C++ how to call header-only library

I learnt C++17 way of dealing with header-only library, by adding inline keyword:

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

struct C {
    static const inline string N {"abc"};
};

int main() {
    cout << C::N << endl;
    return 0;
}

Running above piece of code returns abc as expected.

However, if I tried to use the pre-C++17 style as below:

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

struct C {
    static std::string& N() {
        static std::string s {"abc"};
        return s;
    }
};

int main() {
    cout << C::N << endl;
    return 0;
}

I got result of 1 rather than the expected abc. I'm wondering why and how to fix it?

The problem here is coming from the differences between your two 'N' declarations. In the first case static const inline string N {"abc"}; is a string variable. In the second case static std::string& N() is a function. This is why when you use C::N the first time it works, it is the same as using "abc" in its place because it is just a string variable. This is also why it does not work as expected in the second case: C::N is a function not a variable in the typical sense. So to run the function you must call it by using C::N() which does output a string.

Edit: See @1201ProgramAlarm's comment on your question for the reason you get a 1 when the function isn't called.