Question

#include <string>
#include <iostream>
using namespace std;

struct foo {
    static const int X = 3;
    static char bar(const string &str) {
        // return str[X]; // this works
        return *(str.begin() + X); // this fails
    }
};

int main() {
    cout << foo::bar("abcdefg") << endl;
}

When compiling this, I get a linker error saying “undefined symbol foo::X”. If the previous line is uncommented instead, then it compiles. What makes it different between these two?

Answer 1

You need to define foo::X in addition to declaring it. Do it outside of foo.

const int foo::X;