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\         


        

Answer 1

The forward declaration is an "incomplete type", the only thing you can do with such a type is instantiate a pointer to it, or reference it in a function declaration (i.e. and argument or return type in a function prototype). In line 52 in your code, you are attempting to instantiate an object.

At that point the compiler has no knowledge of the object's size nor its constructor, so cannot instantiate an object.

Answer 2

class tile_tree_apple should be defined in a separate .h file.

tta.h:
#include "tile.h"

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;};       
};

file tt.h
#include "tile.h"

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;};        
};

another thing: returning a tile and not a tile reference is not a good idea, unless a tile is a primitive or very "small" type.

Answer 3

To do anything other than declare a pointer to an object, you need the full definition.

The best solution is to move the implementation in a separate file.

If you must keep this in a header, move the definition after both declarations:

class tile_tree_apple;

class tile_tree : public tile
{
  public:
      tile onDestroy();
      tile tick();
      void onCreate();        
};

class tile_tree_apple : public tile
{
  public:
      tile onDestroy();
      tile tick();
      void onCreate(); 
      tile onUse();       
};

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

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

Important

You have memory leaks:

tile tile_tree::onDestroy() {return *new tile_grass;};

will create an object on the heap, which you can't destroy afterwards, unless you do some ugly hacking. Also, your object will be sliced. Don't do this, return a pointer.

Answer 4

In order for new T to compile, T must be a complete type. In your case, when you say new tile_tree_apple inside the definition of tile_tree::tick, tile_tree_apple is incomplete (it has been forward declared, but its definition is later in your file). Try moving the inline definitions of your functions to a separate source file, or at least move them after the class definitions.

Something like:

class A
{
    void f1();
    void f2();
};
class B
{
   void f3();
   void f4();
};

inline void A::f1() {...}
inline void A::f2() {...}
inline void B::f3() {...}
inline void B::f4() {...}

When you write your code this way, all references to A and B in these methods are guaranteed to refer to complete types, since there are no more forward references!

Answer 5

I had this:

class paulzSprite;
...

struct spriteFrame
{
    spriteFrame(int, int, paulzSprite*, int, int);
    paulzSprite* pSprite; //points to the sprite class this struct frames
    static paulzSprite* pErase; //pointer to blanking sprite
    int x, y;
    int Xmin, Xmax, Ymin, Ymax; //limits, leave these to individual child classes, according to bitmap size
    bool move(int, int);
    bool DrawAt(int, int);
    bool dead;
};

spriteFrame::spriteFrame(int initx, int inity, paulzSprite* pSpr, int winWidth, int winHeight)
{
    x = initx;
    y= inity;
    pSprite = pSpr;
    Xmin = Ymin = 0;
    Xmax = winWidth - pSpr->width;
    Ymax = winHeight - pSpr->height;
    dead = false;
}

...

Got the same grief as in the original question. Only solved by moving the definition of paulzSprite to after that of spriteFrame. Shouldn't the compiler be smarter than this (VC++, VS 11 Beta)?

And btw, I wholeheartedly agree with Clifford's remark above "Pointers don't cause memory leaks, poor coding causes memory leaks". IMHO this is true of many other new "smart coding" features, which should not become a substitute for understanding what you are actually asking the computer to do.

Answer 6

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) ;   
   }