避免遮掩继承而来的名称

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();                   // fine, calls Derived::mf1
d.mf1(x);                  // error! Derived::mf1 hides Base::mf1
d.mf2();                   // fine, calls Base::mf2
d.mf3();                   // fine, calls Derived::mf3
d.mf3(x);                  // error! Derived::mf3 hides Base::mf3

不管是不是virtual，也不管函数参数是否不同，父类的函数名称都被掩盖了

解决办法：
class Derived: public Base {
public:
  using Base::mf1;       // make all things in Base named mf1 and mf3
  using Base::mf3;       // visible (and public) in Derived's scope
  virtual void mf1();
  void mf3();
  void mf4();
  ...
};

Derived d;
int x;
...
d.mf1();                 // still fine, still calls Derived::mf1
d.mf1(x);                // now okay, calls Base::mf1
d.mf2();                 // still fine, still calls Base::mf2
d.mf3();                 // fine, calls Derived::mf3
d.mf3(x);                // now okay, calls Base::mf3

 如果你继承base class并加上重载函数，而你又希望重定义或覆写其中一部分，那么你必须为那些原本会被遮掩的每个名称引入一个using声明，否则某些你希望继承的名称会被遮掩