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Question about classes and RTTI

Started by UriKiller Jan 18, 2007 at 2:10 AM 5 replies 1k views

Original Post

Jan 18, 2007 at 2:10 AM

I've noticed that when I declare a static member inside a class, it will be the same for all the child classes too so if I change it through a child class it will also change for all the others. What I'm trying to do is to somehow give a static id to each class type in a class hierarchy. For example, CLavrador derives from CDog which derives from CAnimal and CCat also derives from CAnimal. I want each one of them to have some integer so I can during runtime check it. If I have a CAnimal *p which points to a CDog and I write p->id it will be the static id of CDog. If virtual data members in a class were allowed, this would be also an easy task because each class would just redeclare the id variable and give it a value in the constructor. [Edited by - UriKiller on January 18, 2007 3:04:35 AM]

Nepherim

Jan 18, 2007 at 2:23 AM

You might want to look into RTTI for this.

But a simple solution you could use is to give each class a virtual function that returns the type ID. So instead of p->id you'd have p->id(). The id method would simply return a single value (the id of the class) and you'd override the function for each class to give a different value.

UriKiller

Jan 18, 2007 at 2:28 AM

Thanks, the virtual methods is a good idea.

UriKiller

Jan 18, 2007 at 2:51 AM

Hmmmmm, I just checked the RTTI.
Something is not working when I use it.

class Base {};class Child : public Base {};void main (){	Base *base = new Base;	Child *child = new Child;	Base *pbase;	pbase = base;	const type_info &t1 = typeid (*pbase);	pbase = child;	const type_info &t2 = typeid (*pbase);}

after this, t1 is equal to t2, but I thought that the whole purpose of this is that it should make t2 contain the name of the Child class, because even though the pointer is a 'Base *' the actual class it is pointing to is Child.

Nitage

Jan 18, 2007 at 2:57 AM

Your base class has no virtual functions, so the typeid is resolved at compile time.

UriKiller

Jan 18, 2007 at 2:59 AM

But still why isn't the typeid returning a type_info with the Child class?

*And I just changed the first line of the code:
class Base {virtual void a (){}};
and there is a runtime error: "Unhandled exception in Kernel32.dll. Microsoft c++ exception."

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