A conforming implementation of ISO C++ is required to document its choice of behavior in each of the areas that are designated “implementation defined”. The following lists all such areas, along with the section numbers from the ISO/IEC 14882:1998 and ISO/IEC 14882:2003 standards. Some areas are only implementation-defined in one version of the standard.
Some choices depend on the externally determined ABI for the platform (including standard character encodings) which GCC follows; these are listed as “determined by ABI” below. See Binary Compatibility, and http://gcc.gnu.org/readings.html. Some choices are documented in the preprocessor manual. See Implementation-defined behavior. Some choices are documented in the corresponding document for the C language. See C Implementation. Some choices are made by the library and operating system (or other environment when compiling for a freestanding environment); refer to their documentation for details.
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https://gcc.gnu.org/onlinedocs/gcc-6.3.0/gcc/C_002b_002b-Implementation.html