c++ primer 学习杂记2【派生类到基类转换的可访问性】

参考：

http://blog.csdn.net/rehongchen/article/details/7930853

http://blog.csdn.net/ming_road/article/details/6953687

http://blog.csdn.net/roden/article/details/5413371

中文版：p489 。对应英文版内容：

Like an inherited member function, the conversion from derived to base may or may not be accessible. Whether the conversion is accessible depends on the access label specified on the derived class' derivation.

If the inheritance is public, then both user code and member functions of subsequently derived classes may use the derived-to-base conversion. If a class is derived using private or protected inheritance, then user code may not convert an object of derived type to a base type object. If the inheritance is private, then classes derived from the privately inherited class may not convert to the base class. If the inheritance is protected, then the members of subsequently derived classes may convert to the base type.

Regardless of the derivation access label, a public member of the base class is accessible to the derived class itself. Therefore, the derived-to-base conversion is always accessible to the members and friends of the derived class itself.

Tips:To determine whether the conversion to base is accessible, consider whether a public member of the base class would be accessible. If so, the conversion is accessible; otherwise, it is not.

提示：要确定到基类的转换是否可访问，可以考虑基类的public成员是否可访问，如果可以，转换是可以访问的，否则，转换是不可访问的。

首先要明白几个概念：用户代码(user code), 后代类(subsequently derived classes),派生类(derived class)

用户代码，指的是除友元函数，成员函数之外的代码。

后代类，不仅仅指第一级派生类，还包括间接派生自基类的后续的派生类。

派生类，这里专指直接继承类。

理解了上面的概念之后，通过代码来说明c++ primer中说到的4点。

class A
{
};

class B:public A
{
public:
    void fun(B&obj)
    {
        A obj1 = (A)obj;
    }
};

class C:protected A
{
public:
    void fun(C&obj)
    {
        A obj1 = (A)obj;
    }
};

class D:private A
{
public:
    void fun(D&obj)
    {
        A obj1 = (A)obj;
    }
};

class E:public B
{
public:
    void fun(B&obj)
    {
        A obj1 = (A)obj;
    }
};

class F:public C
{
    public:
    void fun(C&obj)
    {
        A obj1 = (A)obj;
    }
};

//从private继承类派生的类不能转换为基类。
class H:public D
{
public:
    void fun(D&obj)
    {
        A obj1 = (A)obj;  //error C2247: “A”不可访问，因为“D”使用“private”从“A”继承
                          //error C2243: “类型转换”: 从“D *”到“const A &”的转换存在，但无法访问
    }
};

用户代码中访问的内容：

void inherite_test()
{
    A *pb, *pc, *pe, *pd, *pf, *ph;

    pb = new B; //public
    pc = new C; //protected           error C2243: “类型转换”: 从“C *”到“A *”的转换存在，但无法访问
    pd = new D; //private             error C2243: “类型转换”: 从“D *”到“A *”的转换存在，但无法访问
    pe = new E; //public + public
    pf = new F; //protected + public  error C2243: “类型转换”: 从“F *”到“A *”的转换存在，但无法访问
    ph = new H; //private + public    error C2243: “类型转换”: 从“H *”到“A *”的转换存在，但无法访问
}

解析：

其中类B,C,D分别通过public,protected,private直接继承自A; 类E,F,H则分别public继承自B,C,D类。

各类的成员函数fun中进行派生类到基类的转换操作。

1、pb = new B;  B::fun，E::fun函数说明：

如果是public继承，则用户代码和后代类都可以使用派生类到基类的转换。

If the inheritance is public, then both user code and member functions of subsequently derived classes may use the derived-to-base conversion

2、inherite_test中C,D,F,H类转换的失败都说明了：

如果类是使用private或protected继承派生的，则用户代码不能将派生类型对象转换为基类对象。

If a class is derived using private or protected inheritance, then user code may not convert an object of derived type to a base type object.

3、H类的fun函数发生的错误显示：

从private继承类派生的类不能转换为基类。

If the inheritance is private, then classes derived from the privately inherited class may not convert to the base class.

4、C::fun函数说明：

如果是protected继承，则后续派生类的成员可以转换为基类类型。

If the inheritance is protected, then the members of subsequently derived classes may convert to the base type.

有一点需要说明：对于多级派生的，要多个访问标号综合起来看可访问性。 有个简单的方法，见上面的tips.

例如，C 类protected继承自A，那么A中的public成员在C中变成了protected, F类public继承自C,这样在F中A的public成员fun函数为protected，是可见的。

所以F::fun中派生类到基类转换正确。

但是在用户代码中，是不能访问在C,F中变成了protected的A的public成员的，因此C,F对象转换为类A的对象出错。

=====>感觉fun函数定义的有点不合适，挺奇怪的，但是编译是没问题的，不知道现实生产中有没有这么用的？？？ （待定）