Explain me about std::function handling by class member method

Question

#include <iostream>
#include <functional>

class world
{
public:
    void hello(){
        std::cout << "wow" << std::endl;
    }   
};
int main(void)
{
    std::function<world&,void()> pFn = &world::hello; //is this right???
};

I tried this, but it doesn't work :( how to handle it? please explain me cool way

Answer 1

First, std::function is:

template< class >
class function; /* undefined */
template< class R, class... Args >
class function<R(Args...)>;

Your std::function<world&,void()> doesn't make sense. You are supposed to pick the function type you want to store in the function object.

Next, you need a world to call one of its non-static member functions.

What you can do is the following:

#include <iostream>
#include <functional>

class world
{
public:
    void hello(){
        std::cout << "wow" << std::endl;
    }   
};
int main(void)
{
    //std::function<world&,void()> pFn = &world::hello; //is this right???    
    world w;
    std::function<void()> pFn = [&w](){ w.hello();};
    pFn();
}

You need to make sure w is still alive when you call pFn(). Alternatively you could make pFn take a world as parameter, in either case you need an instance to call a non-static member function.

PS: Note that in the above example there is really no point in using the std::function when you can use the lambda itself. std::function comes for a price that often you do not need to pay.