c++: explain this function declaration

Question

class PageNavigator {
 public:
  // Opens a URL with the given disposition.  The transition specifies how this
  // navigation should be recorded in the history system (for example, typed).
  virtual void OpenURL(const GURL& url, const GURL& referrer,
                       WindowOpenDisposition disposition,
                       PageTransition::Type transition) = 0;
};

I don't understand what is that =0; part...what are we trying to communicate?

Answer 1

It's a pure virtual function - there's no definition in the base class, making this an abstract class, and any instantiable class that inherits from PageNavigator must define this function.

Answer 2

'= 0' means it's a pure virtual method. It must be overriden in inheriting class.

If a class has a pure virtual method it is considered abstract. Instances (objects) of abstract classes cannot be created. They are intended to be used as base classes only.

Curious detail: '= 0' doesn't mean method has no definition (no body). You can still provide method body, e.g.:

class A
{
 public:
  virtual void f() = 0;
  virtual ~A() {}
};

void A::f()
{
  std::cout << "This is A::f.\n";
}

class B : public A
{
 public:
  void f();
}

void B::f()
{
  A::f();
  std::cout << "And this is B::f.\n";
}

Answer 3

The = 0 means the function is a pure virtual or abstract function, which in practice means two things:

a) A class with an abstract function is an abstract class. You cannot instantiate an abstract class.

b) You have to define a subclass which overrides it with an implementation.