design patterns - 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::doSomething ( ClassA* )
{
    if( /* parameter is of type base class */) {

    } else if { /* derived class */ ) {

    }
}

Maybe I could say that the derived class ClassB has some special capabilities. But how do I do that without changing the existing class ClassA ?

Answer 1

It's generally a very bad idea to switch on the exact type like that. By doing this, you are tightly coupling your method to derived classes of ClassA. You should use polymorphism. Introduce a virtual method in class A, override it in class B and simply call it in your method.

Even if I was forced to handle the functionality in the external function itself for some reason, I would do something like:

class ClassA { 
  public: virtual bool hasSpecificFunctionality() { return false; }
};

class ClassB : public ClassA {
  public: virtual bool hasSpecificFunctionality() { return true; }
};

ClassA* SomeClass::doSomething ( ClassA* arg )
{
    if (arg->hasSpecificFunctionality()) {

    } else {

    }
}