How are member unions containing primitives and objects initialized in C++?

Question

Say I have:

class Foo {
  union {
     double a;
     std::string b;
  };
};

What will the default constructor generated by the compiler do? My understanding of the C++ standard is that primitives are not initialized but objects are. So what happens here?

Answer 1

The default constructor (as well as the copy constructor, assignment operator and destructor) for the union is implicitly declared deleted, and so is the constructor of Foo . You'll see this once you try to actually create an instance of Foo .