C++中重载、覆盖和隐藏

一，多态性

1，（1）声明了基类的指针，该指针指向基类，该指针永远调用自己的成员函数，不管函数是否为虚函数。

（2）声明了派生类的指针，该指针指向该派生类，该指针永远调用自己的成员函数，不管函数是否为虚函数。

（3）声明了基类的指针，该指针指向任意一个子类对象，该指针调用一个函数（此函数在基类存在，而且在子类中被重新定义），此时编译系统不清楚该调用哪一个函数，而出错。我们可以将该函数设定为虚函数，则程序运行时候，编译系统会知道调用的是基类函数还是子类函数，此即多态。注：如果调用的函数不是虚函数，则调用自己类中的成员函数。如果指针被声明为基类，尽管它指向子类对象，仍然是基类指针。

2，子类重定义父类成员函数方法，被称为覆盖或重写。覆盖的函数都是虚函数。

3，满足虚函数条件或覆盖的特征是：
   （1）不同的范围（分别位于派生类与基类）；
   （2）函数名字相同；
   （3）参数相同；
   （4）基类函数必须有virtual 关键字。

二，成员函数重载

成员函数被重载的特征：
（1）相同的范围（在同一个类中）；
（2）函数名字相同；
（3）参数不同；
（4）virtual 关键字可有可无。

三，成员函数隐藏

看如下代码：

Base.h

#include <iostream>
using namespace std;

class Base{
public:
    void fn(int x){
        cout << "Base" << x << endl;
    }
    void fn1(int x){
        cout << "Base1" << x << endl;
    }
};

SubClass.h

#include <iostream>
using namespace std;

class SubClass :public Base{
public:
    void fn(float x){
        cout << "SubClass" << x << endl;
    }
    void fn1(int x){
        cout << "SubClass1" << x << endl;
    }
};

main.cpp

#include <iostream>
#include "A.h"
#include "B.h"
#include "C.h"
using namespace std;

int main()
{
    C c();
    cout << c.GetData() << endl;
    cout << c.A::GetData() << endl;
    cout << c.B::GetData() << endl;
    cout << c.C::GetData() << endl;
    cout << c.doGetData() << endl;
    cout << c.A::doGetData() << endl;
    cout << c.B::doGetData() << endl;
    cout << c.C::doGetData() << endl;
    return ;
}

运行结果：

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" alt="" />

fn()函数不符合虚函数条件，不是虚函数。

从SubClass角度来说，其有void fn(float x)函数，从Base继承了void fn(int x)函数，因此这两个函数在SubClass内应该是重载的，因此主函数中SubClass对象sc，sc.fn(3.1f)应该调用void fn(float x)函数，sc.fn(1)应该调用void fn(int x)，但是运行结果显示两个都调用了void fn(float x)函数，这是因为Base中的void fn(int x)函数被隐藏了。

主函数中sc.fn1(2);时候，通过结果知道调用了void fn1(int x)函数，Base中的void fn1(int x)函数被隐藏了。

因此知道符合函数隐藏条件是：

（1）如果派生类的函数与基类的函数同名，但是参数不同。此时，不论有无virtual关键字，基类的函数将被隐藏。注意别与重载混淆。
（2）如果派生类的函数与基类的函数同名，并且参数也相同，但是基类函数没有virtual关键字。此时，基类的函数被隐藏。注意别与覆盖混淆。