交换函数（值传递，地址传递，引用作为函数参数）

本段程序主要是完成值交换函数，包括基于值传递、基于地址传递以及引用作为函数参数三个子函数。
尤其值得关注如何把引用作为函数参数，进而实现数值交换。这一段的代码如下：

void swap_y(int &a, int &b)
{
    int temp = a;
    a = b;
    b = temp;
}

全部代码以及运行效果如下：可以发现，仅仅是值传递，不能使主函数内实现数值交换，其余两种可以实现。

#include <iostream>

using namespace std;

void swap_value(int a, int b)
{
    int temp = a;
    a = b;
    b = temp;
}

void swap_ptr(int *a, int *b)
{
    int temp = *a;
    *a = *b;
    *b = temp;
}

void swap_y(int &a, int &b)
{
    int temp = a;
    a = b;
    b = temp;
}

int main(int argc, char **argv)
{
    int num1 = 10, num2 = 20;
    int x = 1, y = 2;
    int a = 100, b = 200;
    // 1 值传递
    cout << "值传递前：" << endl;
    cout << "num1 = " << num1 << endl;
    cout << "num2 = " << num2 << endl;
    swap_value(num1,num2);
    cout << "值传递后：" << endl;
    cout << "num1 = " << num1 << endl;
    cout << "num2 = " << num2 << endl;
    cout << endl;

    // 2 地址传递
    cout << "地址传递前：" << endl;
    cout << "x = " << x << endl;
    cout << "y = " << y << endl;
    swap_ptr(&x, &y);
    cout << "地址传递后:" << endl;
    cout << "x = " << x << endl;
    cout << "y = " << y << endl;
    cout << endl;

    // 3 引用
    cout << "引用前：" << endl;
    cout << "a = " << a << endl;
    cout << "b = " << b << endl;
    swap_y(a,b);
    cout << "引用后：" << endl;
    cout << "a = " << a << endl;
    cout << "b = " << b << endl;
    cout << endl;

    return 0;
}