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/GCC 7 CPP

3.7 Predefined Macros

Several object-like macros are predefined; you use them without supplying their definitions. They fall into three classes: standard, common, and system-specific.

In C++, there is a fourth category, the named operators. They act like predefined macros, but you cannot undefine them.

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https://gcc.gnu.org/onlinedocs/gcc-7.3.0/cpp/Predefined-Macros.html