copy-list-initialization of non-copyable types

Question

12.6.1 - Explicit initialization

struct complex {
  complex();
  complex(double);
  complex(double,double);
};

complex sqrt(complex,complex);

         


        

Answer 1

The constructor-from-T is not explicit, and copy-list-initialization is not the same as copy-initialization. Both cause "constructors to be considered", but copy-initialization always "considers" the copy con­struc­tor, while list-initialization considers constructors with the list elements filled in (plus some details). To wit:

struct Foo
{
    Foo(int) {}
    Foo(Foo const &) = delete;
};

int main()
{
    Foo f = { 1 };  // Fine
}

(This would fail if the constructor was explicit. Also, Foo x = 1; will of course fail on account of the deleted copy constructor.)

Perhaps an even more enlightening use case:

Foo make() { return { 2 }; }

void take(Foo const &);
take(make());

Everything necessary for this is in 8.5.4/3 and in 13.3.1.7/1.