与 typedef-ed 类交朋友 (c++)

Question

我正在经历一些危机，我需要为一个类型定义的模板类添加好友，但我遇到了很多错误（太多错误，无法粘贴到这里）。我已经把 MRE here 放在了，所以你们不必设置文件。

为了完整起见，这里是 .h 文件：

//Keyboard.h
#pragma once
namespace wm
{
    namespace io
    {
        class Keyboard
        {
            //friend wm::MessageHandler;
        private:
            Keyboard(const Keyboard&) = delete;
            void operator=(const Keyboard&) = delete; 
            Keyboard() = default;

        public:
            static Keyboard& Instance();
            static void SomeServerFunc();

        public:
            static void SomeClientFunc();
        };
    }
}

//Mouse.h
#pragma once
namespace wm
{
    namespace io
    {
        class Mouse
        {
            //friend wm::MessageHandler;
            //friend class wm::MessageHandler; ??
        private:
            Mouse(const Mouse&) = delete;
            void operator=(const Mouse&) = delete;
            Mouse() = default;

        public:
            static Mouse& Instance();
            static void SomeServerFunc();

        public:
            static void SomeClientFunc();
        };
    }
}

//MessageHandler.h

#pragma once
#include "Keyboard.h"
#include "Mouse.h"
namespace wm
{
    template < class Keyboard, class Mouse >
    class _MessageHandler
    {
    public:
        static _MessageHandler& Instance();
        _MessageHandler(const _MessageHandler&) = delete;
        void operator=(const _MessageHandler&) = delete;

    private:
        _MessageHandler() = default;
        static void SomeMsgHandelingFunc(); //calls SomeServerFunc from window and mouse
    };

    typedef _MessageHandler<io::Keyboard, io::Mouse> MessageHandler;
}

事实上，上面的代码编译并运行良好。但我想将键盘和鼠标类中的SomeServerFunc 函数设为私有，并使MessageHandler 成为Keyboard 和Mouse 的朋友。我尝试了不同的方法，但到目前为止没有任何效果。

我在做friend class wm::MessageHandler;，但我得到了详细的类型引用 typedef 错误。 然后我发现在 c++ 11 之后，你可以做 friend wm::MessageHandler; 这会产生很多错误。我也试过了

friend wm::_MessageHandler<io::Keyboard, io::Mouse>;

friend class wm::_MessageHandler<io::Keyboard, io::Mouse>;

friend wm::_MessageHandler<Keyboard, Mouse>;

friend class wm::_MessageHandler<Keyboard, Mouse>;

运气不好。

非常感谢任何帮助。

Answer 1

将键盘和鼠标类中的 SomeServerFunc 函数设为私有，并使 MessageHandler 成为键盘和鼠标的朋友。

我在mouse.h和keyboard.h中添加了类声明，让编译器满意。代码如下：

#pragma once

namespace wm
{
    template < class Keyboard, class Mouse >
    class _MessageHandler;
    namespace io
    {
        class Keyboard;
        class Mouse
        {
        friend class _MessageHandler<io::Keyboard, io::Mouse>;
        private:
            Mouse(const Mouse&) = delete;
            void operator=(const Mouse&) = delete;
            Mouse() = default;

        public:
            static Mouse& Instance();
        private:
            static void SomeServerFunc();

        public:
            static void SomeClientFunc();
        };
    }
}

#pragma once

namespace wm
{
    template < class Keyboard, class Mouse >
    class _MessageHandler;
    namespace io
    {
        class Mouse;
        class Keyboard
        {
        friend class _MessageHandler<io::Keyboard, io::Mouse>;
        private:
            Keyboard(const Keyboard&) = delete;
            void operator=(const Keyboard&) = delete; 
            Keyboard() = default;

        public:
            static Keyboard& Instance();
        private:
            static void SomeServerFunc();

        public:
            static void SomeClientFunc();
        };
    }
}

Answer 2

_MessageHandler 不是一个类，而是一个模板。试试看：

namespace wm
{
    template<class Keyboard, class Mouse> class MessageHandler;

    namespace io
    {
        class Keyboard
        {
            template<class Keyboard, class Mouse> friend class wm::MessageHandler;
            ...
        };
    }
}