朋友功能減速無法訪問功能聲明

Question

我正在爲學校做家庭作業。對於主循環，我循環遍歷對象並調用它們的Update函數，每個對象都有兩個對象指針用於創建鏈表。我將這些指針設爲私有，並使GetNext女巫提供了兩個朋友函數Next指針，SetPointers女巫設置指針。這些函數在ObjectManager類中，它們是私有的。我似乎無法讓對象類訪問其定義而不會導致其他錯誤。

對象類：

namespace Tmpl8{ 
    class Colider; 

    class ObjectManager; 
    class PhysicsManager; 

    class Object{ 
    public: 
     Object(); 
     ~Object(); 

     virtual void Update(); 

    private: 
     Object* Next; 
     Object* Previus; 

     friend Object* ObjectManager::GetNext(Object* object); 
     friend void ObjectManager::SetPointers(Object* object, Object* next, Object* previus); 
    }; 

中的ObjectManager類：

namespace Tmpl8{ 
    class Object; 
    class GameObject; 
    class Colider; 

    class ObjectManager{ 
    public: 
     ObjectManager(); 
     ~ObjectManager(); 

     void Update(); 
     void Remove(Object* object); 
     void Add(Object* object); 

    private: 
     void Resolve(); 
     std::vector<Object*> removeList; 
     std::vector<Object*> addList; 

     Object* first; 
     Object* last; 

     Object* GetNext(Object* object); 
     void SetPointers(Object* object, Object* next, Object* previus); 
    }; 

的的GetNext功能

Object* ObjectManager::GetNext(Object* object) 
    { 
     return object->Next; // this gives the error Tmpl8::Object::Next(delecared at...) is inaccessible 
    } 

生成日誌：

1>c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(25): error C2248: 'Tmpl8::ObjectManager::GetNext' : cannot access private member declared in class 'Tmpl8::ObjectManager' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(29) : see declaration of 'Tmpl8::ObjectManager::GetNext' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(12) : see declaration of 'Tmpl8::ObjectManager' 
    1>c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(26): error C2248: 'Tmpl8::ObjectManager::SetPointers' : cannot access private member declared in class 'Tmpl8::ObjectManager' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(30) : see declaration of 'Tmpl8::ObjectManager::SetPointers' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(12) : see declaration of 'Tmpl8::ObjectManager' 
    1> manager.cpp 
    1>c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(25): error C2027: use of undefined type 'Tmpl8::ObjectManager' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(11) : see declaration of 'Tmpl8::ObjectManager' 
    1>c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(26): error C2027: use of undefined type 'Tmpl8::ObjectManager' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(11) : see declaration of 'Tmpl8::ObjectManager' 
    1>manager.cpp(40): warning C4018: '<' : signed/unsigned mismatch 
    1>manager.cpp(45): warning C4018: '<' : signed/unsigned mismatch 
    1>manager.cpp(57): error C2248: 'Tmpl8::Object::Next' : cannot access private member declared in class 'Tmpl8::Object' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(22) : see declaration of 'Tmpl8::Object::Next' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(14) : see declaration of 'Tmpl8::Object' 
    1> object.cpp 
    1>c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(25): error C2248: 'Tmpl8::ObjectManager::GetNext' : cannot access private member declared in class 'Tmpl8::ObjectManager' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(29) : see declaration of 'Tmpl8::ObjectManager::GetNext' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(12) : see declaration of 'Tmpl8::ObjectManager' 
    1>c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\object.h(26): error C2248: 'Tmpl8::ObjectManager::SetPointers' : cannot access private member declared in class 'Tmpl8::ObjectManager' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(30) : see declaration of 'Tmpl8::ObjectManager::SetPointers' 
    1>   c:\all svn\student.141960.school_projects\programing\block 2\micro machines_02\manager.h(12) : see declaration of 'Tmpl8::ObjectManager' 
    1> Generating Code... 

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Answer 1

有兩種解決方案，我知道（假設你不能把ObjectManager的整個聲明放在Object之前）。簡單的一種是：

friend class ObjectManager; 

你已經邀請好友兩種方法，爲什麼不朋友所有的人？更復雜的一個涉及使用passkey的，在這種情況下，顯然是雪上加霜：

template <typename T> 
class PassKey; 

class ObjectManager; 

class Object { 
public: 
    Object* GetNext(PassKey<ObjectManager>) { return Next; } 
    void SetPointers(PassKey<ObjectManager>, Object*, Object*) { .. } 
}; 

有：

template <typename T> 
class PassKey { 
    friend T; 
    PassKey() { } 
}; 

這裏，GetNext()和SetPointers()上Object公共職能，但它們需要一個PassKey這是私人建造，其唯一的朋友是ObjectManager。因此，ObjectManager可以這樣做：

Object* ObjectManager::GetNext(Object* object) 
{ 
    return object->GetNext(PassKey<ObjectManager>()); 
} 

然而，這真的muddles的界面相比，只邀請好友全班同學在我看來，是什麼沒有好處。