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类的继承
● 通过类的继承（派生）来引入“是一个”的关系（ 17.2 — Basic inheritance in C++）

– 通常采用 public 继承（ struct V.S. class ）
– 注意：继承部分不是类的声明
– 使用基类的指针或引用可以指向派生类对象

struct Base
{};
struct Base2 : Base
{};struct Derive : Base2 //注意，不是声明，该语句等价于struct Derive : public Base，即struct缺省继承方式为public
//class Derive : Base2 //等价于class Derive : private Base，即class缺省继承方式为private
{};int main()
{Derive d;Base& ref = d; //OKBase* ptr = &d; //OK
}

– 静态类型 V.S. 动态类型

struct Base
{void funBase() {} //静态类型
};
struct Base2 : Base
{void funBase2() {} //静态类型
};
struct Derive : Base2
{void funDerive() {} //静态类型
};
struct Derive2 : Base2
{void funDerive2() {} //静态类型
};
int main()
{Derive d;Derive2 d2;Base& ref = d; //静态类型: 即编译期就能决定的类型且不能改变Base* ptr = &d; //动态类型ref.funBase(); //OK，静态类型没有改变ref.funDerive(); //静态类型不能发生改变，Error: No member named 'funDerive' in 'Base'ptr = &d2; //OK，动态类型可以在运行期发生改变return 0;
}

– protected 限定符：派生类可访问

struct Base
{
private:void funBase() {}
};
struct Derive : Base
{void funDerive(){funBase(); //Error: 'funBase' is a private member of 'Base'}
};
int main()
{Base b;b.funBase(); //Error: 'funBase' is a private member of 'Base'return 0;
}

struct Base
{void funBase() {}
};
struct Derive : Base
{void funDerive(){funBase(); //OK}
};
int main()
{Base b;b.funBase(); //OKDerive d;d.funBase(); //OKd.funDerive(); //OKreturn 0;
}

struct Base
{
protected:void funBase() {}
};
struct Derive : Base
{void funDerive(){funBase(); //OK}
};
int main()
{Base b;b.funBase(); //Error: 'funBase' is a protected member of 'Base'Derive d;d.funBase(); //Error: 'funBase' is a protected member of 'Base'd.funDerive(); //OKreturn 0;
}

● 类的派生会形成嵌套域
– 派生类所在域位于基类内部
– 派生类中的名称定义会覆盖基类

struct Base
{int val = 2;
};
struct Derive : Base
{void funDerive(){std::cout << val << std::endl;}int val = 3;
};
int main()
{Derive d;d.funDerive();return 0;
}

struct Base
{int val = 2;
};
struct Derive : Base
{void funDerive(){std::cout << val << std::endl;}//int val = 3;
};
int main()
{Derive d;d.funDerive();return 0;
}

– 使用域操作符显式访问基类成员

struct Base
{int val = 2;
};
struct Derive : Base
{void funDerive(){std::cout << val << std::endl;std::cout << Base::val << std::endl;}int val = 3;
};
int main()
{Derive d;d.funDerive();return 0;
}

– 在派生类中调用基类的构造函数

struct Base
{Base(){std::cout << "Base()\n";}
};
struct Derive : Base
{Derive(){std::cout << "Derive()\n";}
};
int main()
{Derive d;return 0;
}

struct Base
{Base(int){std::cout << "Base()\n";}
};
struct Derive : Base
{Derive(int a) //缺省构造函数调用基类的缺省构造函数，Error: Constructor for 'Derive' must explicitly initialize the base class 'Base' which does not have a default constructor{std::cout << "Derive()\n";}
};
int main()
{Derive d;return 0;
}

struct Base
{Base(int){std::cout << "Base()\n";}
};
struct Derive : Base
{Derive(int a) //缺省构造函数调用基类的缺省构造函数，Error: Constructor for 'Derive' must explicitly initialize the base class 'Base' which does not have a default constructor: Base(a) //显式调用基类的构造函数{//Base::Base(a); //Error: Qualified reference to 'Base' is a constructor name rather than a type in this contextstd::cout << "Derive()\n";}
};
int main()
{Derive d(2);return 0;
}

参考
深蓝学院： C++基础与深度解析
17.2 — Basic inheritance in C++