[SOLVED] State Design Pattern

cazador7907

I'm curious about something that I've started to look at (and ponder) - Finite State Machines. I've been looking at some of the hits returned by different searches to understand how they work and the method used to implement them seems a little confused.

The implementations that I have read through/dissected all seem to implement a weird sort of circular reference between the State abstract class (and any classes based on the State class) and the Player class. How can the c++ compiler handler this sort of implementation. As near as I can tell, to compile the State abstract class, the compiler must have knowledge of the Player class; but, to compile the Player class, it must first compile the State abstract class. I've put a general sort of outline of what I've been seeing below.

I'm sure that something very fundamental is escaping me.

@

class State
{
public:
virtual void Enter(player*);
virtual void Execute(player*);
virtual void Exit(player*);
}

class Player
{
private:
State* m_pCurrentState;

}

@

ixSci

@
class Player;
class State
{
public:
virtual void Enter(Player*);
virtual void Execute(Player*);
virtual void Exit(Player*);
}

class Player
{
private:
State* m_pCurrentState;

}@

vcsala

It is called forward declaration to solve circural references. It is commonly used in Qt applications' header files for types.

lyuts

Consider the code posted by ixSci. When the compiler sees the line 1 it understands, that you are going to use Player type. It is not clear how it is defined and what it looks like, but it is going to find out that later. I would say that forward declarations are used almost everywhere. Another way of using it is to reduce includes in header files, i.e. if you declare a function in a header like:

@void func(CustomType *iParam);@

and CustomType is defined in CustomType.h, then it is better to do a forward declaration like
@
class CustomType;
void func(CustomType *iParam);@

rather than including its header like:
@
#include <CustomType.h>

void func(CustomType *iParam);@

ixSci

Just a bit of adjustment: Forward declaration can be used when the size of a type is not necessary at the moment of a compilation i.e. forward declaration can be used with pointers and references only because its size is predetermined.
The "link":http://www.goingware.com/tips/parameters/notrequired.html seems to be appropriate but I didn't read it myself

cazador7907

Thanks! Just finished reading up on Forward Declarations. Interesting stuff.