Compile with:

clang++ var.cpp main.cpp

var.cpp

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// external linkage
int i = 0;

// internal linkage
// const int i = 0;

// external linkage
// extern const int i = 0;

// internal linkage
// static int i = 0;

main.cpp

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#include <iostream>

// refer to `i`
extern int i;
// may use
// extern const int i;

int main()
{
    std::cout << i;
}