Predefined macros are those that the compiler defines (in contrast to those user defines in the source file). Those macros must not be re-defined or undefined by user.
The following macros are predefined by the C++ standard:
__LINE__ contains the line number of the line this macro is used on, and can be changed by the #line directive.__FILE__ contains the filename of the file this macro is used in, and can be changed by the #line directive.__DATE__ contains date (in "Mmm dd yyyy" format) of the file compilation, where Mmm is formatted as if obtained by a call to std::asctime().__TIME__ contains time (in "hh:mm:ss" format) of the file compilation.__cplusplus is defined by (conformant) C++ compilers while compiling C++ files. Its value is the standard version the compiler is fully conformant with, i.e. 199711L for C++98 and C++03, 201103L for C++11 and 201402L for C++14 standard.__STDC_HOSTED__ is defined to 1 if the implementation is hosted, or 0 if it is freestanding.__STDCPP_DEFAULT_NEW_ALIGNMENT__ contains a size_t literal, which is the alignment used for a call to alignment-unaware operator new.Additionally, the following macros are allowed to be predefined by implementations, and may or may not be present:
__STDC__ has implementation-dependent meaning, and is usually defined only when compiling a file as C, to signify full C standard compliance. (Or never, if the compiler decides not to support this macro.)__STDC_VERSION__ has implementation-dependent meaning, and its value is usually the C version, similarly to how __cplusplus is the C++ version. (Or is not even defined, if the compiler decides not to support this macro.)__STDC_MB_MIGHT_NEQ_WC__ is defined to 1, if values of the narrow encoding of the basic character set might not be equal to the values of their wide counterparts (e.g. if (uintmax_t)'x' != (uintmax_t)L'x')__STDC_ISO_10646__ is defined if wchar_t is encoded as Unicode, and expands to an integer constant in the form yyyymmL, indicating the latest Unicode revision supported.__STDCPP_STRICT_POINTER_SAFETY__ is defined to 1, if the implementation has strict pointer safety (otherwise it has relaxed pointer safety)__STDCPP_THREADS__ is defined to 1, if the program can have more than one thread of execution (applicable to freestanding implementation — hosted implementations can always have more than one thread)It is also worth mentioning __func__, which is not an macro, but a predefined function-local variable. It contains the name of the function it is used in, as a static character array in an implementation-defined format.
On top of those standard predefined macros, compilers can have their own set of predefined macros. One must refer to the compiler documentation to learn those. E.g.: