Macros
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## Macros

All library macros begin with `_GLIBCXX_`.

Furthermore, all pre-processor macros, switches, and configuration options are gathered in the file `c++config.h`, which is generated during the libstdc++ configuration and build process. This file is then included when needed by files part of the public libstdc++ API, like <ios>. Most of these macros should not be used by consumers of libstdc++, and are reserved for internal implementation use. These macros cannot be redefined.

A select handful of macros control libstdc++ extensions and extra features, or provide versioning information for the API. Only those macros listed below are offered for consideration by the general public.

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

`__GLIBCXX__`

  The current version of libstdc++ in compressed ISO date format, form of an unsigned long. For details on the value of this particular macro for a particular release, please consult this document.

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.

`_GLIBCXX_USE_DEPRECATED`

  Defined by default. Not configurable. ABI-changing. Turning this off removes older ARM-style iostreams code, and other anachronisms from the API. This macro is dependent on the version of the standard being tracked, and as a result may give different results for `-std=c++98` and `-std=c++0x`. This may be useful in updating old C++ code which no longer meet the requirements of the language, or for checking current code against new language standards.

`_GLIBCXX_FORCE_NEW`

  Undefined by default. When defined, memory allocation and allocators controlled by libstdc++ call operator new/delete without caching and pooling. Configurable via `--enable-libstdcxx-allocator`. ABI-changing.

`_GLIBCXX_CONCEPT_CHECKS`

  Undefined by default. Configurable via `--enable-concept-checks`. When defined, performs compile-time checking on certain template instantiations to detect violations of the requirements of the standard. This is described in more detail here.

`_GLIBCXX_DEBUG`

  Undefined by default. When defined, compiles user code using the debug mode.

`_GLIBCXX_DEBUG_PEDANTIC`

  Undefined by default. When defined while compiling with the debug mode, makes the debug mode extremely picky by making the use of libstdc++ extensions and libstdc++-specific behavior into errors.

`_GLIBCXX_PARALLEL`

  Undefined by default. When defined, compiles user code using the parallel mode.

`_GLIBCXX_PROFILE`

  Undefined by default. When defined, compiles user code using the profile mode.

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