Question

suppose you have two (or more) classes with private member vectors:

class A {  
private:  
 std::vector<X> priv_vec;  
public:  
 //more stuff  
}  

class B {  
private:  
 std::vector<Y> priv_vec;  
public:  
 //more stuff  
}  

and you have a functor-class which has a state and works on a generic vector (does sorting or counts elements or something like that). The state of the functor is initialized by the first vector the functor is working on. If the functor is applied to another vector later, it will change its behavior depending on the state (sorts in the same way or trims the second vector after as many elements as the first one, etc)

What is the best way to implement such a functor (desgin-pattern or functional interface?) without exposing the private vectors to the other classes or the user of the classes?

for example:
The user would like to initialize this functor with an object of class A and then use this initialized functor for one or more objects of class B. The user isn’t able (and shouldn’t be) to use the private vectors directly as function-arguments for the functor.

Answer 1

Hum, first, beware on states in functors.

Most STL implementation of the algorithms may copy your functors around, therefore you generally have to extract the state in an outer structure.

Now, for the application of functors, well it is simple: have your classes declare a template member function!

class A
{
public:
  template <class Functor>
  Functor Apply(Functor f);

private:
};

class B
{
public:
  template <class Functor>
  Functor Apply(Functor f);
};

// Usage
MyFunctor f;
A a;
B b;
b.Apply(a.Apply(f));

As for the functor, if you need state:

// Alternative 1
class FunctorState {};

class Functor
{
public:
  Functor(FunctorState& state): m_state(state) {}

  // some operator()(...)

private:
  FunctorState& m_state;
};

// Alternative 2
class Functor
{
  struct FunctorState {};
public:
  Functor(): m_state(new FunctorState) {}

  // some operator()(...)

private:
  boost::shared_ptr<FunctorState> m_state;
};

This way, copies of the Functor all points to the same FunctorState instance. Just choose depending if you wish to actually access the state from outside the class or not.