Pragma once replaces #include guard
In the C and C++ programming languages, #pragma once is a non-standard but widely supported preprocessor directive designed to cause the current source file to be included only once in a single compilation. Thus, #pragma once serves the same purpose as include guards, but with several advantages, including: less code, avoidance of name clashes, and sometimes improvement in compilation speed. On the other hand, #pragma once is not necessarily available in all compilers and its implementation is tricky and might not always be reliable.