Question

我正在为学校做家庭作业。对于主循环，我循环通过Objects并调用它们的Update函数，每个Object都有两个Object指针用于制作链表。我将这些指针设为私有，并使两个朋友函数GetNext提供了Next指针，而SetPointers则设置了指针。这些函数在ObjectManager类中，它们是私有的。我似乎无法让Object类访问其定义而不会导致其他错误。

对象类：

   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);
   };

ObjectManager类：

    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);
   };

GetNext函数

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

构建日志：

    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...

==========构建：0成功，1失败，0最新，0跳过==========

Answer 1

我知道有两种解决方案（假设您不能将ObjectManager的整个声明置于Object之前）。简单的是：

friend class ObjectManager;

你已经开始提供两种方法，为什么不和他们一起交朋友呢？更复杂的一个涉及使用passkey，在这种情况下，显然更糟糕：

template <typename T>
class PassKey;

class ObjectManager;

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

使用：

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

此处，GetNext()和SetPointers()是Object上的公共函数，但它们需要PassKey私有构造且其唯一的朋友是ObjectManager。因此，ObjectManager可以：

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

然而，这真的混淆了界面，在我认为与仅仅是为了全班同学交友时没有任何好处。

友元函数减速无法访问函数声明

1 个答案: