C++核心準則邊譯邊學-I.25 在類體系中將抽象類定義成接口會更好

I.25: Prefer abstract classes as interfaces to class hierarchies（在類體系中將抽象類定義為接口會更好）

譯者注：

1. 抽象類：不能直接生成示例的基類。

2. 接口：不包含數(shù)據(jù)成員的抽象類。

Reason（原因）

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

抽象類有很大的可能比包含狀態(tài)的基類更穩(wěn)定。

譯者注：結(jié)合全文，這里的抽象類應該指接口。

Example, bad（反面示例）

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

你只知道Shape會在某處表示:-）

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:

這將強迫每個派生類都要計算中心位置-哪怕是那些特殊且中心從未不用的圖形。類似的，也不是所有的圖形都有顏色，很多圖形最好不要包含一個有Points序列定義的輪廓線。發(fā)明抽象類就是為了鼓勵用戶寫出下面的類：

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（實施建議）

(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.

（簡單）如果指向類C的指針或引用被復制被C的基類類型的指針或引用，而且基類包含數(shù)據(jù)成員時，報警。

譯者注：也就是說，大師的意見是：基類基本上抽象類就不要有數(shù)據(jù)成員了。