How to create a thread inside a class?

Question

class MyClass
{
    public:
        friend void function(MyClass& mc)
        {
            std::cout << "Friend function from thread" << std::endl;
        }    
        void init()
        {
            thr = std::thread(function, this);
            thr.join();
        }

    private:
        std::thread thr;

};

  int main()
   {
    std::cout << "This is main function" << std::endl;
    MyClass nc;
    nc.init();

    return 0;
   }

Error C2065 'function': undeclared identifier

How to create thread inside a class not using any static function?

Answer 1

I do not know why the lookup of your friend function does not work in this context, maybe someone else knows.
But the fastest way to archive what you want is either a lamdba or declare your function.
E.g.

class MyClass;
void function(MyClass& mc);
class MyClass
{
public:
    friend void function(MyClass& mc)
    ...
    void init()
    {
        // either do this
        thr = std::thread([this](){function(*this);});
        // or this note the std::ref. You are passing a reference. Otherwise there will be a copy
        thr = std::thread(&function, std::ref(*this));
        thr.join();
    }

private:
    std::thread thr;

};
....

Answer 2

Well @mkaes beat me to the answer, but I had slightly modified your function() to accept a pointer to the class and not a reference.

• You cannot access a friend function inside a class like you can access other member functions.

  1. The problem was that your friend function had no global declaration even though, you were declaring it as a friend function in the class.
  2. So we define the function outside the class and leave a friend declaration inside.
  3. Now since the function() doesn't know about the class MyClass, we have to forward declare the MyClass class also.

code:

#include <iostream>
#include <thread>

class MyClass;

void function(MyClass* mc)
{
    std::cout << "Friend function from thread" << std::endl;
}

class MyClass
{
public:
    void init()
    {
       thr = std::thread(function, this);
       thr.join();
        // function(this);
    }
    friend void function(MyClass* mc);
private:
    std::thread thr;

};

int main()
{
    std::cout << "This is main function" << std::endl;
    MyClass nc;
    nc.init();

    return 0;
}

Output:

This is main function
Friend function from thread

EDIT:

As per the discussion in the comments, my first code posted here had the problem that you could not call a member function or access a member variable inside the friend function(), since the class was defined afterwards. To address that here is the alternative below. But anyways, @mkaes has already answered it in this way from the beginning.

#include <iostream>
#include <thread>

class MyClass;
void function(MyClass* mc);

class MyClass
{
public:
    void init()
    {
       thr = std::thread(function, this);
       thr.join();
        // function(this);
    }
    friend void function(MyClass* mc)
    {
        std::cout << "Friend function from thread" << std::endl;
        mc->test();
    }
    int test(){}
private:
    std::thread thr;

};



int main()
{
    std::cout << "This is main function" << std::endl;
    MyClass nc;
    nc.init();

    return 0;
}