# Backwards Compatibility

ISO C++
backwards

## 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.

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 `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.

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This project is no longer maintained or supported, and the sources archived.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

This project is no longer maintained or supported, and the sources archived.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

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` (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 <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.

- you cannot do `ostream::operator<<(iterator)` to print the address of the iterator => use `operator<< &*iterator` instead

- you cannot clear an iterator's reference (`iterator = 0`) => use `iterator = iterator_type();`

- `if (iterator)` won't work any more => use `if (iterator != iterator_type())`

### `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, `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:

- `strstream` is considered to be deprecated

- `strstream` is limited to `char`

- with `ostringstream` you don't have to take care of terminating the string or freeing its memory

- `istringstream` can be re-filled (clear(); str(input);)

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

Classes `wstring` and `char_traits<wchar_t>` are not supported.

### No templatized iostreams

Classes `wfilebuf` and `wstringstream` are not supported.

### Thread safety issues

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.

- Our threading expert Loren gives a breakdown of the six situations involving threads for the 3.0 release series.

- This message inspired a recent updating of issues with threading and the SGI STL library. It also contains some example POSIX-multithreaded STL code.

(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.)

## Third

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

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

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
    ])

### No `ios::nocreate/ios::noreplace`.

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)`.

### 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 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.

### 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++11 dialect.

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`.

### `Container::iterator_type` is not necessarily `Container::value_type*`

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

## Bibliography

### Migrating to GCC 4.1

Dan
Kegel

### Building the Whole Debian Archive with GCC 4.1: A Summary

Martin
Michlmayr

### Migration guide for GCC-3.2