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:
- 调用的方法是哪个?
- 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)
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