Create thread inside class with function from same class

Question

I want to be able to define a class with some data members, and a function which has access to those data members, which are to be private.

I then want a public function, which creates some threads, which operate on the data members of the class. I am having some trouble getting my code to compile.

Don't worry about mutex or data protection, this isn't going to be a problem, since this is just some example code for testing.

class foo {
    public:
    void make_foo_func_threads();

    private:
    void foo_func();

    char private_data;
    std::vector<std::thread> some_threads;
}

void foo::foo_func() {
    while(1) {
        private_data = 'A';
    }
}

void foo::make_foo_func_thread() {
    for(...) some_threads.push_back(std::thread(foo_func));
    for(...) some_threads.join();
}

The compiler is giving me the error:

'no matching call to std::thread::thread()'

Apparently there is 'no known conversion for argument 1 from <unresolved overloaded function type> to void (foo::*&&)'.

Erm, yeah, I have no idea what that means apart from the compiler is having trouble understanding how to resolve foo_func - I think.

How can I help the compiler understand what I am trying to do, so it won't bother me with any more errors. No doubt the code I have written is not legal, and if that is the case could someone explain why that is the case to me. Thanks!

Answer 1

foo_func is a (non-static) member function, and it needs an instance of foo on which to operate. This instance must be provided to the thread constructor. If you refer to the std::thread::thread reference page it explains what code is executed in the new thread. The relevant point is that which refers to f being a pointer to member function:

• If f is pointer to a member function of class T, then it is called. The return value is ignored. Effectively, the following code is executed:
  • (t1.*f)(t2, ..., tN) if the type of t1 is either T, reference to T or reference to type derived from T.
  • ((*t1).*f)(t2, ..., tN) otherwise.

so it is clear that the instance is required.

Change to:

for(...) some_threads.push_back(std::thread(&foo::foo_func, this));

Simple example:

#include <iostream>
#include <thread>
#include <vector>

class foo
{
public:
    void make_foo_func_threads()
    {
        for (int i = 0; i < 5; ++i)
            some_threads.push_back(std::thread(&foo::foo_func, this));
        for (auto& t: some_threads) t.join();
    }

private:
    void foo_func() { std::cout << "Hello\n"; }
    std::vector<std::thread> some_threads;
};

int main()
{
    foo f;
    f.make_foo_func_threads();
}