c++ - "The C++ Programming Language"中描述的私有(private)继承用法

Question

The C++ Programming Language，第 4 版，在 §20.5.2“访问基类”(第 605 页)，它说(关于私有(private)继承):

private bases are most useful when defining a class by restricting the interface to a base so that stronger guarantees can be provided.For example, B is an implementation detail of Z .The Vector of pointers template that adds type checking to its Vector base from §25.3 is a good example.

不清楚 Bjarne Stroustrup 在这里想表达什么。如何通过将“接口(interface)”限制为基类来定义类？他所说的“更强的保证”是什么意思？

Answer 1

让我们举一个非常简单的例子:

// A simple class with a *public* member
class A
{
public:
    int a;
};

// Use private inheritance
class B : private A
{
public:
    int b;
};

// Use public inheritance
class C : public A
{
public:
    int c;
};

// ...

B my_b;
my_b.a = 0;  // Invalid, the member a is private due to the private inhericance

C my_c;
my_c.a = 0;  // Valid, because the inheritance is public

private 继承限制 对基类成员的访问。即使 A::a 成员变量是 public，由于 private 继承，它在子类 B。