c++ - Overload resolution: assignment of empty braces

Question

I wrote some code S s; ... s = {};, expecting it to end up the same as S s = {};. However it didn't. The following example reproduces the problem:

#include <iostream>

struct S
{
    S(): a(5) { }
    S(int t): a(t) {}

    S &operator=(int t)  { a = t; return *this; }
    S &operator=(S const &t) = default;

    int a;
};

int main()
{
    S s = {};

    S t;
    t = {};

    std::cout << s.a << '
';
    std::cout << t.a << '
';
}

The output is:

5
0

My questions are:

1. Why is operator=(int) selected here, instead of "ambiguous" or the other one?
2. Is there a tidy workaround, without changing S?

My intent is s = S{}; . Writing s = {}; would be convenient if it worked. I'm currently using s = decltype(s){}; however I'd prefer to avoid repeating the type or the variable name.

Answer 1

Why is operator=(int) selected here, instead of "ambiguous" or the other one?

{} to int is the identity conversion ([over.ics.list]/9). {} to S is a user-defined conversion ([over.ics.list]/6) (technically, it's {} to const S&, and goes through [over.ics.list]/8 and [over.ics.ref] first before coming back to [over.ics.list]/6).

The first wins.

Is there a tidy workaround?

A variation of the trick std::experimental::optional pulls to make t = {} always make t empty. The key is to make operator=(int) a template. If you want to accept int and only int, then it becomes

template<class Int, std::enable_if_t<std::is_same<Int, int>{}, int> = 0>
S& operator=(Int t) { a = t; return *this; }

Different constraints can be used if you want to enable conversions (you'd probably also want to take the argument by reference in that case).

The point is that by making the right operand's type a template parameter, you block t = {} from using this overload - because {} is a non-deduced context.

...without changing S?

Does template<class T> T default_constructed_instance_of(const T&) { return {}; } and then s = default_constructed_instance_of(s);count?