How to initialize static members in the header

Question

Given is a class with a static member.

class BaseClass { public:     static std::string bstring; }; 

String has obviously to be default-initialized outside of the class.

std::string BaseClass::bstring {"."}; 

If I include the above line in the header along with the class, I get a symbol multiply defined error. It has to be defined in a separate cpp file, even with include guards or pragma once.

Isn't there a way to define it in the header?

Answer 1

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.