Pass by Reference

00:00 In this lesson, we’ll define what pass by reference is.

00:05 Pass by reference means that the function receives a reference—link, pointer—to the argument passed as a parameter. The parameter value doesn’t receive a copy of that value. Instead, the parameter variable actually refers to the same piece of memory that the argument referred to. They become two different names for the same object.

00:28 Recall in the example from the last lesson, how the array of numbers—10.0, 2.0, and so on to 7.0—was assigned to the variable numbers, then that array was passed to the function parameter variable a.

00:43 In C++, passing an array like this would have used the mechanism pass by reference. Let’s take a look at how that works in memory. I’ll use that same simplified model of memory to demonstrate what happens with pass by reference. First, before it’s even assigned a name, a space in memory is created to store the array. Because an array and other complex objects have different memory needs, they’re not stored directly with any variable names they’re assigned to.

01:16 The location of the array is given some internal notation. I’m just making something up for the purposes of this example, _array1. The actual process often involves memory addresses in some way, and I didn’t need to get that detailed with this example.

01:36 When it is assigned to the variable numbers, a reference to the array’s location is put in the spot of memory designated for numbers.

01:46 I’ve called that reference &_array1 to indicate that it’s a reference to _array1.

01:55 When arrayMean() is called, that reference to the array is copied to the memory location designated for the parameter a.

02:06 Both numbers and a refer to the one array in memory.

02:12 The array itself isn’t copied, saving both memory and time. This is the payoff of pass by reference.

02:22 So again, pass by reference means the parameter variable is given a reference, not the value, to the argument provided. This has some interesting consequences. For example, let’s say you write a function that modifies the parameter variable Then, those changes will remain after the function ends and will be noticed when you use the argument variable in some way.

02:46 Since the object in memory exists outside of the function, changes would persist once the function ends. This would be an example of something called a side effect.

02:57 You are changing part of the program environment—in this case, the object the argument refers to. Which begs the question: Why would you want to do that?

03:10 Well, once programmers realized that you could impact an argument through a function when pass by reference is being used, they came up with all sorts of good reasons to do so, and here are a couple of the biggest ones.

03:23 Maybe the goal of the function is to make some type of change to a complex object or structure, like adding or removing elements from a given array or collection.

03:33 Many programming languages allow for only one actual return value. So you could pass by reference another argument, then assign that argument to something that could be considered a second return value, in addition to what the return statement would give back to the calling environment.

03:51 Notice, we’re still not talking about Python. Python doesn’t use pass by reference either. We’re almost there, but I want to show you what pass by value and pass by reference look like in C++ and then look at what happens when we compare those behaviors to what Python does. You’ll see that comparison in the next lesson.