问题
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What does the '&' operator do in C++?
In my CS class today the teacher showed us some examples of functions and templates and some of the prototypes for functions had ampersands in the list of parameters like this:
void exchange( T & x, T & y ) ; // prototype
what does that mean? What should I use it for?
回答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.
回答2:
The & means 'reference to' - x is a reference to a T, not a copy.
来源：https://stackoverflow.com/questions/9488894/what-does-a-function-prototype-mean-with-an-ampersand-in-it