error deleted copy const on move

Kai kai_pf@gmx.de
Wed May 12 21:01:46 GMT 2021


G'day!

Today I was confused by an error message, gcc produced on my code.
--- Example:
#include <utility>
class NoCopy
{
public:
    NoCopy( int i ) : ei(i) {}
    NoCopy( NoCopy const& ) = delete;
//    NoCopy( NoCopy&& ) = delete;
//    NoCopy( NoCopy&& ) = default;
    int const ei;
};
void func() {
    NoCopy orig{ 42 };
    auto movy = NoCopy{ std::move(orig) };
}
--- Example end.

gcc complains that I'm trying to call the explicitly deleted copy
constructor here (last line). I checked on compiler explorer that the
latest version shows the same error message; so do clang and msvc, by
the way.

While I'm quite certain, that the error message is technically OK
(because a rvalue ref may also bind to a const lvalue ref, as far as I
remember), it still is a bit confusing for the devloper that is trying
to figure out why he did wrong.

What I'm trying to do here is calling the move constructor. That one was
implicitly deleted by explicitly deleting the copy constructor.
Interestingly enough, when I _ex_plicitly delete the move constructor,
the error message changes to correctly hint to my mishap. (Of course, on
explicitly defaulting the move constructor, the code compiles.)

So, is it a dedicated choice to reference the copy constructor in the
error message, even when the caller offers an (explicit) rvalue? Or
wouldn't it be better, to hint to the implicitly deleted move constructor?

Cheers, Kai
--
"Too proud to beg, too dumb to steal"
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