C++ 类前向声明 [英] C++ class forward declaration

问题描述

当我尝试编译这段代码时，我得到:

When I try to compile this code, I get:

52 C:Dev-CppProjektyyystrategyTiles.h invalid use of undefined type `struct tile_tree_apple' 
46 C:Dev-CppProjektyyystrategyTiles.h forward declaration of `struct tile_tree_apple' 

我的部分代码:

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;};       
};

我真的不知道该怎么办，我搜索了解决方案，但找不到任何与我的问题类似的东西...实际上，我有更多与父级tile"相关的课程.之前还可以...

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...

我决定将所有返回的类型更改为指针以避免内存泄漏，但现在我得到了:

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

27 C:Dev-CppProjektyyystrategyTiles.h ISO C++ forbids declaration of `tile' with no type 
27 C:Dev-CppProjektyyystrategyTiles.h expected `;' before "tick"

仅在基类中，其他一切正常... tile 类中返回 *tile 的每个函数都有此错误...

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

一些代码:

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() {};
};

推荐答案

new T 要编译，T 必须是完整类型.在您的情况下，当您在 tile_tree::tick 的定义中说 new tile_tree_apple 时，tile_tree_apple 是不完整的(它已被前向声明，但是它的定义稍后在您的文件中).尝试将函数的内联定义移动到单独的源文件中，或者至少将它们移动到类定义之后.

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.

类似于:

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() {...}

当您以这种方式编写代码时，这些方法中对 A 和 B 的所有引用都保证引用完整类型，因为不再有前向引用！

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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