C++ class forward declaration

Question

When I try to compile this code, I get:

52 C:\Dev-Cpp\Projektyyy\strategy\Tiles.h invalid use of undefined type `struct tile_tree_apple'  46 C:\Dev-Cpp\Projektyyy\strategy\Tiles.h forward declaration of `struct tile_tree_apple'  

some part of my code:

class tile_tree_apple;  class tile_tree : public tile {       public:           tile onDestroy() {return *new tile_grass;};           tile tick() {if (rand()%20==0) return *new tile_tree_apple;};           void onCreate() {health=rand()%5+4; type=TILET_TREE;};         };  class tile_tree_apple : public tile {       public:           tile onDestroy() {return *new tile_grass;};           tile tick() {if (rand()%20==0) return *new tile_tree;};           void onCreate() {health=rand()%5+4; type=TILET_TREE_APPLE;};            tile onUse() {return *new tile_tree;};        }; 

I dont really know what to do, I searched for the solution but I couldn't find anything simmilar to my problem... Actually, I have more classes with parent "tile" and It was ok before...

EDIT:

I decided to change all returned types to pointers to avoid memory leaks, but now I got:

27 C:\Dev-Cpp\Projektyyy\strategy\Tiles.h ISO C++ forbids declaration of `tile' with no type  27 C:\Dev-Cpp\Projektyyy\strategy\Tiles.h expected `;' before "tick" 

Its only in base class, everything else is ok... Every function in tile class which return *tile has this error...

Some code:

class tile {       public:           double health;           tile_type type;           *tile takeDamage(int ammount) {return this;};           *tile onDestroy() {return this;};           *tile onUse() {return this;};           *tile tick() {return this};           virtual void onCreate() {}; }; 

Answer 1

Use forward declaration when possible.

Suppose you want to define a new class B that uses objects of class A.

1. B only uses references or pointers to A. Use forward declaration then you don't need to include <A.h>. This will in turn speed a little bit the compilation.

   class A ;  class B  {   private:     A* fPtrA ;   public:     void mymethod(const& A) const ; } ; 

2. B derives from A or B explicitely (or implicitely) uses objects of class A. You then need to include <A.h>

   #include <A.h>  class B : public A  { };  class C  {   private:     A fA ;   public:     void mymethod(A par) ;    }