How to fix GCC error: unknown type name ‘size_t’ (STM32)

Problem:

When you try to compile your C/C++ project (typically a STM32 project):

C:/Users/User/MyProject/MyHeader.h:9:7: error: unknown type name 'size_t'
    9 | const size_t MySize = 15;
      |       ^~~~~~

Solution:

At the top of the file where this error occurs, add the following line:

#include <stddef.h>