no matching constructor for initialization of 'std::thread'

Question

I have been working on a rather simple facility: a concurrent for loop construct that takes a list of input elements, an output vector, and a function that computes output elements out of input elements.

I have this snippet that does not compile:

            template<class In, class Out>
            void thread_do(net::coderodde::concurrent::queue<In>& input_queue,
                           Out (*process)(In in),
                           std::vector<Out>& output_vector)
            {
                // Pop the queue, process, and save result.
                ...
            }


                for (unsigned i = 0; i < thread_count; ++i) 
                {
                    thread_vector.push_back(std::thread(thread_do, 
                                                        input_queue,
                                                        process,
                                                        output_vector));
                }

I use -std=c++14.


./concurrent.h:129:45: error: no matching constructor for initialization of 'std::thread'
                    thread_vector.push_back(std::thread(thread_do, 
                                            ^           ~~~~~~~~~~

However, I have no idea how to fix it. Tried to prepend & to the thread_do/appending <In, Out>, yet no to avail.

thread_do is both a template function and a class method. The constructor of std::thread will need to reflect that. — Richard Hodges, Jul 04 '16 at 19:18
Woah, _way_ too much code. Where's the [MCVE] with which you've been debugging so far this month? — Lightness Races in Orbit, Jul 04 '16 at 19:29
_"However, I have no idea how to fix it. Tried to prepend & to the thread_do/appending , yet no to avail."_ You could try reading the documentation, which really should have been your first step instead of wildly guessing. — Lightness Races in Orbit, Jul 04 '16 at 19:30
@LightnessRacesinOrbit Next you suggest reading the entire standard? — coderodde, Jul 04 '16 at 19:36
Funkyfied the question a bit. If I can improve it, please tell me how. — coderodde, Jul 04 '16 at 19:41
Possible duplicate of [Start thread with member function](http://stackoverflow.com/questions/10673585/start-thread-with-member-function) — Stephan Dollberg, Jul 04 '16 at 22:05
@coderodde: For now I would be content if you looked up the functions you use in a reputable reference. That's the minimum you should be doing on a daily basis, and it is definitely a step you must take before asking us to do it for you :) — Lightness Races in Orbit, Jul 04 '16 at 22:09
@LightnessRacesinOrbit And yet, it would take me days to find out **`std::ref`**. — coderodde, Jul 05 '16 at 03:23
@coderodde: And think of all the other interesting things you'd learn while doing so. Looking through documentation for days is a _good_ thing. — Lightness Races in Orbit, Jul 05 '16 at 09:29

score 9 · Answer 1 · answered Jul 04 '16 at 19:23

This minimal, complete example (hint) shows you how to call a template member function in another thread.

#include <thread>

struct X
{

  template<class A, class B> void run(A a, B b)
  {
  }

  template<class A, class B>
  void run_with(A a, B b)
  {
    mythread = std::thread(&X::run<A, B>, this, a, b);
  }

  std::thread mythread;
};

int main()
{
  X x;
  x.run_with(10, 12);
  x.mythread.join();
}

Note that std::thread's constructor is not able to auto-deduce template arguments. You have to be explicit.

score 2 · Accepted Answer · answered Jul 04 '16 at 19:27

You need to instantiate your function:

thread_vector.push_back(std::thread(thread_do<In, Out>,     // you need to instantiate your template function 
                                    std::ref(input_queue),  // pass parameters by ref 
                                    std::ref(process),      // - // -
                                    std::ref(output_vector))// - // -
                                    );

no matching constructor for initialization of 'std::thread'

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