Why does constexpr static member (of type class) require a definition?

==> See the full snippet code and compilation on coliru.

I have a LiteralType class filling constexpr requirements:

struct MyString
{
    constexpr MyString( char const* p=0, int s=0 ) : ptr(p), sz(s) {}
    constexpr char const* data() const { return ptr; }
    constexpr int         size() const { return sz;  }

    char const *ptr = 0;
    int  const  sz  = 0;
};

I use it as a constexpr static member variable:

struct Foo
{
    int foo() { return str_.size(); }

    constexpr static MyString str_{"ABC",3};
};

int main()
{
  Foo bar;
  return ! bar.foo();
}

But the linker says:

^{(Clang-3.5 and GCC-4.9)}

undefined reference to `Foo::str_'

I have to define the constexpr static member!

^{(I do not specify the constructor parameters)}

constexpr MyString Foo::str_;

However if the constexpr static member had been an int the member would not have to be defined outside the class definition. This is my understanding, but I am not sure...

Questions:

• Why int does not need to be defined outside the class declaration but MyString requires this?


• Is there a disadvantage to define a constexpr static member in a header file?
  
  (I provide my library as header files only)