Empty class declaration in header file? [duplicate]

Question

Possible Duplicate:
What is forward declaration in c++?

I just have a question about what a piece of code is doing in this simple example. I've looked up friend classes and understand how they work, but I don't understand what the class declaration at the top is actually doing (i.e. Robot). Does this just mean that the Happy class can use Robot objects but they can't access its private parts, any information would be appreciated.

#include <stdlib.h>
#include <stdexcept>

template <typename T>   // What is this called when included 
class Robot;            // is there a special name for defining a class in this way

template <typename T>
class Happy
{ 
  friend class Joe<T>;  
  friend class Robot<Happy<T> >;
  // ...
};

Answer 1

It's a forward declaration.

It's just there to inform the compiler that a class template named Robot will be defined later and that it should expect that definition.

In the meantime (i.e., until Robot is officially defined), it allows the programmer to refer to that type in the code. In the example you've given, it is necessary to declare Robot as a friend of the Happy class. (Who picked these names?!)

It's a common strategy to avoid circular dependencies and minimize compilation time/overhead.