Dual ABI
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## Dual ABI

In the GCC 5.1 release libstdc++ introduced a new library ABI that includes new implementations of `std::string` and `std::list`. These changes were necessary to conform to the 2011 C++ standard which forbids Copy-On-Write strings and requires lists to keep track of their size.

In order to maintain backwards compatibility for existing code linked to libstdc++ the library's soname has not changed and the old implementations are still supported in parallel with the new ones. This is achieved by defining the new implementations in an inline namespace so they have different names for linkage purposes, e.g. the new version of `std::list<int>` is actually defined as `std::__cxx11::list<int>`. Because the symbols for the new implementations have different names the definitions for both versions can be present in the same library.

The _GLIBCXX_USE_CXX11_ABI macro (see Macros) controls whether the declarations in the library headers use the old or new ABI. So the decision of which ABI to use can be made separately for each source file being compiled. Using the default configuration options for GCC the default value of the macro is `1` which causes the new ABI to be active, so to use the old ABI you must explicitly define the macro to `0` before including any library headers. (Be aware that some GNU/Linux distributions configure GCC 5 differently so that the default value of the macro is `0` and users must define it to `1` to enable the new ABI.)

Although the changes were made for C++11 conformance, the choice of ABI to use is independent of the `-std` option used to compile your code, i.e. for a given GCC build the default value of the _GLIBCXX_USE_CXX11_ABI macro is the same for all dialects. This ensures that the `-std` does not change the ABI, so that it is straightforward to link C++03 and C++11 code together.

Because `std::string` is used extensively throughout the library a number of other types are also defined twice, including the stringstream classes and several facets used by `std::locale`. The standard facets which are always installed in a locale may be present twice, with both ABIs, to ensure that code like `std::use_facet<std::time_get<char>>(locale);` will work correctly for both `std::time_get` and `std::__cxx11::time_get` (even if a user-defined facet that derives from one or other version of `time_get` is installed in the locale).

Although the standard exception types defined in `<stdexcept>` use strings, they are not defined twice, so that a `std::out_of_range` exception thrown in one file can always be caught by a suitable handler in another file, even if the two files are compiled with different ABIs.

### Troubleshooting

If you get linker errors about undefined references to symbols that involve types in the `std::__cxx11` namespace or the tag `[abi:cxx11]` then it probably indicates that you are trying to link together object files that were compiled with different values for the _GLIBCXX_USE_CXX11_ABI macro. This commonly happens when linking to a third-party library that was compiled with an older version of GCC. If the third-party library cannot be rebuilt with the new ABI then you will need to recompile your code with the old ABI.

Not all uses of the new ABI will cause changes in symbol names, for example a class with a `std::string` member variable will have the same mangled name whether compiled with the old or new ABI. In order to detect such problems the new types and functions are annotated with the abi_tag attribute, allowing the compiler to warn about potential ABI incompatibilities in code using them. Those warnings can be enabled with the `-Wabi-tag` option.

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