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In this video, we'll briefly go over the c++ main function.

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Every c++ program must have exactly one main function somewhere.

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A c++ program can consist of hundreds of files,

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but one of those files must contain the main function.

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Note that the name main must be written in lowercase letters.

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When a c++ program executes,

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the main function is called by the operating system,

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and the code between the curly braces executes.

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When execution hits the return statement,

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the program returns the integer value to the operating system.

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If the return value is zero, then the program terminated successfully.

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If the return value is not zero,

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then the operating system can check the value returned and determine what went wrong.

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There are two versions of main,

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and they're both valid as per the c++ specification.

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So far we've only seen the version on the left, which returns an integer

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and has nothing between the two parentheses.

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This version of main expects no information from the operating system in order to run.

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This is the version that we'll use mostly in this course.

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The second version of main is the one on the right. In this version,

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main expects information from the operating system.

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So suppose that when you execute the program,

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you need to pass information into the program from the command line.

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This is very, very common for command line applications.

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In this case, we tell the compiler that we need two pieces of information.

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First, we need the number of pieces of information that were passed in.

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That's argc.

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We can use any variable name here. But historically, we use argc,

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which stands for argument count.

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The second piece of information is the actual arguments that we passed into the program.

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This is captured by argv, which is the argument vector.

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The syntax for argv looks a little strange,

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but it'll make sense as the course progresses.

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It's basically a bunch of strings, such as program.exe,

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argument1 and argument2 in this example.

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We'll talk more about strings and vectors as we move forward,

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but I wanted to show you both versions of the main function

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in case you're looking at code out there and you see the second version,

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both are perfectly valid and both are commonly seen.

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Note that main must always return an integer.

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Main happens to be an example of a function.

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A function is basically a name that we can use to reference a block of code.

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In this example, main is everything between the curly braces.

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Functions are very commonly used

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and we'll write our own functions as the course progresses.

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There'll be an entire section of the course devoted to functions,

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but main is a very special function.

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As we learn c++, our entire program

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will be contained inside the main function.

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And as our programs become more complex,

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we'll write our own functions and then classes

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to better modularize and organize our code.

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In the next video, we'll talk about namespaces

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and see how we can use them in our first program.