Compile Time Error: Union default constructor is deleted

Question

The following C++ code fails to compile. As far as I have looked into this problem, I understand that the problem is because the default constructor of the union has been deleted by the compiler. The online note says the following:

If a union contains a non-static data member with a non-trivial default constructor, the default constructor of the union is deleted by default unless a variant member of the union has a default member initializer.

struct A {
   int val;
   A() : val(0) {}
};

union B
{
   A a;
};

B b;

Why is the default constructor of struct A considered non-trivial? How do I work around this problem to make this code compile successfully?

possible duplicate of https://stackoverflow.com/questions/26572240/why-do-unions-have-a-deleted-default-constructor-if-just-one-of-its-members-does — Spinkoo, Feb 03 '19 at 15:21
I looked at this post before posting this question. However, I am sorry, I could not really understand how to fix the issue. — Rakesh Agarwal, Feb 03 '19 at 15:30
@Spinkoo maybe there is a slight difference. This question asks "What's the problem". And that one asks "Why this rule exists" — con ko, Feb 03 '19 at 15:31
Yeah, I am also wondering what is the easiest fix (if any) to the above issue. — Rakesh Agarwal, Feb 03 '19 at 15:32
am not sure if this fix is the easiest & the most correct but try declaring inside the union like this A a(); instead of A a — Spinkoo, Feb 03 '19 at 15:40
@Spinkoo: That declares a member function, of course. But with `{}`… — Davis Herring, Feb 03 '19 at 15:41
Forget the union, struct A contains an A. Is that even legal? — Michael Surette, Feb 03 '19 at 16:10

eerorika · Accepted Answer · 2019-02-03T15:43:08.453

0

Why is the default constructor of struct A considered non-trivial?

Because it is user-declared.

Examples of classes that do have a trivial constructor:

struct Trivial {
    int val;
};

struct Trivial2 {
    int val;
    Trivial2() = default;
};

As a bonus, this is non-trivial:

struct NonTrivial {
    int val;
    NonTrivial();
};
NonTrivial::NonTrivial = default;

If you want A::val to be zero-initialised however, you'll want to add a default member initialiser to the union:

union B {
   A a = {};
};

edited Feb 03 '19 at 15:43

answered Feb 03 '19 at 15:36

eerorika

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@DavisHerring I'm not sure what you refer to. Is it what I appended to the answer? – eerorika Feb 03 '19 at 15:46
That’s it (or just `A a{};`)—but it’s default-initialized, not zero-initialized, even though `val` does become 0. The important bit is that it makes `B` have a default constructor. – Davis Herring Feb 03 '19 at 16:48

Compile Time Error: Union default constructor is deleted

1 Answers1