Base class pointer vs inherited class pointer?

Question

Suppose I have a class Dog that inherits from a class Animal. What is the difference between these two lines of code?

    Animal *a         


        

Answer 1

The answer to this question is a giant: It depends

There are numerous ways in which the type of the pointer could become important. C++ is a very complex language and one of the ways it shows up is with inheritance.

Lets take a short example to demonstrate one of the many ways in which this could matter.

class Animal {
public:
  virtual void MakeSound(const char* pNoise) { ... }
  virtual void MakeSound() { ... }
};

class Dog : public Animal {
public:
  virtual void MakeSound() {... }
};

int main() {
  Animal* a = new Dog();
  Dog* d = new Dog();
  a->MakeSound("bark");
  d->MakeSound("bark"); // Does not compile
  return 0;
}

The reason why is a quirk of the way C++ does name lookup. In Short: When looking for a method to call C++ will walk the type hierarchy looking for the first type which has a method of the matching name. It will then look for a correct overload from the methods with that name declared on that type. Since Dog only declares a MakeSound method with no parameters, no overload matches and it fails to compile.