strategy pattern and non-public virtual functions

[Maric Michaud]

Le Lundi 05 Juin 2006 16:07, pythoncurious at gmail.com a écrit :
> class Base {
>   public:
>     void f() { this->f_(); }
>   private:
>     virtual void f_() = 0;
> };
>
> class Derived : public Base {
>   private:
>     void f_() { // Do something }
> };
>
> int main() {
>     Derived d;
>     d.f();
> }

This is just polymorphism, not strategy pattern, and I would expect f_ to be 
protected here not private.

You want to ensure derived class will use a given method in  the Base class, 
this could be done explicit with a good naming convention as Duncan said, but 
here is a strategy pattern to ensure a sanity check for example :

class strategyBase(object) :
        def __call__(self, *sa) : raise NotImplementedError('abstract class')

class InputProcessor(object) :

    def sanitize(self, *a) :
	return a

    def f(self, *a) :
        sa = self.sanitize(*a)
        return self.strategy(*sa)

    def __setStrategy(self, strat) :
            if not isinstance(strat, strategyBase) :
                    raise ValueError("strat must be of type strategyBase")
            self.__strat = strat

    strategy = property(fget=lambda s : s.__strat, fset=__setStrategy)

The main purpose of this is to define a common API for all Strategies, and 
this is really useful if you intend to manage many of them.

-- 
_____________

Maric Michaud
_____________

Aristote - www.aristote.info
3 place des tapis
69004 Lyon
Tel: +33 426 880 097