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‫So in this video, we're going to talk about the call stack.

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‫Now, earlier in the course, we had a data structure called a stack and we said it was like a can of

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‫tennis balls.

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‫And a lot of the principles of stacks that we discussed previously will apply to call stacks.

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‫So, for example, you can only get to the item at the top of the call stack, and once that is popped,

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‫you can get to the next item and so on.

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‫So I'm going to take this container and move it over here and bring in a function.

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‫I'm going to call this function one and all it does is output this string one.

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‫But when we call that function, that function gets added to the call stack and then after it outputs

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‫one, this function is done running and function one gets popped from the call stack.

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‫So I'm going to bring this back and put it back on our call stack and we're going to add a line to this

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‫function.

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‫And with that line, we're going to call function two.

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‫Well, once you call function to function two becomes the top item on the call stack.

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‫And once function two is added to the call stack, function, one stops running at pauses and it can

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‫only start running again once function two is popped from the call stack.

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‫So now we're going to add a line to function two where it calls function three.

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‫Now function three is at the top of the call stack, and neither function one nor function two are running.

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‫And we're not going to add anything to this function.

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‫We're just going to have it output three to the screen.

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‫And once it does this, this function is done running and function three is popped from the call stack.

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‫Now function two can run again and function two just got done running this line.

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‫Now it moves to this line and outputs two to the console.

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‫Once it does, this function two will be popped from the call stack.

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‫Now function one can run again.

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‫It just got done running this line of code.

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‫Now it moves to the next line of code and outputs one to the screen and that is popped from the call

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‫stack.

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‫So notice that the functions were called one, two, three, but the output is 3 to 1.

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‫That's because that's the order.

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‫These came off of the call stack.

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‫So I'm going to bring our three functions up and we'll flip over and look at these NVS code.

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‫So there are the three functions there.

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‫And then in our main function, that is where we do the initial function call of function one.

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‫So if we run this, we get 3 to 1 just like we would expect.

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‫But let's take a look at how these go on to and come off of the call stack.

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‫In order to do that, I'm going to come over here.

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‫I'm going to click on this button where it says Run and debug and we'll get this side pane.

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‫And you can see right here it says Call Stack, and then I'm going to come down here to the left of

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‫the number that's next to where it says function one in the main function.

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‫And you can see we get a red dot, a dark red dot.

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‫But if I actually click this, it turns into a bright red dot.

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‫And what that creates is what is called a breakpoint.

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‫So the code will only run up to that point and stop.

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‫And then I'll come up here to this dropdown and I'll select debug C and C++ file.

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‫And when I do this, what I want to do is ignore what is on the terminal there, come up to the three

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‫dots, click on this and choose debug console.

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‫We'll click on that and you're going to get a bunch of information here that you can ignore.

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‫I'm going to right click and say clear console.

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‫And you can see when I did this that this line of code is now highlighted.

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‫That means that's the line of code that is about to run.

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‫And on our call stack we have the main function.

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‫That's the function we're in down here.

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‫And then up here we have this button that says, step into this downward facing button.

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‫So I'll zoom back out here and when I click on this, it's going to run the next line of code, which

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‫is going to be function one down here.

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‫So I'll click on this and you can see that function one is added to the call stack and because it's

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‫at the top of the call stack, that is the function that is running now here because function two is

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‫highlighted.

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‫That means that's the next line of code that's about to run and I'm going to click Step Into Again and

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‫you can see that function two has been added to the call stack.

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‫This is the next line to run function three.

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‫I'll click on Step Into Again and now Function three has been added to the call stack.

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‫The next line to run is going to be this and when I click on step into it will output three to that

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‫debug console.

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‫Now function three is done running and when I click step into a couple more times it is removed from

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‫the call stack.

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‫Now function two picks up where it left off and now it's going to output two.

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‫So I'll click step into again that gets output to the debug console there and then I'll click step into

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‫a couple more times and function two has been removed from the call stack.

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‫So now with this line, we'll output the number one I'll click step into and that has been output up

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‫here and now function one is about to be popped from the call stack.

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‫I'll click on step into a couple of times and now that has been removed and 3 to 1 has been output to

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‫the debug console.

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‫So while this example is not recursive, I wanted to do it this way because it makes it easier to understand

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‫the concept of the call stack.

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‫But in the next video, we will look at a recursive example of how things go onto and come off of the

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‫call stack as we get into the course, especially with the recursive functions.

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‫I'm going to recommend that you create a breakpoint and step through this to see how it is all going

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‫onto and off of the call stack.

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‫But for now, that is our quick introduction of the call stack.