How to initialize static members in the header

In C++17 you can use inline variables, which you can use even outside classes.

The inline specifier, when used in a decl-specifier-seq of a variable with static storage duration (static class member or namespace-scope variable), declares the variable to be an inline variable.

A static member variable (but not a namespace-scope variable) declared constexpr is implicitly an inline variable.⁽¹⁾

For example:

class Someclass {
public:
    inline static int someVar = 1;
};

Or,

namespace SomeNamespace {
    inline static int someVar = 1;
}

⁽¹⁾ https://en.cppreference.com/w/cpp/language/inline

You can't define a static member variable more than once. If you put variable definitions into a header, it is going to be defined in each translation unit where the header is included. Since the include guards are only affecting the compilation of one translation unit, they won't help, either.

However, you can define static member functions! Now, at first sight that may not look as if it could help except, of course, that function can have local static variable and returning a reference to one of these behaves nearly like a static member variable:

static std::string& bstring() { static std::string rc{"."}; return rc; }

The local static variable will be initialized the first time this function is called. That is, the construction is delayed until the function is accessed the first time. Of course, if you use this function to initialize other global objects it may also make sure that the object is constructed in time. If you use multiple threads this may look like a potential data race but it isn't (unless you use C++03): the initialization of the function local static variable is thread-safe.