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  quot tile quot  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  quot tick quot   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

User · Answer

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.

User · Answer

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

User · Answer

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- gt width      Ymax   winHeight - pSpr- gt 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

User · Answer

The problem is that tick   needs to know the definition of tile tree apple  but all it has is a forward declaration of it  You should separate the declarations and definitions like so   tile tree h   ifndef TILE TREE H  define TILE TREE H  include  tile h   class tile tree   public tile   public      tile onDestroy        tile tick        void onCreate         endif   tile tree cpp   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      Except you have a major problem  you   re allocating memory  with new   then copying the allocated object and returning the copy  This is called a memory leak  because there   s no way for your program to free the memory it uses  Not only that  but you   re copying a tile tree into a tile  which discards the information that makes a tile tree different from a tile  this is called slicing   What you want is to return a pointer to a new tile  and make sure you call delete at some point to free the memory   tile  tile tree  tick          if  rand     20    0           return new tile tree apple      Even better would be to return a smart pointer that will handle the memory management for you    include  lt memory gt   std  shared ptr lt tile gt  tile tree  tick          if  rand     20    0           return std  make shared lt tile tree apple gt

User · Answer

To perform  new tile tree apple the constructor of tile tree apple should be called  but in this place compiler knows nothing about tile tree apple  so it can t use the constructor   If you put   tile tile tree  tick    if  rand   20  0  return  new tile tree apple      in separate cpp file which has the definition of class tile tree apple or includes the header file which has the definition everything will work fine

User · Answer

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

User · Answer

Use forward declaration when possible    Suppose you want to define a new class B that uses objects of class A     B only uses references or pointers to A  Use forward declaration then you don t need to include  lt A h gt   This will in turn speed a little bit the compilation      class A    class B      private      A  fPtrA     public      void mymethod const amp  A  const        B derives from A or B explicitely  or implicitely  uses objects of class A  You then need to include  lt A h gt      include  lt A h gt   class B   public A        class C      private      A fA     public      void mymethod A par

User · Answer

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

[c++] C++ class forward declaration

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