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

Regarding

” Isn't there a way to define [the static data member] in the header?

Yes there is.

template< class Dummy >
struct BaseClass_statics
{
    static std::string bstring;
};

template< class Dummy >
std::string BaseClass_statics::bstring = ".";

class BaseClass
    : public BaseClass_statics
{};

An alternative is to use a function, as Dietmar suggested. Essentially that is a Meyers' singleton (google it).

Edit: Also, since this answer was posted we've got the inline object proposal, which I think is accepted for C++17.

Anyway, think twice about the design here. Globals variables are Evil™. This is essentially a global.