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 configured 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, most 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.

One exception type does change when using the new ABI, namely `std::ios_base::failure`. This is necessary because the 2011 standard changed its base class from `std::exception` to `std::system_error`, which causes its layout to change. Exceptions due to iostream errors are thrown by a function inside `libstdc++.so`, so whether the thrown exception uses the old `std::ios_base::failure` type or the new one depends on the ABI that was active when `libstdc++.so` was built, not the ABI active in the user code that is using iostreams. This means that for a given build of GCC the type thrown is fixed. In current releases the library throws a special type that can be caught by handlers for either the old or new type, but for GCC 7.1, 7.2 and 7.3 the library throws the new `std::ios_base::failure` type, and for GCC 5.x and 6.x the library throws the old type. Catch handlers of type `std::ios_base::failure` will only catch the exceptions if using a newer release, or if the handler is compiled with the same ABI as the type thrown by the library. Handlers for `std::exception` will always catch iostreams exceptions, because the old and new type both inherit from `std::exception`.

Some features are not supported when using the old ABI, including:

- Using `std::string::const_iterator` for positional arguments to member functions such as `std::string::erase`.
- Allocator propagation in `std::string`.
- Using `std::string` at compile-time in `constexpr` functions.
- Class `std::chrono::time_zone` and all related APIs.
- The `<syncstream>` header.

### 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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