C++覆盖,隐藏,重载

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定义

• 函数覆盖
  
  (1) 作用域不同
  
  (2) 有virtual关键字
  
  (3) 参数列表/返回值/调用约定必须相同
• 函数隐藏
  
  (1) 作用域不同
  
  (2) 函数名相同
  
  (3) 参数列表/返回值/调用约定不考虑
• 函数重载
  
  (1) 作用域相同
  
  (2) 函数名相同
  
  (3) 参数列表不同,返回值/调用约定不考

练习

class Base
{
public:
    virtual void Handle1(float x)
    {
        cout << "Base::Handle1(float) " << x << endl;
    }
    void Handle2(float x)
    {
        cout << "Base::Handle2(float) " << x << endl;
    }
    void Handle3(float x)
    {
        cout << "Base::Handle3(float) " << x << endl;
    }
};

class Derived : public Base
{
public:
    virtual void Handle1(float x)
    {
        cout << "Derived::Handle1(float) " << x << endl;
    }
    void Handle2(int x)
    {
        cout << "Derived::Handle2(int) " << x << endl;
    }
    void Handle3(float x)
    {
        cout << "Derived::Handle3(float) " << x << endl;
    }
    void Handle3(double x)
    {
        cout << "Derived::Handle3(double) " << x << endl;
    }
};

观察以上类声明及实现，试预测以下main中的执行结果,注意两点：

1. 调用的方法是哪个?
2. 调用造成的原因，是重载、覆盖、隐藏中的哪一种?

 void main(void)
 {
     Derived DervObj;
     Base *pBase = &DervObj;
     Derived *pDerv = &DervObj;

     pBase->Handle1(3.14f);
     /******************************************
     预测：
           输出Derived::Handle1(float)3.14
           是间接调用，覆盖
     运行结果：Derived::Handle1(float)3.14
     ******************************************/

     pDerv->Handle1(3.14f);
     cout << endl;
     /******************************************
     预测：
           输出Derived::Handle1(float)3.14
           是间接调用，隐藏
     运行结果：Derived::Handle1(float)3.14
     ******************************************/

     pBase->Handle2(3.14f);
     /*****************************************
     预测：
           输出Base::Handle2(float)3.14
           是直接调用，隐藏
     运行结果：Base::Handle2(float)3.14
     *****************************************/

     pDerv->Handle2(3.14f);
     /****************************************
     预测：
           输出Derived::Handle2(int)3
           是直接调用，隐藏
     运行结果：Derived::Handle2(int)3
     *****************************************/
     cout << endl;

     pBase->Handle3(3.14f);
     /****************************************
     预测：
           输出Base::Handle3(float)3.14
           直接调用，隐藏
     结果：Base::Handle3(float) 3.14
     }
     ****************************************/

     pDerv->Handle3(3.14f);
     /***************************************
       预测：输出Derived::Handle3(float)3.14
             直接调用，隐藏
       结果：Derived::Handle3(float) 3.14
     }
     ***************************************/

     pDerv->Handle3(3.14);
     /**************************************
       预测：输出Derived::Handle3(double)3.14
             直接调用，隐藏
       结果：Derived::Handle3(double) 3.14
     **************************************/
     cout << endl;

     pDerv->Handle3(3);
     /****************************************
       Handle3没有对应的整数形参，只有float和double
       3既可以转换为float,又可以转换为double,所以编译时报错。
     /****************************************/
     system("pause");
 }

总结

当覆盖，隐藏，重载同时存在时，如何确定调用了某个类的某个函数？

a)查看调用该函数的数据类型，并找到该类型的作用域(函数隐藏)

b)在该作用域中找到同名函数，满足参数列表相同(可以做适当的类型转换),在从这些同名函数中选出最佳的，

如果有多个则产生二义性(函数重载)

c)查看选出的最佳匹配函数是否是虚函数，如果是虚函数并且调用该虚函数的数据类型为指针或引用，

那么就是间接调用，否则就是直接调用(函数覆盖)