The usual way of passing parameters in C++, and the only way in C, is by value. Each parameter becomes a copy of its argument, and changes to in the function body do not make changes in the caller. This is similar to Java. C++ provides reference parameters, which are denoted using an & in front of the parameter name. Reference parameters become aliases for their arguments, and changes to the parameters immediately change the arguments. You cannot send an expression argument to a reference parameter; you must send something which could appear on the left side of an assignment.

The swap function is a classic use of reference parameters. When the reference parameters are exchanged, the arguments are exchanged since the parameters are aliases for them. Also notice that the stats function receives x and y by value, and uses swap to exchange its copies, but this does not effect the values of c and d in the main.

Reference parameters are by far the main use of the reference mechanism in C++. It can also be used for return values, and, though it seems to have no practical use, ordinary variables.