How to force a static member to be initialized?

Question

Consider this example code:

template
char register_(){
    return D::get_dummy(); // static function
}

template
struct Foo{
           


        

Answer 1

We can use a simple trick based on a declaration that must be instantiated with the class:

template<…>
struct Auto {
  static Foo foo;
  static_assert(&foo);
};
template<…> Foo Auto::foo=…;

Note that some compilers warn about the comparison to null; that can be avoided with &foo==&foo, (bool)&foo, or ((void)&foo,true) if needed.

Note also that GCC 9.0–9.2 don’t count this as an odr-use.