kittyPL kittyPL - 1 year ago 87
C++ Question

C++ class forward declaration

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
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
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 couldnt find anything simmilar to my problem... Actually, i have more classes with parent "tile" and It was ok before...
Thanx for any help.


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
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 Source

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!

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