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;
}