All pre-processor switches and configurations are all gathered in the file c++config.h, which is generated during the libstdc++ configuration and build process, and included by files part of the public libstdc++ API. Most of these macros should not be used by consumers of libstdc++, and are reserved for internal implementation use. These macros cannot be redefined. However, a select handful of these macro control libstdc++ extensions and extra features, or provide versioning information for the API, and are able to be used.

All library macros begin with _GLIBCXX_ (except for versions 3.1.x to 3.3.x, which use _GLIBCPP_).

Below is the macro which users may check for library version information.

Below are the macros which users may change with #define/#undef or with -D/-U compiler flags. The default state of the symbol is listed.

“Configurable” (or “Not configurable”) means that the symbol is initially chosen (or not) based on --enable/--disable options at library build and configure time (documented here), with the various --enable/--disable choices being translated to #define/#undef).

ABI means that changing from the default value may mean changing the ABI of compiled code. In other words, these choices control code which has already been compiled (i.e., in a binary such as libstdc++.a/.so). If you explicitly #define or #undef these macros, the headers may see different code paths, but the libraries which you link against will not. Experimenting with different values with the expectation of consistent linkage requires changing the config headers before building/installing the library.