Question

I have written the following wrapper for std::bind and std::queue:

#include "Queue.h"

template<class T>
Queue<T>::Queue(T* input)
{
    instance = input;
}

template<class T> template<typename... Args>
int Queue<T>::push(int (T::*func)(Args... args), Args... args)
{
    queue.push(std::bind(func, instance, args...));
    return queue.size();
}

template<class T>
int Queue<T>::pop()
{
    if(!queue.empty())
    {
        queue.front()();
        queue.pop();
        return queue.size();
    }
    return 0;
}

template<class T>
bool Queue<T>::empty()
{
    return queue.empty();
}

template<class T>
size_t Queue<T>::size()
{
    return queue.size();
}

with the following header:

#ifndef QUEUE_H_
#define QUEUE_H_

#include <functional>
#include <queue>

template <class T>
class Queue
{
    private:
        std::queue<std::function<void()>> queue;    /**< the messaging queue, appended to using enqueue(), popped from using dequeue() */
        T* instance;

    public:

        Queue(T*);

        template<typename... Args>
        int enqueue(int (T::*f)(Args... args), Args... args);

        int dequeue();

        bool empty();

        size_t size();
};

#endif

It allows me to add bound function expressions to a queue and pop them afterwards (queue->push<int>(&Object::jumpAround, 10); and queue->pop()). The problem is, I could not find a generic object- and function-pointer that enabled me to implement this without the <class T> template.

I know that using templates would probably be the safest and best approach here but due to the design of the code implementing this queue I need to get rid of it. Any ideas?

I guess it must be possible somehow because std::bind‘s first parameter can be any function and the second one can be any Object.

Answer 1

If I understand, below is what you require:

class Queue
{
private:
  std::queue<std::function<void()>> queue;    /**< the messaging queue, appended to using enqueue(), popped from using dequeue() */
public:

  // pass in the instance of the object and simply allow the compiler to deduce the function pointer nastiness...
  template<typename T, typename F, typename... Args>
  void enqueue(T instance, F func, Args... args)
  {
    queue.push(std::bind(func, instance, args...));
  }

  int dequeue()
  {
    if(!queue.empty())
    {
      queue.front()();
      queue.pop();
    }
  }
};

Oh and how to use it:

struct foo
{
  void bar(int a)
  {
    std::cout << "foo::bar: " << a << std::endl;
  }
};

struct bar
{
  void foo(int a, int c)
  {
    std::cout << "bar::foo: " << (a  + c)<< std::endl;
  }
};

int main(void)
{
  Queue q;
  foo f;
  bar b;
  q.enqueue(&f, &foo::bar, 10);
  q.enqueue(&b, &bar::foo, 10, 11);

  q.dequeue();
  q.dequeue();
}

Should output:

foo::bar: 10
bar::foo: 21

Or, even better, change your function signature and allow users to enqueue a std::function! This is the “normal” way (see for example, boost::asio::io_service::post.)

EDIT: Here is a simple example:

// Let the compiler do all the hard work for you..
template<typename T>
void enqueue(T f)
{
  queue.push(f);
}

Now to post any function to this queue…

// Here you are posting the functor itself...
q.enqueue(std::bind(&bar::foo, &b, 15, 12));