Defining global constant in C++

Question

I want to define a constant in C++ to be visible in several source files. I can imagine the following ways to define it in a header file:

1. #define GLOBAL_

Answer 1

(5) says exactly what you want to say. Plus it lets the compiler optimize it away most of the time. (6) on the other hand won't let the compiler ever optimize it away because the compiler doesn't know if you'll change it eventually or not.