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

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Linker errors occur when the linker is trying to put all the pieces

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of a program together to create an executable,

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and one or more pieces are missing.

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Typically, this can happen when an object file or libraries can't be found by the linker.

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Fixing linker errors can be tricky.

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Sometimes it's just a matter of getting the paths correct, so that the linker knows where the missing pieces are.

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Let's head over to CodeLite.

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And we'll force a program to have a linker error, so you can see what the error message looks like.

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So I'm in CodeLite. And I've created a project called linker error

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based on the template that we use. So all I've got is a main cpp file

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that says hello world.

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Now we're going to force a linker error here.

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This is not something you want to do normally.

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But I'm going to do it to show you what the error looks like.

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So what I'm going to say is I'm going to say extern int c

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or let's just say x. And I'll explain all this to you in just a second.

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And then in the body of the main, I want to try to use that x right there.

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It's just an integer. And let's say, I want to print it out.

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I just want to print out x, simple as that.

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I'm going to save this.

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Now you'll see if I compile this program. You'll see what happens. I'm going to right click,

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compile.

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I get a clean compile. There's nothing wrong with the program.

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But when I try to build and run it,

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you can see the linker error.

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This is classic linker error,

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undefined reference to something. In this case, it just happens to be x.

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Also you'll notice that the linker returned one here,

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ld returns one, which is the x status. So something

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was wrong with the link.

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Obviously, the compile was fine because we got no compiler errors. Let me do that again.

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There's the compile. No errors. No warnings.

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So let me explain to you what's going on here.

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Let me grab my stylus here real quick.

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And what's going on here is I've got

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my main cpp file here.

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And it's going to be compiled

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to main.o, let's say, or main.object.

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In here, we have a reference to an x.

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You can see it right there. You can see it right there.

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So in here, I've got a reference to an x, which I'm going to use.

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Now we also have our iostream libraries.

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So the linker needs to grab those libraries.

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And I'll just type iostream here. You know what I'm talking about.

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The linker needs to put these two together

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to create my main.exe

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or in this case I believe it would be linker error.exe but you know what I mean.

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So the compiler is fine. We've created our object file.

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This library exists. The problem is that I've told

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the compiler here that this x is not in me.

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It's somewhere else. It's defined by somebody else.

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Well, obviously it isn't defined by someone else because that kind of forced the issue here.

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So now the linker needs to resolve that x, it's saying okay

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where where do I get that value to print because that's what you asked me to do right here right.

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I wanted to print the value of x where where is x. It can't find it.

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That's the linker error. That's exactly what's going on.

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And let me build this. Again, so you can see the error message one more time.

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Let's build it and run it.

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And there's the error message it's saying

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undefined reference to x you can see exactly what's going on.

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It can't find x. Now this is not a compiler error.

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This is a linker error. That's really important to understand.

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Linker errors are tricky to fix.

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A lot of times the files that you need to link are missing.

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They're in the wrong places. You don't have to worry about that using CodeLite.

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In this course, we're going to basically do everything ourselves.

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But if you're using libraries from third parties,

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this is the kind of thing we'll talk about in a more intermediate or advanced course.

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But I just wanted to show you linker errors. In case you do come up with one,

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post it in the forum, and we'll work through it together.