Why is std::pair from anonymous object copying that object instead of moving?

Question

I tried to make std::pair with this style:

#include <iostream>

struct A {
    A() noexcept {
        std::cout << "Created\n";
    }
    A(const A&) noexcept {
        std::cout << "Copy\n";
    }
    A(A&&) noexcept {
        std::cout << "Move\n";
    }
};

int main() {
    std::pair<A, A> a{ {},{} };
    return 0;
}

and got such output:

Created
Created
Copy
Copy

instead of

Created 
Created
Move
Move

But if I define my anonymous object type (e.g. std::pair<A, A> a{A{}, A{}}) or use std::make_pair<A, A>({}, {}) I get right result.

std::pair constructor must use std::forward<U1> and std::forward<U2> to initialize objects, thus I think that my pair uses wrong constructor. Why?

Which compiler are you using? Which version of the compiler? What flags and options do you pass to the compiler when you build? — Some programmer dude, Oct 25 '20 at 19:58
I didn't use any optimization, I compile it on clang and msvc (std::c++17 both) — Shamil Mukhetdinov, Oct 25 '20 at 19:59

score 55 · Accepted Answer · answered Oct 25 '20 at 20:01

There's a problem with how std::pair is defined. I'd even say it's a minor defect in the standard.

It has two constructors that could be used here:

pair(const T1 &x, const T2 &y);, where T1, T2 are template parameters of the pair.
template <class U1, class U2> pair(U1 &&x, U2 &&y);

If you do std::pair<A, A> a{A{}, A{}});, then the second constructor is selected and all is well.

But if you do std::pair<A, A> a{{}, {}};, the compiler can't use the second constructor because it can't deduce U1, U2, because {} by itself has no type. So the first constructor is used and you get a copy.

For it to work properly, std::pair should have an extra non-template constructor pair(T1 &&, T2 &&), and for a good measure two extra constructors: pair(const T1 &, T2 &&) and pair(T1 &&, const T2 &).

Reminds me of Herb Sutter's Twitter question for the CppCon talk: https://twitter.com/herbsutter/status/1305292734966558721 ‍♂️ — Paul, Oct 26 '20 at 16:23
I would rather it have a different one instead of those two it has: `template pair(U1&& x, U2&& y)`. That change might necessitate a new deduction guide though. — Deduplicator, Oct 27 '20 at 10:21

Why is std::pair from anonymous object copying that object instead of moving?

1 Answers1