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If I understand correctly, typeid can determine the actual type in polymorphism, while typeof cannot.

Is it also true that their returns are used for different purposes: the return of typeof is used as type keyword that can define variable, but the return of typeid cannot?

Is there any way to both get the actual type for polymorphism and use the return as type keyword to define another variable? I hope to get the derived class type from a pointer pointing to the base class and define a variable of or a pointer to the derived class. Something like:

baseclass *p = new derivedclass typexxx(*p) *pp = dynamic_cast<typexxx(*p) *> (p); // would like to convert the pointer from pointing to a base class // to its derived class

Thank you very much!

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    Can you give an example of WHY you want to do this? The main point of an inheritance hierarchy is that you normally don't need to know the actual type. Commented Dec 31, 2009 at 22:17
  • Just want to refer to some members defined in the derived class but not in the base class. I am not sure if it is a good idea to add member functions to the base class with same name as those of the derived class and declare them as virtual, since those member functions are too specific to that particular derived class and not required in other derived classes, and the base function will be bigger if I add them to it. Commented Dec 31, 2009 at 22:30
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    OK, you can do what you want with dynamic_cast, but you will need to use an if-ladder to create the derived type pointers. Commented Dec 31, 2009 at 22:39
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    What's the big plan? I don't mean parsing arguments, more abstract than that. Script something? Commented Dec 31, 2009 at 23:12

2 Answers 2

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c++0x will have decltype which can be used like this:

int someInt; decltype(someInt) otherIntegerVariable = 5;

but for plain old c++, unfortunately, no.

I suppose that decltype won't really be much help either though since you want the polymorphic type, not the declared type. The most straight forward way to do what you want is to attempt to dynamic cast to a particular type and check for NULL.

struct A { virtual ~A() {} }; struct B : public A {}; struct C : public A {}; int main() { A* x = new C; if(B* b_ptr = dynamic_cast<B*>(x)) { // it's a B } else if(C* c_ptr = dynamic_cast<C*>(x)) { // it's a C } }
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3 Comments

Thanks Evan! But sometimes I need to determine the type at runtime such as via command line argument. See my post here stackoverflow.com/questions/1984492/…, you will understand.
if (B* b_ptr = dynamic_cast<B*>(x)) and no need for the either the union or mis-scoped variables. Plus you can write more natural else if instead of else { ... if ... }
@Roger: you learn something new every day, I was unaware that you could declare variable inside an if statement. and result in appropriate scope. I'll update my answer.
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Assuming a hierarchy A <- B <- C

A * p = new AorBorC; // create new object of some sort if ( dynamic_cast <C*>(p) ) { C * c = dynamic_cast <C*>(p); c->CFunc(); } else if ( dynamic_cast <B*>(p) ) { B * b = dynamic_cast <B*>(p); b->BFunc(); } else if ( dynamic_cast <A*>(p) ) { A * a = dynamic_cast <A*>(p); a->AFunc(); }

Where AFunc, BFunc, CFunc are specific to their respective classes, and not virtual. Obviously, this can be optimised somewhat.

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2 Comments

if (C* c = dynamic_cast<C*>(p)) { /*use c*/ } No need to cast twice.
Quite - I was trying to make things clearer by being long-winded.

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