#pragma once vs. #ifndef

#pragma once is NOT standard
But #ifndef is standard
#ifndef XXX_YYYY
#define XXX_YYYY


#endif // XXX_YYYY

In the compile-time #pragma once is faster.

https://stackoverflow.com/questions/1143936/pragma-once-vs-include-guards
https://beesbuzz.biz/code/1473-pragma-once-vs-ifndef-define