Re: Identifiying which derived class you have in code
* Christopher:
I've seen various ways of doing this, but what I want is to be able to
take a base class pointer and know which derived class to cast to.
For example I am going to make a base light class, that has a position
and intensity.
A derived classes class may be a point light which also has a falloff
value.
The derived class has all the base class methods and data in common,
but has the additional falloff value get/set methods.
Now my rendering interface takes a base light class pointer
Use a reference, unless you're planning on supporting nullpointer argument.
, but the
body needs to know if it is a point light or not, so it can cast and
get that fall off value.
Ways I've seen this done:
1) dynamic cast and check for NULL;
Well, we could potentially be going through 10 dynamic casts or
more before we know what we have. I heard it was rather expensive to
do that.
2) Put a static const int identifier in every derived class and check
for each in a switch, then you know what to
cast to.
I don't know if this is really more efficient than 1. It also
requires keeping track of all these IDs and which are used already
when you make a new derived class.
3) Any other sugestions?
As I understand it the problem is how to optimize the rendering when the light
is point with no fall off.
Just let the base class provide identification that it is a point.
With that identification of point-ness in hand the rendering code can branch to
treatment as point or treatment as fall-off.
For the 9 or 10 classes with fall-off, provide the fall-off information through
a common interface.
This interface can be present also in the base class, but the rendering code
can, based on identification of point-ness, choose to use more efficient way then.
As always, remember to measure.
It might be that this attempt at optimization will have little, no or even
negative effect (such is difficult to predict, measurement is needed).
Cheers & hth.,
- Alf
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?