c++ - 带有 & 符号的函数原型是什么意思？

Question

可能重复：
'&' 运算符在 C++ 中的作用是什么？

今天在我的 CS 课上，老师向我们展示了一些函数和模板的示例，并且一些函数原型在参数列表中具有与号，如下所示：

void exchange( T & x, T & y ) ; // prototype

这意味着什么？我应该用它做什么？

Answer 1

the & is for reference. In short that's something like a pointer, that can't be NULL. Wikipedia has something on this topic here.

References are cheap when they are used in function/method calls, since the object doesn't need to be copied in your function call. You still have the same syntax as if you had with a copied object. With pointers you would need to handle the special case, that the pointer is NULL.

That is a usual reason to use them. If I guess right and exchange means something like swap the tow objects x and y, the the cost of the function call is directly related to the cost of coping the object, so saving some copies may be a relevant optimization.

Answer 2

The & means 'reference to' - x is a reference to a T, not a copy.