Comments

A comment is a piece of text in source code that isn’t executed - it’s completely ignored by the compiler.

The uses of comments

We can use comments for explaining parts of our code. Because the computer ignores them, we can write anything we want in comments.

Single-line comments

In C, we can create a single-line comment using two forward slashes (//). Anything after the // on that line is ignored by the compiler.

For example:

// This is a comment - the compiler ignores it
int main(int argc, char* argv[]) {
    // This comment is inside the main function
    return 0; // This comment is after a statement
}

Multi-line comments

C also supports multi-line comments. These start with /* and end with */.

Anything between these markers is ignored by the compiler.

For example:

/*
   This is a multi-line comment.
   It can span multiple lines.
*/
int main(int argc, char* argv[]) {
    /* This is another multi-line comment
       inside the main function */
    return 0;
}

You can also use multi-line comments for comments inside a line of code, like this:

int main(int argc, char* argv[]) {
    printf(/*print whatever you like*/ "Hello, World!");
    return 0;
}

Generally, I would discourage using comments inside lines of code, as it can make the code quite messy!

When not to use comments

You shouldn’t use comments to:

• Explain what the code is doing - the code should be clear enough to explain itself.
• Leave notes to yourself or others about how to improve the code - use a version control system like Git for that.
• Comment out large blocks of code that you don’t want to run - use version control for that too!
• Complain about your annoying coworker who keeps breaking the build (well, maybe you can do that sometimes…)

In general, I don’t think you should write comments. Except in very rare circumstances. See this video by CodeAesthetic.