std function constructor initialization

Question

I have started trying to learn std::function after reading Scott Meyers Effective C++. I made the following sample program to give a std::function object to a class

#include <iostream>
#include <string>
#include <functional>

class FlyBehaviour
{
  public :
  void fly()
  {
    std::cout<<"Flying";
    /* Work Here */
  }
};

typedef std::function<void(const FlyBehaviour&)> FlyFunc;

class Duck
{
  public :
  Duck(FlyFunc flyFunc) : _flyFunc(flyFunc){}

  private :

  FlyFunc _flyFunc;
};

int main()
{
  Duck(&FlyBehaviour::fly);
}

Compiling the above code gives me the following error

error: invalid use of qualified-name 'FlyBehaviour::fly'

On research, most of the internet points to using std::bind/boost::bind as a solution. Can someone please tell me how to apply it, especially where do I need to declare the object to which function is binded?

What solutions did your research reveal? You're really close but this has been asked so many times... "where do I need to declare the object" Wherever you like? — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jul 6, 2015 at 23:42

rlbond · Accepted Answer · 2015-07-06 23:55:16Z

2

You need to either bind a FlyBehaviour object to the std::function or supply a FlyBehavior when calling the function. Here is an example of the former (note that I changed the typedef!):

#include <iostream>
#include <string>
#include <functional>

class FlyBehaviour
{
  public :
  void fly()
  {
    std::cout<<"Flying";
    /* Work Here */
  }
};

typedef std::function<void()> FlyFunc;

class Duck
{
  public :
  Duck(FlyFunc flyFunc) : _flyFunc(flyFunc){}
  void run() {_flyFunc();}

  private :

  FlyFunc _flyFunc;
};

int main()
{
    FlyBehaviour f;
  Duck d(std::bind(&FlyBehaviour::fly, f));
  d.run();

Here, FlyBehaviour::fly's signature is void(FlyBehaviour&), but I used std::bind to bind a FlyBehaviour object to the first argument, making it a void() instead.

answered Jul 6, 2015 at 23:55

rlbond

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2 Comments

Pranav Kapoor Over a year ago

Thanks a lot for the answer. In my earlier typedef, was my signature then void (FlyBehaviour&, FlyBehaviour&) ?

rlbond Over a year ago

No, your original typedef would work, but you would need to pass a FlyBehaviour object to call the function.

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