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In this video, I'd like to briefly talk about exceptions in the context of

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c++ class.

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Exceptions can be thrown from class methods,

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constructors and destructors.

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Let's talk about the last one first.

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Do not throw exceptions from a destructor.

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The destructor is marked in c++ as no except by default,

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which means that it does not throw exceptions.

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If a destructor is called as a result of an exception being thrown elsewhere

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and then the destructor throws an exception,

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then there's a problem since the original catch block will never be reached.

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This is a very bad situation.

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The only time a destructor might throw an exception

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is if it handles it itself, and that's a very unlikely use case.

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So remember, best practice is to not throw exceptions from a destructor.

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Okay. So now let's talk about methods and constructors.

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In the case of class methods,

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exception handling here works just as we've seen with regular functions.

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What about constructors?

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Constructors could certainly fail.

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And since constructors don't return values, we can't return a Boolean or an error code.

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Constructors may fail for many reasons. For example, suppose you're allocating

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memory dynamically and you're unable to allocate

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it or if you open a file in the constructor but the file doesn't exist.

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In cases such as these, the constructor can throw an exception.

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The rest works just as you would expect.

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Let's see an example of throwing an exception from a constructor.

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In this slide, we're using the account classes that we've been using throughout the course.

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Suppose we cannot have accounts with negative balances.

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So in the account class constructor,

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we can check the balance and if it's less than 0,

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throw an illegal balance exception object.

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In this case, we're assuming that we've created our own illegal balance exception class.

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That's it. Now let's see how we can catch this exception if it occurs.

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So here we have a try block. And in that try block, we're declaring a unique pointer to an account object.

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And using make unique to create a checking account object for Moe

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with a negative balance on the heap.

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Since the account balance is negative,

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the account constructor is going to throw an illegal balance exception,

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which we're catching in our catch block.

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We display an error, and our program continues.

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Now there's a lot more to class level exceptions.

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But for the purpose of this course, this is the basic information necessary to use

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and understand class level exceptions.

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In the next video, we'll look at the exception class hierarchy that the c++

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standard library uses internally,

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and we'll see how we can tap into that hierarchy for our own exception classes.

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But first, let's head over to the IDE and let's throw a class level exception

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from our account class.

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Welcome back. I'm in the IDE, and I'm in the still in the section 18 workspace,

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and I'm in the ConstructorException project.

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And what I've done here is I've copied all those files

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that we created for the account class that we've been using throughout the term.

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So I've got accounts and checking accounts and printables

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and basically, everything we've done, this is the latest version of it.

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But what I've done is I've added a new

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header file right here called illegal balance exception.

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And i don't need the cpp file because everything can be done in this header file.

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Notice what I've done here. I've created a class called illegal balance exception,

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which represents the condition where we've got an illegal balance, in this case,

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a negative value.

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This is the exception object we're going to throw.

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And then we've got a constructor and a destructor.

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And I've tagged them both as default that way we're using just regular default constructor

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and regular default destructor. That's it.

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Now what we want to do is go to our account cpp file.

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And what we're going to do is in the account constructor right here

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we're going to check to see if the balance is less than 0.

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If the balance is negative, then we're going to throw in an illegal balance exception.

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That's that class that we just created. This will throw an object of that class.

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Okay.

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Now I mentioned that there was a lot more to constructor level

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exception handling.

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One of the things that you can do with constructors is you can try

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remember we have an initializer list here.

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We could try to execute some of that.

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So it's possible to put try statements in there as well.

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In this case, we wouldn't do that, but you might want to do that in subclasses.

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Again, that's a little bit beyond the scope of the class, but I just wanted to bring it to your attention

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in case you see that out there.

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So this is it here, right. We've got an account. If the balance is negative, we're going to throw an exception.

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So now let's look at our main and let's test this out.

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So here's my main. And notice what I've done in the main.

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I've got a try block right here. Let me grab my pen so I can mark this up

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and make it a little clearer.

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I've got a try block right here. And in the try block,

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I've got a unique pointer to the base class.

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I call it Moe's account.

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And I'm using std make unique to create a checking account on the heap.

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That checking account contains the name Moe

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and notice that negative balance right there.

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Okay. That's going to throw an exception. We're using this in the try block

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because that constructor for the account class. In this case, remember, it's checking account,

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but it's calling the base class constructor as well. And that's the guy that's going to throw the exception

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So in this case, this will throw an exception.

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It'll throw a illegal balance exception which I'm catching here

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by const ref.

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If it doesn't throw an exception, then we simply de-reference Moe's account and print

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it out to stand it out.

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Okay. If it does throw an exception, we're going to catch it right here.

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And we're going to deal with it. How do we deal with it? We're just going to say couldn't create account, negative balance.

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That's it. But the point is our program completes successfully. This will execute.

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It doesn't crash. Okay. Now let's do this two different ways.

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Let's run this test case. And first, let's create it with a positive balance,

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a 10 dollar balance.So let me build and run,

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and the output says checking account Moe 10 dollars,

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program completed successfully, just what we'd expect.

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But now let's create the negative balance, and we'll build and run again.

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And notice the output now says couldn't create account negative balance.

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And again, program completed successfully. That's pretty cool.

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So that works really, really well. And that pretty much does it.

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For the section challenge in this section, it's actually a pretty easy challenge.

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Although some of the things will be a little bit advanced, the challenge is pretty easy to do.

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What I'd like you to do for that challenge, and I'll talk about it in the challenge video

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is to provide an exception

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when we've got insufficient funds. Okay. Remember, before we created

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right here this withdrawal that was true false.

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Well, rather than do true false, let's throw an exception and catch it,

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but we'll get to that soon.

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In the next video, we'll talk about the c++ standard exception class hierarchy.