Question

I googled const + extern on the internet, but it seems there isn’t really a good answer for my question.

const alone means internal linkage, but if I want to share a const variable among compilation units. Is extern the best choice?

Common solution would be:

//g.h
extern const int MAX;

// g.c
extern const int MAX = 3;

However, this solution has a drawback, like below:

// Say, I want to use this MAX in the same header file.
// g.h
extern const int MAX;
class AClass
{
public: 
    AClass (): i(MAX){}
private:
    int i;
};

Compiler will complain like:”error C2057: expected constant expression”.

Is there a solution for this?

Answer 1

if you want to be able to use your constant at compile time (i.e. size an array by it, without using VLA) it has to be known at compile time, i.e. it cannot have external linkage.

However, you could just declare your constant in your header file, and make it available to anyone including it. Still, that won’t have the exact same effect as an external linkage.

// a.h
const int MAX = 3;

// a.cpp
#include "a.h"
int b[a];