c++ - 关于 C++ 编程语言的问题

Question

在 The C++ Programming Language 一书的第 24.3.4 章中说

class Cfield : public Field{ /*...*/ }

This expresses the notion that a Cfield really is a kind of Field, allows notational convenience when writing a Cfield function that uses a member of the Field part of the Cfield, and - most importantly - allows a Cfield to override Field virtual functions. The snag is that the Cfield* to Field* conversion implied in the declaration of Cfield defeats all attempts to control access to the Field:

void g(Cfield* p)
{
    *p = "asdf"; // access to Field controlled by Cfield's assignment operator:
                 // p->Cfield::operator=("asdf")

    Field* q = p; // implicit Cfield* to Field* conversion
    *q = "asdf"   // OOPS! no control
}

我在这里不明白的是加粗的句子。 Cfield 如何阻止控制对 Field 的访问的尝试？

实际上，最后一行代码:

*q = "asdf"; 

将调用 Field::operator=("asdf")，那么 Cfield 是如何阻止控制对 Field 的访问的呢？

Answer 1

CField 实例应该提供对 Field 的受控(即通过 CField::operator=() 以某种方式检查)访问基类数据。但是，如果您将 CField * 隐式转换为 Field *，然后取消引用它，那么该控件就会消失，因为您选择了 Field::operator=( )。

我同意这不是 BS 的著作中最清晰的，在我看来这有点无关紧要——只要你用心去做，你总是可以“不受控制地访问”任何东西。