Backwards Compatibility

The latest version of this document is always available at http://gcc.gnu.org/onlinedocs/libstdc++/17_intro/backwards_compatibility.html.

To the libstdc++ homepage.


First.

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.

Known Issues include many of the limitations of its immediate ancestor.

Portability notes and known implementation limitations are as follows.

No ios_base

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.

No cout in ostream.h, no cin in istream.h

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This project is no longer maintained or supported, and the sources archived.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


Second.

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 this library are based on SGI/HP STL release 3.11.

Portability notes and known implementation limitations are as follows.

Namespace std:: not supported

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 (ie, 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 <luc@spaceroots.org>
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
])
Illegal iterator usage

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

isspace from <cctype> is a macro

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>).

No vector::at, deque::at, string::at

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.

No std::char_traits<char>::eof

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
No string::clear

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

Removal of ostream::form and istream::scan extensions

These are no longer supported. Please use stringstreams instead.

No basic_stringbuf, basic_stringstream

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".

Little or no wide character support
No templatized iostreams
Thread safety issues

This project is no longer maintained or supported, and the sources archived.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


Third.

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

The subset commonly known as the Standard Template Library (chapters 23 through 25, 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.

Pre-ISO headers moved to backwards or removed

The pre-ISO C++ headers (iostream.h, defalloc.h etc.) are available, unlike previous libstdc++ versions, but inclusion generates a warning that you are using deprecated headers.

This compatibility layer is constructed by including the standard C++ headers, and injecting any items in std:: into the global namespace.

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 item [27.4].

Some include adjustment may be required. What follows is an autoconf test that defines PRE_STDCXX_HEADERS when they exist.

# 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 <new.h>
  #include <iterator.h>
  #include <alloc.h>
  #include <set.h>
  #include <hashtable.h>
  #include <hash_set.h>
  #include <fstream.h>
  #include <tempbuf.h>
  #include <istream.h>
  #include <bvector.h>
  #include <stack.h>
  #include <rope.h>
  #include <complex.h>
  #include <ostream.h>
  #include <heap.h>
  #include <iostream.h>
  #include <function.h>
  #include <multimap.h>
  #include <pair.h>
  #include <stream.h>
  #include <iomanip.h>
  #include <slist.h>
  #include <tree.h>
  #include <vector.h>
  #include <deque.h>
  #include <multiset.h>
  #include <list.h>
  #include <map.h>
  #include <algobase.h>
  #include <hash_map.h>
  #include <algo.h>
  #include <queue.h>
  #include <streambuf.h>
  ],,
  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
])

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.

Extension headers hash_map, hash_set moved to ext or backwards

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 move deprecate these files, and suggest using TR1's unordered_map and unordered_set instead.

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
])
No ios::nocreate/ios::noreplace.

The existence of ios::nocreate being used for input-streams has been confirmed, most probably because the author thought it would be more correct to specify nocreate explicitly. So it can be left out for input-streams.

For output streams, "nocreate" is probably the default, unless you specify std::ios::trunc ? To be safe, you can open the file for reading, check if it has been opened, and then decide whether you want to create/replace or not. To my knowledge, even older implementations support app, ate and trunc (except for app ?).

No stream::attach(int fd)

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 filedescriptors), 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.

Support for C++98 dialect.

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
])
Support for C++TR1 dialect.

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
])
Support for C++0x dialect.

Check for baseline language coverage in the compiler for the C++0xstandard.

# AC_COMPILE_STDCXX_OX
AC_DEFUN([AC_COMPILE_STDCXX_0X], [
  AC_CACHE_CHECK(if g++ supports C++0x features without additional flags,
  ac_cv_cxx_compile_cxx0x_native,
  [AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  AC_TRY_COMPILE([
  template <typename T>
    struct check 
    {
      static_assert(sizeof(int) <= sizeof(T), "not big enough");
    };

    typedef check<check<bool>> right_angle_brackets;

    int a;
    decltype(a) b;

    typedef check<int> check_type;
    check_type c;
    check_type&& cr = c;],,
  ac_cv_cxx_compile_cxx0x_native=yes, ac_cv_cxx_compile_cxx0x_native=no)
  AC_LANG_RESTORE
  ])

