What are C++ functors and their uses?

Question

I keep hearing a lot about functors in C++. Can someone give me an overview as to what they are and in what cases they would be useful?

Answer 1

To add on, I have used function objects to fit an existing legacy method to the command pattern; (only place where the beauty of OO paradigm true OCP I felt ); Also adding here the related function adapter pattern.

Suppose your method has the signature:

int CTask::ThreeParameterTask(int par1, int par2, int par3)

We will see how we can fit it for the Command pattern - for this, first, you have to write a member function adapter so that it can be called as a function object.

Note - this is ugly, and may be you can use the Boost bind helpers etc., but if you can't or don't want to, this is one way.

// a template class for converting a member function of the type int        function(int,int,int)
//to be called as a function object
template
class mem_fun3_t
{
  public:
explicit mem_fun3_t(_Ret (_Class::*_Pm)(_arg1,_arg2,_arg3))
    :m_Ptr(_Pm) //okay here we store the member function pointer for later use
    {}

//this operator call comes from the bind method
_Ret operator()(_Class *_P, _arg1 arg1, _arg2 arg2, _arg3 arg3) const
{
    return ((_P->*m_Ptr)(arg1,arg2,arg3));
}
private:
_Ret (_Class::*m_Ptr)(_arg1,_arg2,_arg3);// method pointer signature
};

Also, we need a helper method mem_fun3 for the above class to aid in calling.

template
mem_fun3_t<_Ret,_Class,_arg1,_arg2,_arg3> mem_fun3 ( _Ret (_Class::*_Pm)          (_arg1,_arg2,_arg3) )
{
  return (mem_fun3_t<_Ret,_Class,_arg1,_arg2,_arg3>(_Pm));
}

Now, in order to bind the parameters, we have to write a binder function. So, here it goes:

template
class binder3
{
public:
//This is the constructor that does the binding part
binder3(_Func fn,_Ptr ptr,_arg1 i,_arg2 j,_arg3 k)
    :m_ptr(ptr),m_fn(fn),m1(i),m2(j),m3(k){}

 //and this is the function object 
 void operator()() const
 {
        m_fn(m_ptr,m1,m2,m3);//that calls the operator
    }
private:
    _Ptr m_ptr;
    _Func m_fn;
    _arg1 m1; _arg2 m2; _arg3 m3;
};

And, a helper function to use the binder3 class - bind3:

//a helper function to call binder3
template 
binder3<_Func, _P1, _arg1, _arg2, _arg3> bind3(_Func func, _P1 p1,_arg1 i,_arg2 j,_arg3 k)
{
    return binder3<_Func, _P1, _arg1, _arg2, _arg3> (func, p1,i,j,k);
}

Now, we have to use this with the Command class; use the following typedef:

typedef binder3 ,T* ,int,int,int> F3;
//and change the signature of the ctor
//just to illustrate the usage with a method signature taking more than one parameter
explicit Command(T* pObj,F3* p_method,long timeout,const char* key,
long priority = PRIO_NORMAL ):
m_objptr(pObj),m_timeout(timeout),m_key(key),m_value(priority),method1(0),method0(0),
method(0)
{
    method3 = p_method;
}

Here is how you call it:

F3 f3 = PluginThreadPool::bind3( PluginThreadPool::mem_fun3( 
      &CTask::ThreeParameterTask), task1,2122,23 );

Note: f3(); will call the method task1->ThreeParameterTask(21,22,23);.

The full context of this pattern at the following link