Question

我有以下两个类：

struct Entity 
{
    unsigned id;
    Entity(unsigned id);
    Entity();
};

class EntityManager
{
public:
    Entity create();
    ...

private:
    Entity make_entity(unsigned index, unsigned generation);
};

此刻此工作正常。问题是封装。我不想允许直接创建类Entity。

因此，我的目标是使Entity的构造函数成为私有的。然后我可以（根据我的理解）通过EntityManager Entity friend EntityManager来struct Entity { unsigned id; private: Entity(unsigned id); Entity(); }; class EntityManager { friend struct Entity; public: Entity create(); private: Entity make_entity(unsigned index, unsigned generation); };中的mantain功能。

所以做出改变就是结果：

entity_manager.cpp: In member function ‘Entity EntityManager::make_entity(unsigned int, unsigned int)’:
entity_manager.cpp:12:1: error: ‘Entity::Entity(unsigned int)’ is private
 Entity::Entity(unsigned id) : id(id) {}
 ^
entity_manager.cpp:19:21: error: within this context
     return Entity(id);
                     ^

这打破了代码。我得到的错误就是这个：

Entity::Entity() : id(0) {}
Entity::Entity(unsigned id) : id(id) {}

Entity EntityManager::make_entity(unsigned idx, unsigned  generation)
{
    unsigned id = 0;
    id = ...
    return Entity(id);
}

Entity EntityManager::create()
{
    ...
    return make_entity(..., ...);
}

实现文件就像这样：

Entity(id)

我有什么明显的遗失吗？我还尝试在Entity::Entity(id)上调用entity_manager.cpp: In member function ‘Entity EntityManager::make_entity(unsigned int, unsigned int)’: entity_manager.cpp:19:29: error: cannot call constructor ‘Entity::Entity’ directly [-fpermissive] return Entity::Entity(id); ^ entity_manager.cpp:19:29: note: for a function-style cast, remove the redundant ‘::Entity’ entity_manager.cpp:12:1: error: ‘Entity::Entity(unsigned int)’ is private Entity::Entity(unsigned id) : id(id) {} ^ entity_manager.cpp:19:29: error: within this context return Entity::Entity(id);，但后来又出现了另一个错误：

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Answer 1

您向前发送friend声明。您需要在Entity结构中包含此行：

friend class EntityManager;

Answer 2

朋友声明需要进入Entity课程，而不是EntityManager课程。否则，任何类都可以通过在其声明中放置friend class X来访问另一个类的私有数据。希望共享其私有数据的类必须明确声明。

Answer 3

尝试以下

struct Entity;

class EntityManager
{
public:
    Entity create();

private:
    Entity make_entity(unsigned index, unsigned generation);
};

struct Entity 
{
private:
    unsigned id;
    Entity(unsigned id);
    Entity();
    friend Entity EntityManager::make_entity(unsigned index, unsigned generation);
};

Answer 4

您向前发送了friend声明。友谊类包含可由专业人士使用的私人（和/或受保护）成员。

struct Entity 
{
    unsigned id;
private:
    Entity(unsigned id);
    Entity();
    friend class EntityManager;
};

无法访问Friend类的私有构造函数

4 个答案: