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-

Answer 1

To keep the definition of a static value with the declaration in C++11 a nested static structure can be used. In this case the static member is a structure and has to be defined in a .cpp file, but the values are in the header.

class BaseClass
{
public:
  static struct _Static {
     std::string bstring {"."};
  } global;
};

Instead of initializing individual members the whole static structure is initialized:

BaseClass::_Static BaseClass::global;

The values are accessed with

BaseClass::global.bstring;

Note that this solution still suffers from the problem of the order of initialization of the static variables. When a static value is used to initialize another static variable, the first may not be initialized, yet.

// file.h
class File {
public:
  static struct _Extensions {
    const std::string h{ ".h" };
    const std::string hpp{ ".hpp" };
    const std::string c{ ".c" };
    const std::string cpp{ ".cpp" };
  } extension;
};

// file.cpp
File::_Extensions File::extension;

// module.cpp
static std::set headers{ File::extension.h, File::extension.hpp };

In this case the static variable headers will contain either { "" } or { ".h", ".hpp" }, depending on the order of initialization created by the linker.