Is a queue of lambdas a good design pattern for a work queue in C++11?

Question

I have to make a thread-safe work queue which can have work popped on in different threads and it will process it on a worker thread. The work can be very generic so I was thinking using lambdas with capture as a good way to allow this. I have the following code as a starter:

#include <iostream>
#include <vector>
#include <functional> 
typedef std::function<void()> Task;
typedef std::vector<Task> TaskQueue;

class Queue 
{
   public:
    void flush() {
        for (auto it : m_queue) {
            it();
        }
    }
   // will add thread safe locks later.        
    void queue(Task task) {
        m_queue.push_back(task);
    }

private:
    TaskQueue m_queue;
};
Queue q;

class WorkMaker
{
public: 

    WorkMaker(int inA) : m_a(inA) {}

    void MakeWork() {
        q.queue([&]{
            std::cout << this->m_a << std::endl;
        });
    }


private:
    int m_a;
};

int main()
{
  WorkMaker w1(1);
  WorkMaker w2(2);
  w1.MakeWork();
  w2.MakeWork();
  q.flush();
  return 0;
}

Is there something inherently unperformant about this code or will the compiler optimize it out? Also is passing a lambda into a std::function argument by value copying the lambda or just the pointer to it?

EDIT:

I think i can solve the problem of memory ownership by using shared_ptr's and passing them into the lambda instead. Consider the following modification:

typedef std::function<void()> Task;
typedef std::deque<Task> TaskQueue;

class Queue 
{
   public:
    void flush() {
        while (!m_queue.empty()) {
            auto it = m_queue.front();
            m_queue.pop_front();
            it();
        }
    }
   // will add thread safe locks later.        
    void queue(Task task) {
        m_queue.push_back(task);
    }

private:
    TaskQueue m_queue;
};
Queue q;


class WorkMaker : public std::enable_shared_from_this<WorkMaker>
{
public: 

    WorkMaker(int inA) : m_a(inA) {}
    ~WorkMaker() { std::cout << "Destroy " << m_a << std::endl;  }
    void MakeWork() {
        std::shared_ptr<WorkMaker> self = shared_from_this();
        q.queue([self]{
             std::cout << self->m_a << std::endl;
        });
    }
    int m_a;
};

int main()
{
  {
    auto w1 = std::make_shared<WorkMaker>(1);   
    auto w2 = std::make_shared<WorkMaker>(2);    
    w1->MakeWork();
    w2->MakeWork();
  }
  q.flush();
  return 0;
}

I get the desired output as :

1
Destroy 1
2
Destory 2

Answer 1

The std::function will make a private copy of the function pointer, lambda or whatever it refers to. Typically, this copy is referenced from the std::function object so that further copying is later avoided.

There is nothing particularly slow with using std::function objects this way. However, you should probably think about replacing the std::vector by a std::deque .