Pass by Reference vs Pass by Value

00:00 In this lesson, you’ll compare the notions of pass by value and pass by reference, and later, you’ll compare them to Python’s argument passing mechanism.

00:10 Much of this lesson is taken from another Real Python course I’ve done over functions in general. In this lesson, we’re going to look at a program which does the following: First, it’s going to create a variable and assign it a value.

00:25 In these examples, I’ll be calling the variable x. That variable will then be used as the argument to a function called f. Really inventive variable names, I know. That function will then take the associated parameter value called fx—to mean the variable x in the function f—and reassign it.

00:47 It will print the value of that parameter variable both before and after it’s changed, and the main program will print the value of x before and after the function is called.

00:59 I’ll show this to you using pass by value in C++, pass by reference in C++, and in the following lesson, you’ll see how this looks in Python. So, here is the pass by value version in C++.

01:16 You can see at the top, our function f(), taking a single parameter called fx. Because fx is one of C++’s, simple types, an integer, this will use pass by value.

01:32 fx will get a copy of the value provided as an argument.

01:39 Here’s C++’s weird output statement. First, there is a label to describe what’s about to be displayed, followed by the value of the parameter fx.

01:54 The parameter variable is reassigned a new value, then its new value is displayed with an appropriate label. Down here is the main() function.

02:06 It creates a variable x and assigns it a value. That value is displayed with the label using cout. It then calls the function f(), passing x as an argument, and finally displays the value of x again, after the function was called. I can go into my terminal shell, compile it, and show you how it runs.

02:49 Since the argument was passed by value, the function f() has no knowledge of the variable x. It simply received a value of 5.

03:01 The value of fx was indeed changed to 10, but the original variable x was unaffected by that.

03:11 This next version uses pass by reference. There’s only one change from the previous version to this one. That’s the use of an ampersand (&) in front of the parameter name, fx.

03:24 See, everything else is the same

03:29 as I switch back and forth between the two file listings. Except, of course, the name. Everything else is the same. But the & is what tells C++ to use pass by reference for this parameter.

03:45 So this time when we use x as the argument in the function call, the parameter fx will be referring to the exact same piece of memory that x is using.

03:57 That means when it changes the value of fx here, it is also changing the value of x. You can see that when I compile and run this program as well.

04:27 Notice this time that x now has a value of 10 after the function is finished. That’s because it was pass by reference. f() received a reference to x, which it called fx, so anything done to fx in the function affected the value of x outside the function. In your next lesson, you’ll see what Python’s mechanism is and compare it to both of these.