Re: Overriding base class

From:

Michael DOUBEZ <michael.doubez@free.fr>

Newsgroups:

comp.lang.c++

Date:

Wed, 25 Jul 2007 16:55:44 +0200

Message-ID:

<46a762ed$0$16398$426a74cc@news.free.fr>

James Emil Avery a ?crit :

Hi all,

I have a problem where

1. I have many derived classes of a base class.
2. The base class has many functions implementing common behaviour.
3. The code should be the same for all the derived classes, but the
    unctions depend on *static, constant data* that is specific to each
    erived class.
4. I cannot declare this static data virtual, because I must also be able
    to instantiate the base class; I.e. I must have the static data in the
    base class as a fall-back.

My problem: I override the data in the derived classes, but the common
functions still use the data from the base class.

I do not wish to duplicate the base-class functions to all the many
derived classes, because of the unnecessary blow-up of code size,
decreased readability and maintenance. I also want to keep the data
static, since memory requirements would otherwise drastically increase.

Is there any way in C++ that makes this setup possible? I.e. have code
that is shared between all derived classes, but which depends on data
that is specific to each derived class?

That looks like a job for the "Curiously recurring template pattern"
(CRTP). There is an extensive litteratur about it, just google for it.

Here is an example:
template <class Derived>
struct base
{
     int value()
     {
         return Derived::value;
     }

  //other function to factor out
  // as a bonus, you can call derived function
  // static_cast<Derived*>(this)->implementation();
  // this is static polymorphism
};

struct derived : base<derived>
{
     enum{value=1};
};

int main()
{
     derived a;

     std::cout<<a.value()<<std::endl;

     return 0;
}

Michael