C++11 multithreading with class member function

Question

I want to use multithreading in C++11 to call a class member function in its own thread. I have been able to get this to work with a global function:

#include <thread>
#include <iostream>

void Alpha(int x)
{
    while (true)
    {
        std::cout << x << std::endl;
    }
}

int main()
{
    std::thread alpha_thread(Alpha, 5);
    alpha_thread.join();

    return 0;
}

However, I cannot get it to compile with a class member function:

#include <thread>
#include <iostream>

class Beta
{
public:
    void Gamma(int y)
    {
        while (true)
        {
            std::cout << y << std::endl;
        }
    }
};

int main()
{
    Beta my_beta;
    std::thread gamma_thread(my_beta.Gamma, 5);
    gamma_thread.join();

    return 0;
}

The compile error is:

no matching function for call to 'std::thread::thread(<unresolved overloaded function type>)'
 std::thread gamma_thread(my_beta.Gamma, 5);
                                    ^

What am I doing wrong?

score 18 · Accepted Answer · answered Apr 20 '15 at 17:29

18

You need to pass two things: a pointer-to-member, and the object. You cannot call a non-static member function (like Gamma) in C++ without an object. The correct syntax would be:

std::thread gamma_thread(&Beta::Gamma, // the pointer-to-member
                         my_beta,      // the object, could also be a pointer
                         5);           // the argument

You can think of my_beta here as being the first argument to Gamma(), and 5 as the second.

answered Apr 20 '15 at 17:29

Barry

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If I want to create a `thread` inside the current object constructor, should I change `my_beta` to `this`? – Michel Feinstein Sep 27 '19 at 16:26
1

@mFeinstein If you're calling your own member function, yeah. – Barry Sep 27 '19 at 16:48

score 3 · Answer 2 · answered Apr 20 '15 at 17:30

3

You need to name the function, then pass the object on which to call it as an explicit implicit this parameter. :)

std::thread gamma_thread(&Beta::Gamma, my_beta, 5);

This is a bit of an abstraction leak, granted.

answered Apr 20 '15 at 17:30

Lightness Races in Orbit

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2

I chuckled at "explicit implicit `this`" – Barry Apr 20 '15 at 17:32

score 2 · Answer 3 · edited Apr 20 '15 at 17:39

You have multiple problems in your program

As your compile error says, you need to pass the address of the function &Beta::Gamma.
You need to pass the object as a parameter considering this is an implicit parameter of a member function

Modified source

#include <thread>
#include <iostream>

class Beta
{
public:
    void Gamma(int y)
    {
        while (true)
        {
            std::cout << y << std::endl;
        }
    }
};

int main()
{
    Beta my_beta;
    std::thread gamma_thread(&Beta::Gamma, my_beta, 5);
    gamma_thread.join();

    return 0;
}

C++11 multithreading with class member function

3 Answers3

Modified source

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