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

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.

Answer 2

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 <memory>

std::shared_ptr<tile> tile_tree::tick() {
     if (rand() % 20 == 0)
         return std::make_shared<tile_tree_apple>();
}