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In this video, we'll learn about the c++11 override specifier.

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We know that we can override base class virtual functions in our derived classes.

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We also know that we must provide the exact function signature

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in the derived class, including the return type.

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But what if we don't?

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What if we inadvertently make a small change to the function signature by mistake?

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If this happens, then the function we intended to override doesn't get overwritten at all.

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Instead it gets redefined,

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and we know that redefined functions are statically bound, not dynamically bound.

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This type of error is really hard to find sometimes

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since the function signatures may look very, very similar,

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and it may not be easy to spot the difference.

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C++11 addresses this issue

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by providing the override specifier.

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You can add this specifier to the method you're writing in your derived class,

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and the c++ compiler will ensure that you're indeed overriding and not redefining.

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This is a simple addition to the language but a very useful one.

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Let's see what it looks like in code.

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Here we have two simple classes in a class hierarchy.

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The base class has a virtual function named "say hello".

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It returns nothing, expects nothing and is a const function.

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In the derived class, I wanted to override this function.

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But when I wrote the function prototype, I forgot the const at the end of the prototype.

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Since the function signatures are not exactly, the same

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the c++ compiler considers this function redefinition and not overriding.

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It will compile just fine since this is perfectly legal.

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Let's see the difference between the methods.

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Here it might be easier to see that the function signatures are indeed different.

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So the say hello function of the derived class will be statically bound,

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not dynamically bound, as we expected.

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This will cause unexpected behavior when we use a base class pointer.

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Here we have two pointers to a base class object, P1 and p2.

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P1 is pointing to a base object, and p2 is pointing to a derived object.

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When I call say hello via p1,

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this will be bound dynamically, and it will call bases "say hello".

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However, when I call say hello via pointer 2,

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I expect it to display hello I'm a derived class object

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because I expected dynamic binding,

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but I messed up when i created the function by not matching the signature exactly.

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So the function call is statically bound to base say hello.

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We can easily prevent this error by using the c++11 override specifier.

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Notice the say hello function of the derived class.

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We can place the override specifier after the function prototype.

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This tells the compiler that you intend to override this function,

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and it should make sure that you do so.

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In this case, the compiler will produce an error, and you can add that const to fix the problem.

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Let's head over to the IDE and see this example in live code.

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I'm in the IDE, and I'm on the section 16 workspace

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in the UsingOverride project.

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You can see here that I've got a simple base class.

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And all it has inside is a virtual function right here

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called say hello. And notice that this is a const function.

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And all it says is hello I'm a base class object.

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Now I've got a derived class here that's derived publicly from the base class.

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And what I meant to do here was to override this say hello function right here.

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But I messed up, and I forgot that const right there.

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So it should have had a const there, but it doesn't. So now as far as the compiler is concerned,

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this function, right here in my derived class,

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is redefining the base class function, not overriding it.

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So it's going to statically bind it rather than dynamically bind it.

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Okay. So let's take a look at the main,

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and we'll walk through this you can see exactly what's going to happen.

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Okay, so here's p1 and p1 is a pointer to a base object.

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And I'm instantiating a base object and pointing right to it.

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So now when I call say hello via that p1 pointer,

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obviously, this is going to do dynamic binding,

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and it's going to bind it to my base class since that's what I'm pointing to so

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this will call base say hello.

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In this example, this is a little different than what we've seen.

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Here I have p2 is a pointer to a derived object,

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and I'm instantiating a derived object.

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So in this case, what's going to happen is this is not a base class pointer,

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this is just a regular old pointer to an object.

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So in this case, it's going to be statically bound, and we're going to call derived say hello.

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This is the problematic one right here, p3.

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The problem here is since we did not override instead we redefined,

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this will not be bound dynamically, it'll be bound statically.

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So as far as the compiler is concerned,

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I've got a base pointer right here, so it's going to bind it to base say hello.

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But I really wanted it to bind to derive say hello.

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All right. So let's run this, and then what we'll do is we'll fix this with the override keyword.

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So let me run this.

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And here's our output. And you can see our main. Let me scroll up just a little bit right here.

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So you can see here p1 points to a base and it's looking at a base.

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So hi, I'm a base class object.

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In the case of p2,

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we're statically binding because we're pointing to a derived object and that's exactly what we have.

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So it's going to say hello, I'm a derived class object.

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Here's the problematic one on line 30.

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P3 is pointing to a base object, and it's currently

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pointing to the derived, right.

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Here's where the problem is because

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I redefined that function so it's going to be statically bound rather than dynamically bound,

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so it's going to call base say hello and you can see right there,

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Hi, I'm a base class object. That's not what I wanted.

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I wanted it to say "hello, I'm a derived class object"

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because I wanted it to use dynamic polymorphism.

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So how do we fix this? Well, the fix is easy, right. We just come up here and put const in there.

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But we can also use the override keyword in c++11,

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which is really handy. You really should do this every time that you're going to overwrite a function in a derived class,

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simply put the override word in there.

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Right there, you can put the override specifier right at the end of the function prototype.

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So now when you compile, let me just build this real quick,

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now you get an error. And the error says virtual void derive say hello marked as override,

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but it does not override.

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That gives you a really good clue as to you're not overriding anything here.

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So you can come in here and say oh okay, I got it. There's my const.

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You can put your const in there now. And then when you run,

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you can see that now -- let me scroll up just a little bit,

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now you can see that third statement it says "hello, I'm a derived class object". That's what we want.

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So now we've got our polymorphic behavior.

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So real handy, really easy to do, use it all the time,

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and it'll save you a lot of grief with debugging.