C++ – Start thread with member function

cc++11multithreading

I am trying to construct a std::thread with a member function that takes no arguments and returns void. I can't figure out any syntax that works – the compiler complains no matter what. What is the correct way to implement spawn() so that it returns a std::thread that executes test()?

#include <thread>
class blub {
  void test() {
  }
public:
  std::thread spawn() {
    return { test };
  }
};

Best Answer

#include <thread>
#include <iostream>

class bar {
public:
  void foo() {
    std::cout << "hello from member function" << std::endl;
  }
};

int main()
{
  std::thread t(&bar::foo, bar());
  t.join();
}

EDIT: Accounting your edit, you have to do it like this:

  std::thread spawn() {
    return std::thread(&blub::test, this);
  }

UPDATE: I want to explain some more points, some of them have also been discussed in the comments.

The syntax described above is defined in terms of the INVOKE definition (ยง20.8.2.1):

Define INVOKE (f, t1, t2, ..., tN) as follows:

  • (t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is an object of type T or a reference to an object of type T or a reference to an object of a type derived from T;
  • ((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is not one of the types described in the previous item;
  • t1.*f when N == 1 and f is a pointer to member data of a class T and t 1 is an object of type T or a
    reference to an object of type T or a reference to an object of a
    type derived from T;
  • (*t1).*f when N == 1 and f is a pointer to member data of a class T and t 1 is not one of the types described in the previous item;
  • f(t1, t2, ..., tN) in all other cases.

Another general fact which I want to point out is that by default the thread constructor will copy all arguments passed to it. The reason for this is that the arguments may need to outlive the calling thread, copying the arguments guarantees that. Instead, if you want to really pass a reference, you can use a std::reference_wrapper created by std::ref.

std::thread (foo, std::ref(arg1));

By doing this, you are promising that you will take care of guaranteeing that the arguments will still exist when the thread operates on them.

Note that all the things mentioned above can also be applied to std::async and std::bind.

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