How to have static data members in a header-only library?

C++17 and above

Use inline static variables for non-dynamic initialization:

struct Foo
{
    inline static int I = 0;
};

And use function local static variables otherwise:

struct Foo
{
    static std::string& Bar()
    {
        static std::string S = compute();
        return S;
    }
};

C++14 and below

Use function local statics, as they are plain easier to use.

If for some reason you really wish for a static data member, then you can use the template trick:

template <typename T = void>
struct Foo
{
     static int I = 0; // inline initialization only for simple types.
};

template <typename T>
int Foo<T>::I;

On local statics

For resources which require dynamic initialization, it is best to use a local static.

The order in which file-scope or class-scope statics are dynamically initialized is undefined, in general, leading to the Static Initialization Order Fiasco when you try to read a uninitialized static as part of the initialization of another. Local static solve the issue by being initialized lazily, on first use.

There is some slight overhead to using local statics, however. From C++11 onwards, the initialization is required to be thread-safe, which typically means that any access is gated by an atomic read and well-predicted branch.

My own solution is to use a templated holder class, as static members work fine in templates, and use this holder as a base class.

template <typename T>
struct static_holder
{
    static T static_resource_;
};

template <typename T>
T static_holder<T>::static_resource_;

Now use the holder class:

class expensive_resource { /*...*/ };

class i_want_a_static_member : private static_holder<expensive_resource>
{
public:
    void foo()
    {
        static_resource_.bar();
    }
};

But as the name of the member is specified in the holder class, you can't use the same holder for more than one static member.

As of C++ 17. You can now use inline variables to do this:

static const inline float foo = 1.25f;