C++ 中的参数传递方式有2中，pass by value 和 pass by reference，传递数值和传递引用，两者最主要的区别在于
pass by value: 将数值copy一份给形参，形参数值的改变不影响形参；
pass by reference : 形式参数能访问参数，形式参数和被穿参数为同一内存地址，形参的改变会引起被传递参数的改变；
如果不希望改变被传递参数的数值，建议在函数定义的时候函数头中采用 constant 关键字，表示被传递参数数值不会改变。

pass by value

int max( int x, int y)
{
if(x<=y)
{
x = y;
}
return x;

}

int main(void )
{
int x = 7,y =9;
cout<<"before" << endl;
cout<< "x value: " << x << " y value: " << y << endl;
cout<< "max : "<< max(x,y) << endl;
cout<<"after"<< endl;
cout<< "x value: " << x << " y value: " << y << endl;
return 0;

}

pass by reference

一种采用引用的方式调用；另一种采用指针调用。

引用

int max( int& x, int& y)
{
if(x<=y)
{
x = y;
}
return x;

}

int main(void )
{
int x = 7,y =9;
cout<<"before" << endl;
cout<< "x value: " << x << " y value: " << y << endl;
cout<< "max : "<< max(x,y) << endl;
cout<<"after"<< endl;
cout<< "x value: " << x << " y value: " << y << endl;
return 0;

}

指针

int max( int* x, int* y)
{
if(x<=y)
{
*x = *y;
}
return *x;

}

int main(void )
{
int x = 7,y =9;
cout<<"before" << endl;
cout<< "x value: " << x << " y value: " << y << endl;
cout<< "max : "<< max(&x,&y) << endl;
cout<<"after"<< endl;
cout<< "x value: " << x << " y value: " << y << endl;
return 0;

}

小结

1. 若在调用函数不希望改变被穿参数，采用pass by value 并在 函数定义时添加 constant 修饰符；
2. pass by reference 有两种参数传递方法，一种通过 引用，另一种是指针；
3. 若被传递参数数据量比较大，建议采用pass by reference.