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) 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).

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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.