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In this video, we'll talk about compiler errors and why they can happen.

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Programming languages have very specific rules about how to write code,

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so that the compiler can translate that code to machine code.

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These rules involve both structure and meaning.

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For example, c++ says that all variable declarations

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must end with a semicolon.

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If we don't add the semicolon at the end or we put it in the middle instead,

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then we have a syntax error.

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The compiler is complaining that the structure is not correct.

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In the first example, we didn't terminate the string error

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with a trailing quote character.

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In the second example, we left out the semicolon at the end of the return 0 statement.

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Both of these are syntax errors. There's something wrong with the structure of the code.

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Now these syntax errors are pretty easy to fix, and the compiler errors

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usually give us pretty good information about what's going on.

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Semantic errors are a little different.

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In this case, the syntax or structure is correct.

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But the meaning of the code doesn't make sense to the compiler.

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Suppose we want to add two variables, a and b.

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If the variables are both integers, then this makes perfect sense.

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But what happens if one of the variables

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is a person and the other variable is an integer.

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Does it make sense to add a person and an integer?

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Probably not. In this case, the compiler generates an error.

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When we think of compiler errors, we really think about both of these classes of errors.

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Remember, compilers are very, very picky. Why,

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because their job is to translate your code that you wrote into machine

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code that functions as you said.

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If the compiler has any problem understanding the meaning of your code,

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then it won't guess, it will generate an error.

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Let's head over to CodeLite and see some of these errors.

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So now I'm in CodeLite and I created a project called compiler errors.

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And I created it right from our template, so it's just a simple

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hello world program which is printing out hello world.

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Now what we want to do is we want to introduce some compiler errors into this program,

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so you can see what they're like. Now let's do some --

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first let's do some syntax errors.

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Simple stuff like okay we forgot this quote right here.

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And I can save and compile,

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and you'll see that I'll get an error that says

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missing terminating character and if you click on here it takes you here.

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Okay. Now in this case we've got another error

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that says expected primary expression before return.

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This happens sometimes.

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The compiler gets confused it sees that first error and.

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Then it's trying to figure out what's going on and it goes to the next line

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and it says i don't know where I'm at it creates an error.

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Many times when you fix that first error, many of the subsequent errors go away.

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So let's put that quote back in.

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We'll save, and let's recompile.

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Now we've got a clean compile.

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So that would be my advice. When you're first learning,

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start out at the beginning, get that first syntax error fixed, recompile

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and very likely a whole bunch of other errors just went away.

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Okay. So that's an example of a syntax error.The compiler is expecting

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a string and it knows it's got a rule that says all strings have to have a quote character,

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a bunch of characters and then a quote character at the end to terminate it.

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It didn't see that. It didn't like that. And there's the syntax error, really easy to fix.

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Other times you may have things that are misspelled.

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So for example, let's say I spelt end

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l here wrong, I put an extra l in there. Now when i compile,

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it's saying error. And ll

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is not a member of the standard namespace.

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So it knows that cout is and endl is

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but endll is not.

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So in this case,

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this error message kind of leads you to the place

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where the problem is. And just oh of course I have a typo. So you can fix that as well.

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Okay. Another example of a compiler error

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that's a syntax error is forgetting the semicolon here let's say

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at the return 0.

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All statements have to end with a semicolon, so we just forgot it

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that violates one of the rules that the compiler

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knows about and it doesn't like it. So what we'll do here again we'll compile

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and you can see it says error expecting a semicolon

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before that curly.

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So there's the curly, and it's expecting a semicolon

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sure is because of that.

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Okay. Now suppose we just say return semicolon.

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Now this is problematic because we've told the compiler

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in our code that main returns an integer.

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But here, we're just returning. We're not returning an integer. So let's see what it says.

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Let's compile,

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and it says error return statement with no value

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in function returning integer. Pretty good statement, right.

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It's telling you I've got a function. You told me it's returning an integer, but you're not returning any

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value now. So I can

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return 0.

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Okay. So let me show you an example of

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a semantic error, something like this.

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This is perfectly valid. I can return an integer. I can return a string.

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I can return a lot of things. The problem is

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it doesn't make sense to the compiler because you've told it here

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that you're returning an integer but now you're returning a string.

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So let's run that and see what you get.

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Here you go. Error, invalid conversion from a character star, which is a string

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to an integer. So the compiler is trying to figure out

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how do I convert that Joe to an integer, I don't know.

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I don't have -- I have no idea. I'll just throw the error.

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Okay. And that's up to you now to fix it. So in this case, again, I'll fix it to return 0.

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Now let's do another one. Let's get rid of that first curly.

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This is a real interesting compiler error. And I'll compile it, and then we'll talk about it.

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Let's compile, and you can see that I've got three errors from that.

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What's happening is the compiler is totally lost here.

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The compiler is seeing int main and then it's expecting a curly

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and it's processing more and more stuff kind of hoping to see that

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left curly eventually,

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and it's not seeing it so it's seeing other things and it's throwing errors it's saying

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I don't know what's going on before the std.

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I've got an unexpected, unqualified something before return.

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I'm lost. Okay.

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So when you see these kinds of of errors, I mean this is a tiny little program,

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and I've got three compiler errors.

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Sometimes you'll compile, something you'll get 20, 40 errors.

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And many times when you fix that first one, most of them will go away.

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So in this case, all that's missing is that curly.

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So I'll compile,

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and we should have a clean compile now that we do.

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Sometimes you'll add extra curlies at the end,

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which again is illegal. So I'll compile

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and there it is.

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An error I'm expecting something before that token. It doesn't know what you're doing.

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So we'll get rid of that.

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Okay. So let's do one more semantic error. Let's say that we've got hello world in here.

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And I'm going to put parentheses around it, and I'm going to divide it by

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125. And I'll put a parenthesis around that, just simple math.

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Okay, that's pretty weird.

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I've got something here

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divided by something here. That's valid. I can divide two things

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the division operator which is that slash is a binary operator.

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It works on two things, right. So 4 divided by 2 is 2.,

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8 divided by 2 is 4 and so forth. But in this case,

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the syntax is correct.

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But the compiler is going to say I have no idea what you want me to do here.

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You're telling me to divide hello world by 125,

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I don't even know what that means. So let's compile that

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and the error is something like error invalid operands.

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So the division operator, the slash operator

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has got invalid operands.

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There's two operands, right.

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One is a string, the other one is an integer. And it's saying I don't know what to do here.

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So these are some simple examples of compiler errors.

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Don't be afraid of the compiler errors. You're going to get them.

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Don't let them frustrate you. If you have any that you can't figure out,

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please post to the forums and I'll be glad to help you out.

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It's just something that we have to learn to work with because we're going to get compiler errors.

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We all make mistakes.

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Sometimes we're prototyping. We're trying new things, and we're getting errors.

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Sometimes they're just silly mistakes and they're hard to find sometimes.

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Sometimes you'll be looking at your own code. I can tell you this,

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this little error here with the the end double ll,

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this is a good error message that you got. But if you're looking at this code

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and you've been coding for a few hours,

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I don't think you'll see those two ll, most likely they look like one l.

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So this you know this is the kind of thing that programming is about.

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One of the important things is getting rid of all these compiler errors.

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Once we get rid of the compiler errors, then we can test our code,

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and we can look for logic errors and so forth.

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Okay. So that wraps up this video. In the next video,

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we'll talk a little bit about compiler warnings.

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They're a little bit different from errors because they're not true errors

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but they're telling you something that you should look out for.