C++核心准则边译边学-I.25 在类体系中将抽象类定义成接口会更好

I.25: Prefer abstract classes as interfaces to class hierarchies（在类体系中将抽象类定义为接口会更好）

译者注：

1. 抽象类：不能直接生成示例的基类。

2. 接口：不包含数据成员的抽象类。

Reason（原因）

Abstract classes are more likely to be stable than base classes with state.

抽象类有很大的可能比包含状态的基类更稳定。

译者注：结合全文，这里的抽象类应该指接口。

Example, bad（反面示例）

You just knew that Shape would turn up somewhere :-)

你只知道Shape会在某处表示:-）

class Shape {  // bad: interface class loaded with datapublic:    Point center() const { return c; }    virtual void draw() const;    virtual void rotate(int);    // ...private:    Point c;    vector outline;    Color col;};

This will force every derived class to compute a center -- even if that's non-trivial and the center is never used. Similarly, not every Shape has a Color, and many Shapes are best represented without an outline defined as a sequence of Points. Abstract classes were invented to discourage users from writing such classes:

这将强迫每个派生类都要计算中心位置-哪怕是那些特殊且中心从未不用的图形。类似的，也不是所有的图形都有颜色，很多图形最好不要包含一个有Points序列定义的轮廓线。发明抽象类就是为了鼓励用户写出下面的类：

class Shape {    // better: Shape is a pure interfacepublic:    virtual Point center() const = 0;   // pure virtual functions    virtual void draw() const = 0;    virtual void rotate(int) = 0;    // ...    // ... no data members ...    // ...    virtual ~Shape() = default;};

Enforcement（实施建议）

(Simple) Warn if a pointer/reference to a class C is assigned to a pointer/reference to a base of C and the base class contains data members.

（简单）如果指向类C的指针或引用被复制被C的基类类型的指针或引用，而且基类包含数据成员时，报警。

译者注：也就是说，大师的意见是：基类基本上抽象类就不要有数据成员了。