What are rules for a class static variable initialization?

Question

I have some legacy code and I need to add a new class for the message (which is irrelevant to my question). But it turns out that I need to declare an empty constructor in o

Answer 1

If you leave A constructor defaulted and never call it then there is no need to generate it and therefore no need to create test. If you explicitly default it at definition, call A constructor or access A::test it will be initialized properly.

12.1 Constructors [class.ctor]

7 A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used (3.2) to create an object of its class type (1.8) or when it is explicitly defaulted after its first declaration.

struct A : public Message
{
    A() = default; // no constructor is emitted unless A is instantiated

    A(); // declaration
};

A::A() = default; // explicit default definition

int
main()
{
    A a; // instantiation
    A::test; // just explicitly access test so it is initialized regardless of A constructor
}