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

In earlier versions of the standard, <fstream.h>, <ostream.h> and <istream.h> used to define cout, cin and so on. ISO C++ specifies that one needs to include <iostream> explicitly to get the required definitions.

Some include adjustment may be required.

This project is no longer maintained or supported, and the sources archived. For the desperate, the GCC extensions page describes where to find the last libg++ source. The code is considered replaced and rewritten.

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

The following sections list some possible solutions to support compilers that cannot ignore std::-qualified names.

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 std::, as the compilers use -fno-honor-std (ignore std::, :: = std::) by default. That is, the responsibility for enabling or disabling std:: is on the user; the maintainer does not have to care about it. This probably applies to some other compilers as well.

Second, experiment with a variety of pre-processor tricks.

By defining std as a macro, fully-qualified namespace calls become global. Volia.

#ifdef WICKEDLY_OLD_COMPILER
# define std
#endif

Thanks to Juergen Heinzl who posted this solution on gnu.gcc.help.

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

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 <[email protected]>
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 <iostream>
		  std::istream& 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
])

The following illustrate implementation-allowed illegal iterator use, and then correct use.

Glibc 2.0.x and 2.1.x define <ctype.h> functionality as macros (isspace, isalpha etc.).

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

#include <cctype>
int main() { std::isspace('X'); }

Results in something like this:

std:: (__ctype_b[(int) ( ( 'X' ) )] & (unsigned short int) _ISspace ) ;

A solution is to modify a header-file so that the compiler tells <ctype.h> to define functions instead of macros:

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

Then, include <ctype.h>

Another problem arises if you put a using namespace std; declaration at the top, and include <ctype.h>. This will result in ambiguities between the definitions in the global namespace (<ctype.h>) and the definitions in namespace std:: (<cctype>).

One solution is to add an autoconf-test for this:

AC_MSG_CHECKING(for container::at)
AC_TRY_COMPILE(
[
#include <vector>
#include <deque>
#include <string>

using namespace std;
],
[
deque<int> test_deque(3);
test_deque.at(2);
vector<int> test_vector(2);
test_vector.at(1);
string test_string(“test_string”);
test_string.at(3);
],
[AC_MSG_RESULT(yes)
AC_DEFINE(HAVE_CONTAINER_AT)],
[AC_MSG_RESULT(no)])

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

Use some kind of autoconf test, plus this:

#ifdef HAVE_CHAR_TRAITS
#define CPP_EOF std::char_traits<char>::eof()
#else
#define CPP_EOF EOF
#endif

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

void
clear() { _M_mutate(0, this->size(), 0); }

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

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

These are no longer supported. Please use stringstreams instead.

Although the ISO standard i/ostringstream-classes are provided, (<sstream>), for compatibility with older implementations the pre-ISO i/ostrstream (<strstream>) interface is also provided, with these caveats:

You can then use output-stringstreams like this:

#ifdef HAVE_SSTREAM
# include <sstream>
#else
# include <strstream>
#endif

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

oss << "Name=" << m_name << ", number=" << m_number << std::endl;
...
#ifndef HAVE_SSTREAM
  oss << 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

Input-stringstreams can be used similarly:

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

int i;
iss >> i;

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

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

If you don't care about speed, you can put these conversions in a template-function:

template <class X>
void fromString(const string& input, X& any)
{
#ifdef HAVE_SSTREAM
std::istringstream iss(input);
#else
std::istrstream iss(input.c_str());
#endif
X temp;
iss >> temp;
if (iss.fail())
throw runtime_error(..)
any = temp;
}

Another example of using stringstreams is in this howto.

There is additional information in the libstdc++-v2 info files, in particular “info iostream”.

Classes wstring and char_traits<wchar_t> are not supported.

Classes wfilebuf and wstringstream are not supported.

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.

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.

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 fast 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 same definition that SGI 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.

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.

(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.)

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.