  AC_CACHE_CHECK(if g++ supports C++0x features with -std=c++0x,
  ac_cv_cxx_compile_cxx0x_cxx,
  [AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  ac_save_CXXFLAGS="$CXXFLAGS"
  CXXFLAGS="$CXXFLAGS -std=c++0x"	
  AC_TRY_COMPILE([
  template <typename T>
    struct check 
    {
      static_assert(sizeof(int) <= sizeof(T), "not big enough");
    };

    typedef check<check<bool>> right_angle_brackets;

    int a;
    decltype(a) b;

    typedef check<int> check_type;
    check_type c;
    check_type&& cr = c;],,
  ac_cv_cxx_compile_cxx0x_cxx=yes, ac_cv_cxx_compile_cxx0x_cxx=no)
  CXXFLAGS="$ac_save_CXXFLAGS"
  AC_LANG_RESTORE
  ])

  AC_CACHE_CHECK(if g++ supports C++0x features with -std=gnu++0x,
  ac_cv_cxx_compile_cxx0x_gxx,
  [AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  ac_save_CXXFLAGS="$CXXFLAGS"
  CXXFLAGS="$CXXFLAGS -std=gnu++0x"	
  AC_TRY_COMPILE([
  template <typename T>
    struct check 
    {
      static_assert(sizeof(int) <= sizeof(T), "not big enough");
    };

    typedef check<check<bool>> right_angle_brackets;

    int a;
    decltype(a) b;

    typedef check<int> check_type;
    check_type c;
    check_type&& cr = c;],,
  ac_cv_cxx_compile_cxx0x_gxx=yes, ac_cv_cxx_compile_cxx0x_gxx=no)
  CXXFLAGS="$ac_save_CXXFLAGS"
  AC_LANG_RESTORE
  ])

  if test "$ac_cv_cxx_compile_cxx0x_native" = yes || 
     test "$ac_cv_cxx_compile_cxx0x_cxx" = yes || 
     test "$ac_cv_cxx_compile_cxx0x_gxx" = yes; then
    AC_DEFINE(HAVE_STDCXX_0X,,[Define if g++ supports C++0x features. ])
  fi
])

Check for library coverage of the C++0xstandard.


# AC_HEADER_STDCXX_0X
AC_DEFUN([AC_HEADER_STDCXX_0X], [
  AC_CACHE_CHECK(for ISO C++ 0x include files,
  ac_cv_cxx_stdcxx_0x,
  [AC_REQUIRE([AC_COMPILE_STDCXX_0X])
  AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  ac_save_CXXFLAGS="$CXXFLAGS"
  CXXFLAGS="$CXXFLAGS -std=gnu++0x"	

  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 <cstdarg>
    #include <cstdbool>
    #include <cstddef>
    #include <cstdint>
    #include <cstdio>
    #include <cstdlib>
    #include <cstring>
    #include <ctgmath>
    #include <ctime>
    #include <cwchar>
    #include <cwctype>

    #include <algorithm>
    #include <array>
    #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 <random>
    #include <regex>
    #include <set>
    #include <sstream>
    #include <stack>
    #include <stdexcept>
    #include <streambuf>
    #include <string>
    #include <tuple>
    #include <typeinfo>
    #include <type_traits>
    #include <unordered_map>
    #include <unordered_set>
    #include <utility>
    #include <valarray>
    #include <vector>
  ],,
  ac_cv_cxx_stdcxx_0x=yes, ac_cv_cxx_stdcxx_0x=no)
  AC_LANG_RESTORE
  CXXFLAGS="$ac_save_CXXFLAGS"
  ])
  if test "$ac_cv_cxx_stdcxx_0x" = yes; then
    AC_DEFINE(STDCXX_0X_HEADERS,,[Define if ISO C++ 0x 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_0X])
  AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  ac_save_CXXFLAGS="$CXXFLAGS"
  CXXFLAGS="$CXXFLAGS -std=gnu++0x"	
  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_0X])
  AC_LANG_SAVE
  AC_LANG_CPLUSPLUS
  ac_save_CXXFLAGS="$CXXFLAGS"
  CXXFLAGS="$CXXFLAGS -std=gnu++0x"	
  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
])
Container iterator_type is not necessarily container value_type*

Fourth, and future


Links

Migrating to gcc-4.1, by Dan Kegel.

Building the whole Debian archive with GCC 4.1: a summary, by Martin Michlmayr

Migration guide for GCC-3.2