前瞻性声明循环依赖 [英] Forward declaration & circular dependency

问题描述

我有两个类，Entity和Level。两者都需要访问彼此的方法。因此，使用#include，出现循环依赖的问题。因此为了避免这种情况，我试图在Entity.h中转发declare Level：

  class Level {}; 
  

但是，由于Entity需要访问Level中的方法，所以它不能访问这些方法，知道他们存在。有没有办法解决这个问题，而不重新声明Entity中的大多数级别？

解决方案

  class Level; 
  

注意缺少花括号。这告诉编译器有一个名为 Level 的类，但是没有内容。然后你可以自由地使用指针（ Level * ）和引用（ Level& p>

请注意，由于编译器需要知道类的大小来创建变量，因此不能直接实例化 Level 。

  class Level; 
 
 class Entity 
 {
级别&level; // legal 
级别; // illegal 
}; 
  

为了能够使用 Level code> Entity 的方法，你最好在单独的<$>中定义 Level c $ c> .cpp 文件，并且只在标题中声明。从定义中分离声明是一个C ++最佳实践。

  // entity.h 
 
 class Level; 
 
 class Entity 
 {
 void changeLevel（Level&）; 
}; 
 
 
 // entity.cpp 
 
 #includelevel.h
 #includeentity.h
 
 void Entity :: changeLevel（level& level）
 {
 level.loadEntity（* this）; 
} 
  

I've got two classes, Entity and Level. Both need to access methods of one another. Therefore, using #include, the issue of circular dependencies arises. Therefore to avoid this, I attempted to forward declare Level in Entity.h:

class Level { };

However, as Entity needs access to methods in Level, it cannot access such methods, since it does not know they exist. Is there a way to resolve this without re-declaring the majority of Level in Entity?

解决方案

A proper forward declaration is simply:

class Level;

Note the lack of curly braces. This tells the compiler that there's a class named Level, but nothing about the contents of it. You can then use pointers (Level *) and references (Level &) to this undefined class freely.

Note that you cannot directly instantiate Level since the compiler needs to know the class's size to create variables.

class Level;

class Entity
{
    Level &level;  // legal
    Level level;   // illegal
};

To be able to use Level in Entity's methods, you should ideally define Level's methods in a separate .cpp file and only declare them in the header. Separating declarations from definitions is a C++ best practice.

// entity.h

class Level;

class Entity
{
    void changeLevel(Level &);
};


// entity.cpp

#include "level.h"
#include "entity.h"

void Entity::changeLevel(Level &level)
{
    level.loadEntity(*this);
}

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