[問題] C++ Primer 4/e ch14問題

[Zephyr750 (紅蓮西風750)]

使用平台：VC++6

//範例中沒有，隨意定義內容的Screen class
class Screen
{
public:
        int a;
};

class ScrPtr
{
        friend class ScreenPtr;
        Screen *sp;
        size_t use;
    ScrPtr(Screen *p):sp(p), use(1){}
        ~ScrPtr(){delete sp;}
};

class ScreenPtr
{
public:
        //沒有default建構式：ScreenPtrs必須綁定於某個物件上
        ScreenPtr(Screen *p):ptr(new ScrPtr(p)){}
        //拷貝成員並累加參用計數
        ScreenPtr(const ScreenPtr &orig):ptr(orig.ptr){ ++ptr->use; }
        ScreenPtr& operator=(const ScreenPtr&);
        //如果參用計數為0，刪除StrPtr物件
        ~ScreenPtr(){ if(--ptr->use == 0) delete ptr; }
→      const Screen& operator*() const{ return *ptr->sp;}
//錯誤訊息出現時指著這一行
        const Screen* operator->() const{ return ptr->sp;}
        Screen& operator*(){ return *ptr->sp;}
        Screen* operator->(){ return ptr->sp;}
private:
        ScrPtr *ptr;//指向被計數的ScrPtr
};

error訊息：
--------------------Configuration: ch14 - Win32 Debug--------------------
Compiling...
main.cpp
d:\c++primer4\ch14\screenptr.h(11) : error C2440: 'return' : cannot convert from 'class Screen' to 'const class ScreenPtr &'
        Reason: cannot convert from 'class Screen' to 'const class ScreenPtr'
        No constructor could take the source type, or constructor overload resolution was ambiguous
d:\c++primer4\ch14\screenptr.h(12) : error C2440: 'return' : cannot convert from 'class Screen *' to 'const class ScreenPtr *'
        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
d:\c++primer4\ch14\screenptr.h(13) : error C2440: 'return' : cannot convert from 'class Screen' to 'class ScreenPtr &'
        A reference that is not to 'const' cannot be bound to a non-lvalue
d:\c++primer4\ch14\screenptr.h(14) : error C2440: 'return' : cannot convert from 'class Screen *' to 'class ScreenPtr *'
        Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
Error executing cl.exe.

ch14.exe - 4 error(s), 0 warning(s)

請問是發生了什麼問題？？
是不是const 的refrence和const的derefrence不可以放non-const呢？
不過已經重載了，怎麼還會？？


--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 60.248.125.78
※ 編輯: Zephyr750       來自: 60.248.125.78        (08/11 14:55)
※ 編輯: Zephyr750       來自: 60.248.125.78        (08/11 14:58)
→ PkmX:你應該要return ptr->sp;吧 ScreenPtr的ctor是吃Screen*
→ PkmX:不過這寫法怎覺得邏輯怪怪的~"~
→ PkmX:撤回一樓 那樣會return reference to temporary object...
→ DarkNT:*和->的優先權確認一下
→ Zephyr750:->優先於*
→ Zephyr750:ptr->sp是指向ScrPtr.sp, *ptr->sp是Screen
→ Zephyr750:找到問題了！純粹是看錯字！key錯了！sorry
※ 編輯: Zephyr750       來自: 59.120.98.237        (08/11 16:00)
→ Zephyr750:ScreenPtr改成Screen就好了！返回型別打錯字！XDDD
→ PkmX:所以說應該要return Screen&或Screen*嘛
→ PkmX:一直return自己難怪邏輯很怪XD
※ 編輯: Zephyr750       來自: 60.244.112.65        (08/11 16:23)
→ Zephyr750:純排版修改