Defining global constant in C++

(5) is "better" than (6) because it defines GLOBAL_CONST_VAR as an Integral Constant Expression (ICE) in all translation units. For example, you will be able to use it as array size and as case label in all translation units. In case of (6) GLOBAL_CONST_VAR will be an ICE only in that translation unit where it is defined and only after the point of definition. In other translation units it won't work as ICE.

However, keep in mind that (5) gives GLOBAL_CONST_VAR internal linkage, meaning that the "address identity" of GLOBAL_CONST_VAR will be different in each translation unit, i.e. the &GLOBAL_CONST_VAR will give you a different pointer value in each translation unit. In most usage cases this doesn't matter, but if you'll need a constant object that has consistent global "address identity", then you'd have to go with (6), sacrificing the ICE-ness of the constant in the process.

Also, when the ICE-ness of the constant is not an issue (not an integral type) and the size of the type grows larger (not a scalar type), then (6) usually becomes a better approach than (5).

(2) is not OK because the GLOBAL_CONST_VAR in (2) has external linkage by default. If you put it in header file, you'll usually end up with multiple definitions of GLOBAL_CONST_VAR, which is an error. const objects in C++ have internal linkage by default, which is why (5) works (and which is why, as I said above, you get a separate, independent GLOBAL_CONST_VAR in each translation unit).

Starting from C++17 you have an option of declaring

inline extern const int GLOBAL_CONST_VAR = 0xFF;

in a header file. This gives you an ICE in all translation units (just like method (5)) at the same time maintaining global address identity of GLOBAL_CONST_VAR - in all translation units it will have the same address.


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;

(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.

Tags:

C++

Recent Posts