# 8.7. Call by reference¶

An alternative parameter-passing mechanism that is available in C++ is called “pass by reference.” This mechanism makes it possible to pass a structure to a procedure and modify it.

For example, you can reflect a point around the 45-degree line by swapping the two coordinates. The most obvious (but incorrect) way to write a reflect function is something like this:

void reflect (Point p) {      // WRONG !!
double temp = p.x;
p.x = p.y;
p.y = temp;
}


But this won’t work, because the changes we make in reflect will have no effect on the caller.

Instead, we have to specify that we want to pass the parameter by reference. We do that by adding an ampersand (&) to the parameter declaration:

void reflect (Point& p) {
double temp = p.x;
p.x = p.y;
p.y = temp;
}


Now we can call the function in the usual way:

printPoint (blank);
reflect (blank);
printPoint (blank);


Take a look at the active code below. reflect passes the parameter p by reference. Notice that the output of this code matches what we expect it to be.

(3, 4)
(4, 3)


Here’s how we would draw a stack diagram for this program:

The parameter p is a reference to the structure named blank. The usual representation for a reference is a dot with an arrow that points to whatever the reference refers to.

The important thing to see in this diagram is that any changes that reflect makes in p will also affect blank.

Passing structures by reference is more versatile than passing by value, because the callee can modify the structure. It is also faster, because the system does not have to copy the whole structure. On the other hand, it is less safe, since it is harder to keep track of what gets modified where. Nevertheless, in C++ programs, almost all structures are passed by reference almost all the time. In this book I will follow that convention.