Check for derived type (C++)

Question

How do I check at runtime if an object is of type ClassA or of derived type ClassB? In one case I have to handle both instances separately

ClassA* SomeClass:         


        

Answer 1

Others have pointed out that switching on types is usually a bad idea, so I won't. If you really have to do it, you can use the typeid operator to switch on the dynamic type of an object:

ClassA* SomeClass::doSomething ( ClassA* a )
{
    if (typeid(*a) == typeid(ClassA)) {
        /* parameter is of type base class */
    } else if (typeid(*a) == typeid(ClassB)) {
        /* a specific derived class */ 
    } else {
        /* some other derived class */
    }
}

dynamic_cast is similar, but tests for convertibility, not equality:

ClassA* SomeClass::doSomething ( ClassA* a )
{
    if (ClassB *b = dynamic_cast(a)) {
        /* parameter is, or is derived from, ClassB */
    } else {
        /* parameter is, or is derived from, ClassA but not ClassB */ 
    }
}

These only work if ClassA is polymorphic (that is, it has at least one virtual function).