Initializing a union with a non-trivial constructor

Question

I have a structure which I create a custom constructor to initialize the members to 0\'s. I\'ve seen in older compilers that when in release mode, without doing a memset to

Answer 1

AFAIK union members may not have constructors or destructors.

Question 1: no, there's no such guarantee. Any POD-member not in the constructor's initialization list gets default-initialized, but that's with a constructor you define, and has an initializer list. If you don't define a constructor, or you define a constructor without an initializer list and empty body, POD-members will not be initialized.

Non-POD members will always be constructed via their default constructor, which if synthesized, again would not initialize POD-members. Given that union members may not have constructors, you'd pretty much be guaranteed that POD-members of structs in a union will not be initialized.

Question 2: you can always initialize structures/unions like so:

struct foo
{
    int a;
    int b;
};

union bar
{
    int a;
    foo f;
};

bar b = { 0 };