C++多态（二）——函数重载（overloading）和操作符重载

   任何函数都能重载。

一、普通函数的重载

C语言中一个函数只能处理一个类型的数据，不可能兼顾两种或多种数据类型；C++使用使用同一名称的函数来处理多个类型的数据。

#include <iostream>
#include <vector>
using namespace  std;

double sq(double y)     //fun1
{
    return y*y;
}

int sq(int y)           //fun2
{
    return y*y;
}

double sq(int y)        //fun3
{
    return (double)y*y;
}

void main()
{
    int i=;
    double d=5.5;
    cout<<sq(i)<<endl;
    cout<<sq(d)<<endl;
}

其中的fun2和fun3冲突，因为两者的参数列表相同，不满足重载的条件。如果删掉fun3，得到答案25和30.25

所以，重载的依据是参数列表。

二、构造函数的重载

#include <iostream>
using namespace std;

class A
{
    int a, b;
public:
    A( );
    A(int i, int j);
    ~A( );
    void Set(int i, int j)
    {a=i; b=j;}
};
A::A( )
{
    a=b=;
    cout<<"No_arg_cons: a="<<a<<", b="<<b<<endl;
}
A::A(int i, int j)
{
    a=i;
    b=j;
    cout<<"Constructor: a="<<a<<", b="<<b<<endl;
}
A::~A( )
{
    cout<<"Destructor: a="<<a<<", b="<<b<<endl;
}
void main( )
{
    cout<<"Starting first round:\n";
    A     obja[];
    for(int i=; i<; i++)
        obja[i].Set(i, i+);
    cout<<"Finishing first round:\n";
    cout<<"Starting second round:\n";
    A     objb[]={A(,),A(,),A(,)};
    cout<<"Finishing second round.\n";
    cout<<"Program being terminated!\n";
}

Result：
Starting first round:
No_arg_cons: a=0, b=0
No_arg_cons: a=0, b=0
No_arg_cons: a=0, b=0
Finishing first round:
Starting second round:
Constructor: a=5, b=6
Constructor: a=7, b=8
Constructor: a=9, b=10
Finishing second round.
Program being terminated!
Destructor: a=9, b=10
Destructor: a=7, b=8
Destructor: a=5, b=6
Destructor: a=2, b=3
Destructor: a=1, b=2
Destructor: a=0, b=1

三、运算符重载

在面向对象程序设计语言中，运算符也是一种函数，所以运算符也能像函数一样给予重载，以便完成更多功能。

运算符重载有两种实现方式，一种作为友元函数，另一种是成员函数。

1.友元函数方式

#include <iostream>
using namespace std;

class point
{
    int x, y;
public:
    point (int vx=, int vy=)
    {
        x = vx;
        y = vy;
    }
    friend point operator + (point & p1, point & p2);
    friend point operator - (point & p1, point & p2);
    void print()
    {
        cout<<x<<" "<<y<<endl;
    }
};

point operator +(point & p1, point & p2)
{
    point p;
    p.x = p1.x + p2.x;
    p.y = p1.y + p2.y;
    return p;
}         //不能返回[局部变量的]引用
point operator -(point & p1, point & p2)
{
    point p;
    p.x = p1.x - p2.x;
    p.y = p1.y - p2.y;
    return p;
}
void main()
{
    point p1(,),p2(,);
    p1.print();
    p1 = p1 + p2;         //即p1= operator + (p1, p2)
    p1.print();
    p2 = p1 - p2;             //即p2= operator - (p1, p2)
    p2.print( );
}

2.成员函数方式

#include <iostream>
using namespace std;

class point
{
    int x, y;
public:
    point() { x = ; y = ; }
    point (int vx, int vy)
    { x = vx; y = vy; }
    point operator + (point & p);
    point operator - (point & p);
    void print() {cout<<x<<" "<<y<<endl; }
};
point point::operator +(point & p)
{
    point temp;
    temp.x = x + p.x;    //就是temp.x = this->x + p.x;两者相加
    temp.y = y + p.y;
    return temp;
}
point point::operator -(point & p)
{
    point temp;
    temp.x = x - p.x;
    temp.y = y - p.y;
    return temp;
}
void main()
{
    point  p1(,), p2(,), p3;
    p3.print();
    p3 = p1 + p2;        //即p3 = p1.operator + (p2);
    p3.print();
    p3 = p3 - p1;        //即p3 = p3.operator - (p1);
    p3.print();
}

 25

   30.25