I spend an hour wiggling down a large class I needed to pass a member function of to a minimal example. The compiler error given is:

error C2664: 'B::B(const B &)': cannot convert argument 1 from '_Ty' to 'const B &'

Below is a minimal example and the Code compiles fine when using a pointer to the mutex. Can somebody help me understand what went wrong here and why and how I could have gotten hints towards that from the error message in 'tuple'?

Thanks everyone!

#include <functional>
#include <iostream>
#include <string>
#include <mutex>


class A  {
public:
    std::string Init(std::function<std::string(void)> f) {
        return f();
    }
};

class B {
    std::mutex lock;
    std::string member = "Hello World!";

public:
    std::string foo() {
        return member;
    }
};

int main() {
    auto callback3 = std::bind(&B::foo, B());
    auto instance = A();
    std::cout << instance.Init(callback3) << "\n";
}

CodePudding user response：

A std::mutex is non-copyable, its copy constructor is deleted. There are no defined semantics for copying a mutex, what does that mean? If a mutex has locked something, does it mean that, somehow, two mutexes managed to lock the same object, the original and the copy, and both must be unlocked. For this reason a std::mutex is not copyable.

This makes B non-copyable too.

std::bind requires a copyable callable object to work with, which results in this compilation error.