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_CONST_VAR 0xFF
2. int GLOBAL_CONST_VAR = 0xFF;
3. Some function returing the value (e.g. int get_GLOBAL_CONST_VAR())
4. enum { GLOBAL_CONST_VAR = 0xFF; }
5. const int GLOBAL_CONST_VAR = 0xFF;
6. extern const int GLOBAL_CONST_VAR; and in one source file const int GLOBAL_CONST_VAR = 0xFF;

Option (1) - is definitely not the option you would like to use

Option (2) - defining instance of the variable in each object file using the header file

Option (3) - IMO is over killing in most cases

Option (4) - in many cases maybe not good since enum has no concrete type (C++0X will add possibility to define the type)

So in most cases I need to choose between (5) and (6). My questions:

1. What do you prefer (5) or (6)?
2. Why (5) is ok, while (2) is not?

Answer 1

Definitely go with option 5 - it's type safe and allows compiler to optimize (don't take address of that variable :) Also if it's in a header - stick it into a namespace to avoid polluting the global scope:

// header.hpp
namespace constants
{
    const int GLOBAL_CONST_VAR = 0xFF;
    // ... other related constants

} // namespace constants

// source.cpp - use it
#include <header.hpp>
int value = constants::GLOBAL_CONST_VAR;