Because comments are such an important part of programming, Emacs provides special commands for editing and inserting comments.
The comment commands in this table insert, kill and align comments. They are described in this section and following sections.
Insert or realign comment on current line; alternatively, comment or uncomment the region (comment-dwim).
Kill comment on current line (comment-kill).
Set comment column (set-comment-column).
Like RET followed by inserting and aligning a comment (indent-new-comment-line).
Add or remove comment delimiters on all the lines in the region.
The command to create or align a comment is M-; (comment-dwim). The word "dwim" is an acronym for "Do What I Mean"; it indicates that this command can be used for many different jobs relating to comments, depending on the situation where you use it.
If there is no comment already on the line, M-; inserts a new comment, aligned at a specific column called the comment column. The new comment begins with the string Emacs thinks comments should start with (the value of comment-start; see below). Point is after that string, so you can insert the text of the comment right away. If the major mode has specified a string to terminate comments, M-; inserts that too, to keep the syntax valid.
If the text of the line extends past the comment column, then the comment start string is indented to a suitable boundary (usually, at least one space is inserted).
You can also use M-; to align an existing comment. If a line already contains the comment-start string, M-; reindents it to the conventional alignment and moves point after it. (Exception: comments starting in column 0 are not moved.) Even when an existing comment is properly aligned, M-; is still useful for moving directly to the start of the text inside the comment.
C-u M-; kills any comment on the current line, along with the whitespace before it. To reinsert the comment on another line, move to the end of that line, do C-y, and then do M-; to realign it.
Note that C-u M-; is not a distinct key; it is M-; (comment-dwim) with a prefix argument. That command is programmed so that when it receives a prefix argument it calls comment-kill. However, comment-kill is a valid command in its own right, and you can bind it directly to a key if you wish.
M-; does two other jobs when used with an active region in Transient Mark mode (Section 11.2). Then it either adds or removes comment delimiters on each line of the region. (If every line is a comment, it removes comment delimiters from each; otherwise, it adds comment delimiters to each.) If you are not using Transient Mark mode, then you should use the commands comment-region and uncomment-region to do these jobs (Section 24.7.2). A prefix argument used in these circumstances specifies how many comment delimiters to add or how many to delete.
Some major modes have special rules for indenting certain kinds of comments in certain contexts. For example, in Lisp code, comments which start with two semicolons are indented as if they were lines of code, instead of at the comment column. Comments which start with three semicolons are supposed to start at the left margin. Emacs understands these conventions by indenting a double-semicolon comment using TAB, and by not changing the indentation of a triple-semicolon comment at all.
;; This function is just an example ;;; Here either two or three semicolons are appropriate. (defun foo (x) ;;; And now, the first part of the function: ;; The following line adds one. (1+ x)) ; This line adds one.
In C code, a comment preceded on its line by nothing but whitespace is indented like a line of code.
If you are typing a comment and wish to continue it on another line, you can use the command C-M-j (indent-new-comment-line). This terminates the comment you are typing, creates a new blank line afterward, and begins a new comment indented under the old one. When Auto Fill mode is on, going past the fill column while typing a comment causes the comment to be continued in just this fashion. If point is not at the end of the line when C-M-j is typed, the text on the rest of the line becomes part of the new comment line.
To turn existing lines into comment lines, use the M-x comment-region command. It adds comment delimiters to the lines that start in the region, thus commenting them out. With a negative argument, it does the opposite--it deletes comment delimiters from the lines in the region.
With a positive argument, comment-region duplicates the last character of the comment start sequence it adds; the argument specifies how many copies of the character to insert. Thus, in Lisp mode, C-u 2 M-x comment-region adds ;; to each line. Duplicating the comment delimiter is a way of calling attention to the comment. It can also affect how the comment is indented. In Lisp, for proper indentation, you should use an argument of two, if between defuns, and three, if within a defun.
The comment column is stored in the variable comment-column. You can set it to a number explicitly. Alternatively, the command C-x ; (set-comment-column) sets the comment column to the column point is at. C-u C-x ; sets the comment column to match the last comment before point in the buffer, and then does a M-; to align the current line's comment under the previous one.
The variable comment-column is per-buffer: setting the variable in the normal fashion affects only the current buffer, but there is a default value which you can change with setq-default. Section 32.2.4. Many major modes initialize this variable for the current buffer.
The comment commands recognize comments based on the regular expression that is the value of the variable comment-start-skip. Make sure this regexp does not match the null string. It may match more than the comment starting delimiter in the strictest sense of the word; for example, in C mode the value of the variable is "/\\*+ *", which matches extra stars and spaces after the /* itself. (Note that \\ is needed in Lisp syntax to include a \ in the string, which is needed to deny the first star its special meaning in regexp syntax. Section 14.5.)
When a comment command makes a new comment, it inserts the value of comment-start to begin it. The value of comment-end is inserted after point, so that it will follow the text that you will insert into the comment. In C mode, comment-start has the value "/* " and comment-end has the value " */".
The variable comment-padding specifies how many spaces comment-region should insert on each line between the comment delimiter and the line's original text. The default is 1.
The variable comment-multi-line controls how C-M-j (indent-new-comment-line) behaves when used inside a comment. If comment-multi-line is nil, as it normally is, then the comment on the starting line is terminated and a new comment is started on the new following line. If comment-multi-line is not nil, then the new following line is set up as part of the same comment that was found on the starting line. This is done by not inserting a terminator on the old line, and not inserting a starter on the new line. In languages where multi-line comments work, the choice of value for this variable is a matter of taste.
The variable comment-indent-function should contain a function that will be called to compute the indentation for a newly inserted comment or for aligning an existing comment. It is set differently by various major modes. The function is called with no arguments, but with point at the beginning of the comment, or at the end of a line if a new comment is to be inserted. It should return the column in which the comment ought to start. For example, in Lisp mode, the indent hook function bases its decision on how many semicolons begin an existing comment, and on the code in the preceding lines.