inline variables & Meyer Singleton

If we want to share a variable between multiple .cpp files we could do the following:

//.h:
extern int a;
//.cpp
int a = 1;

But this could be a bit messy, what if we wanted a header-only approach? This solution is called the Meyer’s Singleton approach:

//.h:
inline int& Instance()
{
    static int x = 1;
    return x;
}

Now, since C++17 we have a better solution, leveraging the inner workings to the compiler.

inline int x = 1;

We also had the same problem pre-C++17 in structs and classes:

//.h:
struct S
{
    static int x;
};

//.cpp:
int S::x = 1;

Now we can just do:

struct S
{
    inline static int x = 17;
};