Re: [boost] Re: Virtual Constructor submission (a.k.a. ctor_fn())

From: Anthony Liguori (anthony_at_[hidden])
Date: 2003-05-19 20:12:23

>
>
>Is seems to be some unusual implementation of Factory Pattern. Could you
>please elaborate little more here ?
>  
>
    struct A {
       virtual int my_func() {
          return 3;
       }
    };

    struct B : public A {
       virtual int my_func() {
          return 4;
       }
    };

    B b;
    A &a = b;

    assert(a.my_func() == 4);

In the above exampe, my_func is a virtual method.  The caller only needs 
to know the interface of the base class and is able to class function of 
a class he knows absolutely nothing about.  This encapsulation allows a 
great number of design patterns to be implemented.

    struct A {
       A(int, int) {}
    };

    struct B : public A {
       B(int, int) {}
    };

    boost::function<A *(int, int)> func = boost::ctor_fn<B *(int, int)>();

    A *a = func(1, 2);

In this example, func is a virtual constructor.  The caller only needs 
to know the interface of the base class to create instances of its 
subclasses.

The factory pattern a much more specific usage of this concept.  The 
factory pattern allows for a single class to create any number of 
subclasses based on an inputted key.  One could use boost::ctor_fn to 
implement a factory, but it has many uses outside of factories.

One would use this library any time a class needed to be able to create 
objects but didn't know what particular type of object it needed to 
create at compile time.

Regards,
Anthony Liguori