Re: polymorphism; dummy parameters

"Victor Bazarov" <>
Thu, 3 May 2007 16:26:55 -0400
vsgdp wrote:

Say you have 3 child classes A, B, C deriving from a parent class P
with pure virtual function f.

Suppose A::f(x1); // A::f depends only on x1
Suppose B::f(x1, x2); // B::f depends on x1 and x2
Suppose C::f(x1, x2, x3); // C::f depends on x1, x2, x3

Is it bad form to define P::f(x1, x2, x3), and then A::f and B::f just
ignore the parameters it does not need?

Yes. But you'll be better off asking about it in 'comp.object'.
Here, in the multi-paradigm, "do as I please as long as it's legal",
land, it's perfectly OK, if it works for you.

In some sense this could be justified; you could say that for A::f, x2
and x3 are just "zero" or some identity value that doesn't actually
affect the output.

Any thoughts on this?

Not many, no. Depends on the problem you're solving, I guess.

I think it is okay occasionally if not abused often, but I would like
the opinion of the C++ programming community.

For those that want some context, basically I have a Light class and I

DirectionalLight, PointLight, SpotLight, AreaLight. The area light
implementation requires some special data the others do not; this data
also varies over time so I can't just set it at construction time.

So, it seems that you need a specific "set my stuff" function, which
should probably be very generic and implement some kind of "property"

can't set the area light's specific data at runtime either without
knowing its specific type (defeats polymorphism). So I am thinking
the best solution would be to just pass the data to the pure virtual
function, even though only area lights will use the data.

Take a look at "properties". If you give your properties names, by
which you will be able to differenciate them, you will be able to set
them independently. You can have a single 'set(string, void*)' function
or add another 'set_many(vector<string>,vector<void*>)'.

I am not sure it's better than stuffing the base class with the arguments
it's not going to use (that's true for any pure virtual function, no?);
at least it's a bit more generic (and you can probably find some more
or less generalized implementations out there in the open).

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