question regarding virtual methods

[rippin]

I understand what a virtual function is:

class Base
{
public:
  virtual int DoSomething();
  int DoSomethingElse();
};

class Derived : public Base
{
public:
  int DoSomething();
  int DoSomethingElse();
};

int main(void)
{
  Derived* d = new Derived;
  Base* b = d;

  d->DoSomething();  // Calls Derived::DoSomething()
  b->DoSomething();  // Calls Derived::DoSomething()

  d->DoSomethingElse();  // Calls Derived::DoSomethingElse()
  b->DoSomethingElse();  // Calls Base::DoSomethingElse()
  d->Base::DoSomethingElse();  // Calls Base::DoSomethingElse()

  delete d;

  return 0;
}

My question is: Why wouldn't you make all functions virtual?
I am just learning C++ and this concept doesn't make alot of sense right now. Also, does a virtual function have to be overriden by a derived class???

Thanks for any help

[parksie]

Sometimes you want to call the function related to that pointer's type, or you may want to specifically call a base class function on a derived class. That's why there's a choice.

Virtual functions don't have to be overridden unless they're Pure Virtual Functions, the presence of which create an abstract class that can't be used, only derived from:

class Base {
public:
    int MyFunction(int x) = 0; // Pure virtual
};