How do you check object's type.

This is a discussion on How do you check object's type. within the C++ Programming forums, part of the General Programming Boards category; In implementing a Radiosity renderer I am iterating over a list of objects with the following: Code: for (Quad* o ...

  1. #1
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    How do you check object's type.

    In implementing a Radiosity renderer I am iterating over a list of objects with the following:

    Code:
    for (Quad* o = scn.obj; o; o=o->next)
    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.

  2. #2
    Confused Magos's Avatar
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    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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  3. #3
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    You cannot check types directly.
    What about typeid():

    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.

  4. #4
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    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.

    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);      
    }
    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).

  5. #5
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    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.

  6. #6
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    What the heck is rtti? And, why should a standard language feature depend on a compiler setting?

  7. #7
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    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.

  8. #8
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    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.

  9. #9
    Super Moderator VirtualAce's Avatar
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    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.

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