[問題] 有關C++Primer中文版,p461頁(12.4.4)

[easterday (....)]

上次經過大家的幫忙,終於讓我比較了解這一節在做甚麼
但是又有新的疑問了...
#include <string>
#include <iostream>
using namespace std;
class Sales_item
{
public:
        friend istream& operator>>(istream& in, Sales_item& s);
        std::string isbn;
        int unit_sold;
        double revenue;
        Sales_item(const std::string &book="")
                :isbn(book),unit_sold(0),revenue(0.0)  {}
        Sales_item(istream& is);
        bool same_isbn(const Sales_item& that) const;
};
Sales_item::Sales_item(istream& is)
{
        is>> *this;
}
bool Sales_item::same_isbn(const Sales_item& that) const
{
        return (isbn==that.isbn);
}
istream& operator>>(istream& in, Sales_item& s)
{
        double price;
        in>> s.isbn >> s.unit_sold >> price;
        if (in)
                s.revenue= s.unit_sold * price;
        else
                s=Sales_item();
        return in;
}
class ESales_item
{
public:
        friend istream& operator>>(istream& in, ESales_item& s);
        std::string isbn;
        int unit_sold;
        double revenue;
        explicit ESales_item(const std::string &book="")
                :isbn(book),unit_sold(0),revenue(0.0)  {}
        explicit ESales_item(istream& is);
        bool same_isbn(const ESales_item& that) const;

};
ESales_item::ESales_item(istream& is)
{
        is>> *this;
}
bool ESales_item::same_isbn(const ESales_item& that) const
{
        return (isbn==that.isbn);
}
istream& operator>>(istream& in, ESales_item& s)
{
        double price;
        in>> s.isbn >> s.unit_sold >> price;
        if (in)
                s.revenue= s.unit_sold * price;
        else
                s=ESales_item();
        return in;
}

int main()
{
        Sales_item item(string("9"));
        string null_book = "9";
//      cout << item.same_isbn("999")<<endl;  //compiling err
        cout << item.same_isbn(null_book)<<endl;
//      cout<< item.same_isbn(cin)<<endl;     //good

        ESales_item item2(string("9"));
        //item2.same_isbn(null_book);                           //compling err
        //item2.same_isbn(cin);                                 //compilng err
        ESales_item another("9");
        //explicit的功用應該是禁止隱式轉換,為什麼又把const char* ==>string了??
        cout << item2.same_isbn(another) <<endl;

        getchar();
        return 0;
}
explicit的功用應該是禁止隱式轉換
但是為什麼又把const char*轉換成了string??

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推 Feis:Explicit 的不是string(const char*)
→ masan22305:說個無關緊要的  可以貼程式碼在http://ideone.com/
→ easterday:我觀察到的現象是,沒有explicit時
→ easterday:const char*不能轉換成string
推 purpose:ESales_item another("9"); 這裡的隱式轉換是將 "9" 轉成
→ purpose:std::string 物件，所以是歸 class string 管，所以可以過
→ easterday:那為什麼Sales_item item2("9");這樣不能過?
→ easterday:我想我知道問題了,item2名子衝到了...