Reassigning std::bind using auto: which compiler gets it right?

February 24, 2023

MSVC compiles the following code, while GCC and Clang don’t.

#include <iostream>
#include <functional>

class Class
{
    public:
        void display() { std::cout << "display" << std::endl; }
        void store() { std::cout << "store" << std::endl; }
};

int main()
{
    Class instance;
    // std::function<void(void)> f; // this will work
    auto f = std::bind(&Class::display, instance);
    f();
    f = std::bind(&Class::store, instance);
    f();
}

_{Godbolt conformance view}

It seems that GCC and Clang have the copy assignment operator deleted.

Which compilers get it more correct from a standard point of view?

>Solution :

I don’t have the exact wording in the standard, but the compiler error together with cppreference give a pretty good idea of what is happening:

According to the standard, the implicitly defined copy constructor is deleted, if the class implements a move constructor. If a default copy constructor exists, it will not be deleted https://en.cppreference.com/w/cpp/language/copy_constructor.

The return type of std::bind is an unspecified copy-constructible type (if all arguments are copy-constructible) and move-constructible otherwise. https://en.cppreference.com/w/cpp/utility/functional/bind.

So I suppose all compilers are correct in having freedom for the unspecified part of the standard.

On a side note, I would capture the instance in a lambda instead of using std::bind.