Why do unions have a deleted default constructor if just one of its members doesn't have one?

Question

N3797::9.5/2 [class.union] says:

If any non-static data member of a union has a non-trivial default constructor (12.1), copy c

Answer 1

In your example, the problem is not that your code has no non trivial defautl constructor, but that it has a copy constructor.

But generally, a union has several members: "at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time" (9.5/1).

Suppose you have a union with several members some of them having non trivial constructors or copy constructors:

union W {
    A a; 
    int i; 
};

When you would create an object:

W w;  

how should this object be default constructed ? Which member should be the active one ? How should such an object be default-copied ? Is it the A or the int that should be constructed/copied ?

This is why the standard foresses that your union has a deleted default constructor (copy constructor in your case). It should be sufficient to user-provide the missing default function.

union W
{
    int i;
    A a;
    W() { /*...*/ }
    W(const W&c) { /*...*/ }
};

This paper explains the rationale and the wording of C++11 on the topic in its full details.

Important remark: Unfortunately, unrestricted unions are not supported by MSVC13 : it still does not accept ANY member having ANY of the non-trivial default function user defined. GCC accepts it since 4.6 and clang since 3.1.