第五章：隐藏实现 Hiding the Implementation

第五章：隐藏实现 Hiding the Implementation

Setting limits：设置限制

library creator 和 client programmer之间的关系：便于作者修改，有些东西客户不该碰到

C++ access control：C++的访问控制

C++加入了访问控制：public，private，protected

public的所有人可见（struct的成员默认为public）

private意味着只有此类型的creator才能访问，client programmer是不能访问的

protected除了继承的情况下与private的表现一样

Friends：友元

可以通过关键字friend进行特别授权

可以声明一个：Global friend 全局函数

              Struct member friend 另外一个结构的成员（函数）

              Entire struct 整个结构

    为当前结构的友元

Nested friends：嵌套友元

内嵌的结构不会自动拥有对private成员的访问权限

要想获取相应权限可以如下操作：首先声明，其次声明其为友元，再次定义。（first, declare (without defining) the nested structure, then declare it as a friend, and finally define the structure. ）

Is it pure?：友元破坏了C++的面向对象特性（C++的设计初衷是实用性^_^）

Object layout：object的布局

由于访问控制符的原因，C++里不同成员块（以访问控制为标准）的位置未必与声明时的一样（同一访问控制符内的顺序还是一定的），所以不应该依赖于位置来访问object的成员

The class

Access control 通常被称作隐藏实现。

它分离了实现implementation与接口interface

Handle classes：句柄类

隐藏实现，减少重编译

人们经常使用一种叫做handle classes or the “Cheshire cat”的方式来减少重编译：即头文件里只包含实现的指针（只需部分声明，前置声明），而其他部分都在实现文件里，这样就可以保证头文件没有发生改变。

Summary

Access control in C++ gives valuable control to the creator of a class. The users of the class can clearly see exactly what they can use and what to ignore. More important, though, is the ability to ensure that no client programmer becomes dependent on any part of the underlying implementation of a class. If you know this as the creator of the class, you can change the underlying implementation with the knowledge that no client programmer will be affected by the changes because they can’t access that part of the class.

When you have the ability to change the underlying implementation, you can not only improve your design at some later time, but you also have the freedom to make mistakes. No matter how carefully you plan and design, you’ll make mistakes. Knowing that it’s relatively safe to make these mistakes means you’ll be more experimental, you’ll learn faster, and you’ll finish your project sooner.

The public interface to a class is what the client programmer does see, so that is the most important part of the class to get “right” during analysis and design. But even that allows you some leeway for change. If you don’t get the interface right the first time, you can add more functions, as long as you don’t remove any that client programmers have already used in their code.