Make sure public inheritance models “is-a”
public inheritance 意味着 “is-a” 的关系。
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class Bird {};
class FlyingBird : public Bird {
public:
virtual void fly();
};
// Penguin is a bird, but cannot fly
class Penguin: public Bird {};
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代码通过编译并不代表就可以正确运作
- “public 继承” 意味着 “is-a”。适用于 base classes 身上的每一件事情一定也适用于 derived classes 身上，因为每个 derived class 对象都是一个 base class 对象。
Avoid hiding inherited names
内层作用域的名称会遮掩外围作用域的名称。
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// a normal case
class Base {
private:
int x;
public:
virtual void mf1() = 0;
virtual void mf2();
void mf3();
};
class Derived: public Base {
public:
virtual void mf1();
void mf4();
}
void Derived::mf4() {
mf2(); // find the function in Base class
}
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作用域的查询路径: Derived class -> Base class -> Base class’s namespace -> global
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// override case
class Base {
private:
int x;
public:
virtual void mf1() = 0;
virtual void mf1(int);
virtual void mf2();
void mf3();
void mf3(double);
};
class Derived: public Base {
public:
virtual void mf1();
void mf3();
void mf4();
}
Derived d;
int x;
d.mf1(); // ok, call Derived::mf1()
d.mf1(x); // wrong, Derived::mf1() override Base::mf1()
d.mf2(); // ok, Base::mf2()
d.mf3(); // ok, Derived::mf3()
d.mf3(x); // wrong, Derived::mf3() override Base::mf3()
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- derived classes 内的名称会遮掩 base classes 内的名称
- 可以使用 using 声明式或 forwarding 函数
Differentiate between inheritance of interface and inheritance of implementation
- 成员函数的接口总会被继承
- pure virtual 函数：必须被任何"继承了它们"的具象 class 重新声明，它们在抽象 class 中通常没有定义。
- 声明一个 pure virtual 函数的目的是为了让 derived classes 只继承函数接口
- 声明简朴的 impure virtual 函数的目的是让 derived classes 继承函数的接口和缺省实现
- 声明 non-virtual 函数的目的是为了令 derived classes 继承函数的接口及一份强制性实现
- non-virtual 函数代表的意义是不变性凌驾特异性，绝不该在 derived classes 中被重新定义
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class Shape {
public:
virtual void draw() const = 0;
virtual void error(const std::string& name);
int ObjectID() const;
};
class Rectangle: public Shape{};
class Circle: public Shape{};
Shape* ps = new Shape; // wrong, Shape is abstract
Shape* ps1 = new Rectangle; // OK
ps1->draw(); // call Rectagle::draw
Shape* ps2 = new Circle;
ps2->draw(); // call Circle::draw
ps1->Shape::draw(); // call Shape::draw
ps2->Shape::draw(); // call Shape::draw
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- 接口继承和实现继承不同。在 public 继承下，derived classes 总是继承 base classes 的接口
- pure virtual 函数只具有指定接口继承
- 简朴的 impure virtual 函数具体指定接口函数继承及缺省实现继承
- non-virtual 函数具体指定接口继承及强制性实现继承
Consider alternatives to virtual functions
基于 Non-Virtual Interface 手法实现 Template Method 模式
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class GameCharacter {
public:
int healthValue() const {
int retVal = doHealthValue();
return retVal;
}
private:
virtual int doHealthValue() const {
...
}
};
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基于 Function Pointers 实现 Strategy 模式
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class GameCharacter; // forward declaration
int defaultHealthCalc(const GameCharacter& gc);
// Strategy
class GameCharacter {
public:
typedef int (*HealthCalcFunc) (const GameCharacter&);
explicit GameCharacter(HealthCalcFunc hcf = defaultHealthCalc) : healthFunc(hcf) {}
int healthValue() {
return healthFunc(*this);
}
private:
HealthCalcFunc healthFunc;
};
class EvilBadGuy : public GameCharacter {
public:
explicit EvilBadGuy(HealthCalcFunc hcf = defaultHealthCalc) : GameCharacter(hcf) {}
};
int loseHealthQuickly(const GameCharacter&);
int loseHealthSlowly(const GameCharacter&);
// 同一人物类型可以有不同的健康计算函数
EvilBadGuy softEnemy(loseHealthQuickly);
EvilBadGuy toughEnemy(loseHealthSlowly);
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Nerver redefine an inherited non-virtual function
- non-virtual 函数是静态绑定
- virtual 函数是动态绑定
Nerver redefine a function’s inherited default parameter value
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class Shape {
public:
enum ShapeColor {Red, Green, Blue};
virtual void draw(ShapeColor color = Red) const = 0;
};
class Rectangle : public Shape {
public:
// apply with different default parameter
virtual void draw(ShapeColor color = Green) const;
};
class Circle : public Shape {
public:
virtual void draw(ShapeColor color) const;
// 静态绑定下，该函数不从其base class继承缺省参数值
// 若以指针(或 reference)调用该函数，可以不指定参数值。动态绑定会继承缺省值
};
Shape* ps; // static Shape
Shape* pc = new Circle; // static Shape
Shape* pr = new Rectangle; // static Shape
ps = pc; // ps dynamic Circle
ps = pr; // ps dynamic Rectangle
pc->draw(Shape::Red); // call Circle::draw(Shape::Red)
pr->draw(Shape::Red); // call Rectangle::draw(Shape::Red)
pr->draw(); // call Rectangle::draw(Shape::Red);
// 使用NVI手法: 令base class内的一个public non-virtual函数调用private virtual函数
class Shape {
public:
enum ShapeColor {Red, Green, Blue};
// non-virtual, must not override
void draw(ShapeColor color = Red) {
doDraw(color);
}
private:
virtual void doDraw(ShapeColor color) const = 0;
};
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Model “has-a” or “is-implemented-in-terms-of” through composition
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class Address {};
class PhoneNumber {};
// compostion class
class Person {
public:
virtual std::string some() const = 0;
private:
std::string name;
Address addr;
PhoneNumber phone;
};
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Use private inheritance judiciously
如果 classes 之间的继承关系是 private，编译器不会自动将一个 derived class 对象转换为一个 base class 对象
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class Timer {
public:
explicit Timer(int tickFreq);
virtual void onTick() const;
};
// Widget is not a timer, but can onTick
class Widget : private Timer {
private:
virtual void onTick() const;
};
// use composition
class Widget {
private:
class WidgetTimer: public Timer {
public:
virtual void onTick() const;
};
WidgetTimer timer;
};
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Use multiple inheritance judiciously
- 多重继承比单一继承复杂
- virtual 继承会增加大小、速度、初始化(及赋值)复杂度等成本。如果 virtual base classes 不带任何数据，将是最具实用价值的情况
- 多重继承的正确用途
- public 继承某个 interface class
- private 继承某个协助实现的 class
