI STL.

Header files `<hash_map>` and `<hash_set>` moved to `<ext/hash_map>` and `<ext/hash_set>`, respectively. At the same time, all types in these files are enclosed in `namespace __gnu_cxx`. Later versions deprecate these files, and suggest using TR1's `<unordered_map>` and `<unordered_set>` instead.

The extensions are no longer in the global or `std` namespaces, instead they are declared in the `__gnu_cxx` namespace. For maximum portability, consider defining a namespace alias to use to talk about extensions, e.g.:

          #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;

This is a bit cleaner than defining typedefs for all the instantiations you might need.

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

    # 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
    ])

    # 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
    ])

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

When one of these flags is needed a possible alternative is to attempt to open the file using `std::ifstream` 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 `std::istream` 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 `open(2)`.

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.

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

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

For another example of this, refer to fdstream example by Nicolai Josuttis.

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

    # 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
    ])

Check for library coverage of the TR1 standard.

    # 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
    ])

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

    # 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
    ])

    # 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
    ])

Check for baseline language coverage in the compiler for the C++11 standard.

    # 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
    ])

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++).

    # 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
    ])

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

    # 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
    ])

    # 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
    ])

Some C++11 features first appeared in GCC 4.3 and could be enabled by `-std=c++0x` and `-std=gnu++0x` 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 `-std=c++0x` and `-std=gnu++0x`.

This is a change in behavior from older versions. Now, most `iterator_type` typedefs in container classes are POD objects, not `value_type` pointers.