关于passing by value 和 passing by reference 我始终不能理解
请高手指点
 参照下边的程序
 #include <iostream>
using namespace std;
void swapr(int & a, int & b);  
void swapp(int * p, int * q);   
void swapv(int a, int b);      
int main()
{
    int wallet1 = 300;
    int wallet2 = 350;

    cout << "wallet1 = $" << wallet1;
    cout << " wallet2 = $" << wallet2 << "\ n";

    cout << "Using references to swap contents:\ n";
    swapr(wallet1, wallet2);   
    cout << "wallet1 = $" << wallet1;
    cout << " wallet2 = $" << wallet2 << "\ n";

    cout << "Using pointers to swap contents:\ n";
    swapp(&wallet1, &wallet2);
    cout << "wallet1 = $" << wallet1;
    cout << " wallet2 = $" << wallet2 << "\ n";

    cout << "Trying to use passing by value:\ n";
    swapv(wallet1, wallet2);   
    cout << "wallet1 = $" << wallet1;
    cout << " wallet2 = $" << wallet2 << "\ n";
    return 0;
}

void swapr(int & a, int & b)    {
    int temp;

    temp = a;      
    a = b;
    b = temp;
}

void swapp(int * p, int * q)   
{
    int temp;

    temp = *p;      
    *p = *q;
    *q = temp;
}

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