When to use #define?

Question

for ex- #define PI 3.14

Answer 1

https://stackoverflow.com/questions/6274008/why-would-someone-use-define-to-define-constants?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

But also useful to stop a .hpp or .h file being processed more than once.

#ifndef __fred__h__
#define __fred__h__

// put innards of .h file here.

#endif