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Let's say I have class SuperClass { public: int a; } and class SubClass : SuperClass { public: int b; } and I took a pointer to an instance of the SubClass SubClass *subPointer and addressed that pointer to a SuperClass pointer SuperClass *superPointer = subPointer. Now of course I can always cast the superPointer object to a pointer of SubClass because the only thing it stores is an adress. But how would I know if the object superPointer is pointing to an instance of SubClass or is just a SuperClass pointer?

Monet R. Adams
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  • This can't be done in general, though RTTI will tell you in most cases if you compile with it. – Antimony Jul 12 '13 at 02:30
  • In addition to @Antimony's suggestions there are ways to build this knowledge into the objects themselves, but they generally require that everything inherits from some god class (yet another `TObject`, anyone?). Most people will try to dissuade you from doing this. – dmckee --- ex-moderator kitten Jul 12 '13 at 02:34
  • possible duplicate of [Finding the type of an object in C++](http://stackoverflow.com/questions/351845/finding-the-type-of-an-object-in-c) – jogojapan Jul 12 '13 at 02:36
  • As the proposed duplicate indicates, the easiest thing to do is to add a virtual destructor to `SuperClass`, and then test the result of `dynamic_cast(superPointer)`. – jxh Jul 12 '13 at 03:06
  • See it in action here: http://ideone.com/hUiIfE – paddy Jul 12 '13 at 03:10

2 Answers2

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You usually don't want to use typeid for this.

You usually want to use dynamic_cast instead:

if (SubClass *p = dynamic_cast<SubClass *>(SuperClassPtr))
    // If we get here (the `if` succeeds) it was pointing to an object of 
    // the derived class and `p` is now pointing at that derived object.

A couple of notes though. First of all, you need at least one virtual function in the base class for this to work (but if it doesn't have a virtual function, why are you inheriting from it?)

Second, wanting this very often tends to indicate design problems with the code. In most cases, you want to define a virtual function in the base class, which you (if necessary) override in the derived class to do whatever's needed so you can just use a pointer to the base class throughout.

Finally, as it stands right now, most of the conversions will fail -- you've used the default (private) inheritance, which prevents the implicit conversion from derived * to base * that you'd normally expect to see happen (you probably want class SubClass : public SuperClass).

Jerry Coffin
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Use RTTI machanism. Like:

if(typeid(*superPointer) == typeid(SuperClass)) superPointer->dosomething();
if(typeid(*superPointer) == typeid(SubClass)) superPointer->dosomethingelse();
lulyon
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    Have you tested this? I have come across issues where this returns the superPointer's information every time. – Sellorio Jul 12 '13 at 02:40