Attempting to reference a deleted function when using a mutex

Question

I'm getting a weird error working on a project. I've created a super simple example to recreate the error.

I've created a class. What I'd like to do in this class, is to have a sort of 'getter' function for my class that fills values of a struct. In the main application, the user would instantiate this struct, pass it to a member function, and be able to read the values in the struct upon return. Because of the design of the actual class, this has to happen in a separate thread. Here's what I have:

myClass.h:

#ifndef __MY_CLASS_H__
#define __MY_CLASS_H__

#include <mutex>

class myClass {
public:
    struct my_struct_s {
        int field1;
        short field2;
    };

    int get_data(my_struct_s & my_struct);

private:

};

#endif /* __MY_CLASS_H__ */

myClass.cpp:

#include "myClass.h"

int myClass::get_data(struct my_struct_s & my_struct)
{
    int var1 = 5;
    char var2 = 2;

    my_struct.field1 = var1;
    my_struct.field2 = var2;

    return 0;
}

Main.cpp:

#include "myClass.h"
#include <iostream>
#include <thread>
#include <Windows.h>

bool thread_running;
std::thread thread;

void run_thread(myClass & lmyClass)
{
    myClass::my_struct_s my_struct;

    while (thread_running) {
        lmyClass.get_data(my_struct);

        std::cout << my_struct.field1 << std::endl;
        std::cout << my_struct.field2 << std::endl;

        Sleep(100);
    }
}

int main(int argc, char *argv[])
{
    myClass lmyClass; 

    thread_running = true;
    thread = std::thread(run_thread, lmyClass);

    Sleep(1000);
    thread_running = false;

    if (thread.joinable()) {
        thread.join();
    }

    getchar();

    return 0;
}

It works as expected. However, because of the asynchronous nature of the class, I need mutexes to protect data being handled in different threads within the class.

If I add a std::mutext as a private member of my class, I receive the following when I try to run the code:

Error 1 error C2280: 'std::mutex::mutex(const std::mutex &)' : attempting to reference a deleted function ...

1) I'm trying to understand why I'm receiving this error.

2) (this part is a little more opinion-based), given the information, is this 'getter' way of filling out a public struct so that someone implementing my class can mess with the variables within it a good design? Is there a better way of doing it?

Answer 1

You don't have a copy constructor in myclass, so the compiler gives it a default copy constructor. This will attempt to copy all members, including your std::mutex, which is not copyable. As the compiler error says, it's marked as deleted.

You need to define your own copy constructor; most likely you'll want it to acquire the mutex stored in the instance being copied, then copy all of it's other members to the new instance.