In implementing a Radiosity renderer I am iterating over a list of objects with the following:
Quad is a subclass of GeomObj. Since the scene I am rendering should only be comprised of Quads, this code should not crash in the context I need it, however it throws a warning during compilation. I'm just wondering how check the type of the object 'o' so I can just skip over any GeomObjs if they don't happen to be Quads.Code:for (Quad* o = scn.obj; o; o=o->next)
You cannot check types directly. You could use some kind of variable that is inherited:
Code:class GeomObj { ... bool IsQuad; } class Quad : public GeomObj { public: Quad() { IsQuad = true; } } class NonQuadOrWhatever : public GeomObj { public: NonQuadOrWhatever() { IsQuad = false; } }
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What about typeid():You cannot check types directly.
http://msdn.microsoft.com/library/de...d_operator.asp
I can't get it to work--not even MS's simple example--but according to the description, it seems like it should work. This is the error I get:
warning C4541: 'typeid' used on polymorphic type 'class Base' with /GR-; unpredictable behavior may result
...which I thought was the whole point.
Last edited by 7stud; 03-04-2005 at 04:06 PM.
magos, you actually can check types, with some restrictions though. By using typeid you can compare the type of two objects that have a virtual method.
tejj, you could use rtii. But you might also want to consider something like this.
You'd then pay the price of two virtual calls, comparative to the one virtual call and one comparison you'd have using typeid(which requires accessing the typeid/virtual table anyhow).Code:class GeomVisitor { public: virtual void visit(Quad* quad) = 0; virtual void visit(GeoObj* geomObj) = 0; }; // ... // class ScreenRender : public GeomVisitor { public: // ... // virtual void visit(Quad* quad); virtual void visit(GeoObj* geoObj); }; class GeomObj { public: virtual void accept(GeomVisitor* v) { v->visit(this); } }; class Quad : public GeomObject { public: virtual void accept(GeomVisitor* v) { v->visit(this); } }; // ... // ScreenRender render; for (Quad* o = scn.obj; o; o=o->next) { o->accept(&render); }
7stud, when using MSVC++ the rtti can be turned on in the project properties->C++->language page
Last edited by okinrus; 03-04-2005 at 04:23 PM.
What the heck is rtti? And, why should a standard language feature depend on a compiler setting?
Ok, after looking in VC6 help, I found out what rtti is: run time type information. Is it off by default because it slows things down?
Here is the example I was envisioning:
Code:#include <iostream> #include <typeinfo> using namespace std; class GeomObj { public: virtual void display() { cout<<"I am type GeomObj."<<endl; } }; class Quad : public GeomObj { public: virtual void display() { cout<<"I am type Quad."<<endl; } }; class NotQuad : public GeomObj { public: virtual void display() { cout<<"I am type NotQuad."<<endl; } }; int main() { GeomObj* array[2]; array[0] = &Quad(); array[1] = &NotQuad(); for(int i=0; i<2; i++) { if(typeid(*array[i])==typeid(Quad)) { array[i]->display(); } } return 0; }
Last edited by 7stud; 03-04-2005 at 06:13 PM.
There's some storage cost involved with using rtti. For each class, it must store the name of a class and the other data inside the type info. The virtual table, too, must have a pointer to the type info.
RTTI is extremely useful for debugging purposes and is useful for programs when you really must know the type of object - much of this is generic programming but RTTI enables you to check the actual type even if it is a user-defined class. Very handy at times.
