Why does C++ implicit casting work, but explicitly casting throws an error (specific example)?

Question

I was trying to overload the casting operator in C++ for practice, but I encountered a problem and I can't figure out the issue. In the example, you can implicitly cast fine, but it causes an error when you try to explicitly cast.

struct B
{
    B() = default;
    B( B& rhs ) = default;
};

struct A
{
    operator B()
    {
        return B();
    }
};

int main()
{
    A a;
    B example = a;    //fine
    B example2 = static_cast<B>(a);    //error
}

The error is:

error C2440: 'static_cast': cannot convert from 'A' to 'B'

message : No constructor could take the source type, or constructor overload resolution was ambiguous

The problem only appears if you define the copy constructor in the B structure. The problem goes away, though, if you define the move constructor, too, or make the copy constructor take in a const B& ( B( const B& rhs ) ).

I think the problem is that the explicit cast is ambiguous, but I don't see how.

I was looking at a similar problem, here, but in that case I could easily see how the multiple options for casting led to ambiguity while I can't here.

Answer 1

static_cast<B>(a);

This expression is an rvalue, more loosely described as a temporary value.

The B class has no suitable constructor. The B( B &rhs) constructor is not suitable, mutable lvalue references don't bind to temporaries, hence the compilation failure.