static const double cannot have an in-class initializer. why is it so?

Question

The problem with the following code is static member of type "const double" cannot have an in-class initializer. Why is applicable only for a \'const double\'in th

Answer 1

The compiler offered me to use constexpr instead of const:

static_consts.cpp:3:29: error: ‘constexpr’ needed for in-class initialization of static data member ‘const double sample::md’ of non-integral type [-fpermissive]
static_consts.cpp:7:22: error: ‘constexpr’ needed for in-class initialization of static data member ‘const double sample::md’ of non-integral type [-fpermissive]

I've just accepted the offer:

class sample{
   static const char mc = '?';
   static constexpr double md = 2.2;
   static const bool mb = true;
};
const char sample::mc;
const bool sample::mb;
int main(){
}

And now it compiles just fine (C++11).