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This video we'll briefly go over the C++ pre-processor

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and pre-processor directives.

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What is the C++ pre-processor?

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The C++ preprocessor is a program

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that processes your source code before the compiler sees it.

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The C++ pre-processor first strips all the comments from the source file

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and replaces each comment with a single space.

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Then it looks for pre-processor directives and executes them.

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Pre-processor directors are lines in the source code that begin with a pound

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or hashtag symbol.

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As you can see in this slide, there are quite a few pre-processor directives available.

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The most commonly used pre-processor directive is the include directive.

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When the preprocessor sees this directive,

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it replaces the pound include line

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with the file that it's referring to

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that it recursively processes that file as well.

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So by the time the compiler sees the source code,

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all comments are stripped out and all preprocessor directives have been

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processed and removed.

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Preprocessor directives have commonly used to conditionally compile code.

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For example, suppose you only want to compile a portion of your source code

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if you're on the windows operating system.

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In this case, you would use a pre-processor directive

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to test to see if you are on windows

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and then perhaps includes some windows-specific libraries.

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If you're not on windows, then maybe you want to include some macOS 10 libraries

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or use the error directive to abort the compile with an error message.

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Conditional compilation is beyond the scope of this course,

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but we'll use several preprocessor directives as we move on in the course.

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One final note that's very important to understand.

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The C++ preprocessor does not understand C++.

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Let me say that again, the C++ pre-processor

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does not understand C++.

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It simply follows the preprocessor directives and gets the source code

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ready for the compiler.

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The compiler is the program that does understand C++.

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Okay. That wraps up this video. In the next video, we'll talk about comments

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and C++.