Unions in C++11: default constructor seems to be deleted

Question

I am trying to understand how unions were extended by C++11. One thing that changed is the ability to use now non-static data members with non-trivial special member functions.

Answer 1

The cppreference quote is unclear. What happens is that if ANY memeber of the union defines ANY of those non-trivial special member functions, then ALL of them will be deleted by default in the union.

So since you have a non-trivial destructor for X, the U default constructor is deleted.

Answer 2

X is not a pod type because is not trivially copiable as it have destructor Also U is not a pod type.

U s2; try to call the default costructor that is deleted so the error

U s1 {}; use member wise initialization and don't call any costructor

In union with non pod member the default costructor of union is deleted because it would call the default costructor of members ie the compiler doesn't know which member to call the default costructor

 Union XX{
   string m1; 
   vector <int> m2;
}

default costructor of XX cannot call default costructor of m1 AND m2 so it is deleted