Null-defined header file in C + +

Null is not a C-language base type, which is defined in the Stddef.h file as the most basic language-dependent macro exists. However, with the development of C + +, many files will declare or include NULL as a standard macro as long as the system or standard operation is involved. So almost any C + + standard header file will introduce this macro. It is defined in the Cstddef file. In fact, the compiler has built-in null types, such as __null, etc.

Null-defined header file in C + +