Re: Dynamic polymorphism vs. Static polymorphism
"Krivenok Dmitry" <dima@icebrains-soft.com> wrote in message
news:1149006864.596733.38880@i40g2000cwc.googlegroups.com...
Hello all!
Perhaps the most important feature of dynamic polymorphism is
ability to handle heterogeneous collections of objects.
("C++ Templates: The Complete Guide" by David Vandevoorde
and Nicolai M. Josuttis. Chapter 14.)
How to implement analogue of this technique via static polymorphism?
Perhaps there is special design pattern for this purpose...
Thanks!
All elements of a container must be of the same type. If it is a pointer or
smart pointer to a common base class we have dynamic polymorphism. Static
polymorphism can be accomplished using a flag to determine the actual type:
struct clumsy
{
int flag;
union (...} other_stuff;
};
You can have an array of these but you must set the flag to the right value
for each type of data stored in the union and go through a switch or similar
logic structure every time you use it. If you want to add a new type to the
union you have to find all the logic structures and update them, so this
scheme is not very maintainable.
A variation on this theme is:
struct bummer
{
int flag;
};
struct radical
{
int flag;
double value;
};
radical r;
r.flag = 2;
r.value = 76.3;
bummer *p = (bummer*) &r;
Now you can have an array of bummer pointers, check the flag, and cast to
the actual type. This has all the problems of the union version but might
save a little memory in some cases I suppose.
Void pointers offer a sort of static polymorphism. I sometimes use them at C
interfaces where they are actually pointing to C++ classes. If you cast
carefully that can work.
Also see boost::any.
All and all when I look over these alternatives I certainly appreciate
dynamic polymorphism.
Cy