How to disable auto insertion of comment header when pressing enter, but keep auto insertion of header when auto wrapping comment?

Question

When editing c and cpp files, this is the behavior I want to achieve:

• When typing a comment line of the form // Line..., pressing <CR> should not add the comment header // in the next line.
• However, if while typing the // Line..., the line length exceeds the tw I set, auto wrap should break the line, and should insert // in the next line.
• The auto comment insertion behavior of /* * * */ style comments should not change. That is, after typing in /*, both explicitly hitting <CR> and auto wrap should insert * into the next line, produce the single space indentation, and if I type /, it should close the comment block with */, properly indented.

I have fo=tcrqn and comments=s1:/*,mb:*,ex:*/,:// set for c and cpp files.

I tried the following:

• Following the suggestion in this superuser answer, modified for inoremap instead of nnoremap:

inoremap <silent> <cr> <esc>:set paste<cr>o<esc>:set nopaste<cr>:startinsert<cr>`

This does not work because although the desired behavior for // style single line comments are achieved, it destroys the behavior of /* * * */ style block comments, preventing auto insertion of * characters when I press <cr> in a comment block.

• Then I tried modifying it to the following:

inoremap <silent> <cr> <esc>:set comments-=://<cr>:set comments+=f://<cr>o<esc>:set comments-=f://<cr> set comments+=://<cr>:startindent!<cr>

This mostly works, except that when the * is inserted by pressing <cr> in the block /* * * */ comments, the inserted indentation is now missing. That is, representing the cursor position with |, after pressing <cr> I want something like:

/*
 * Line ... here I press <cr>
 * | <- Notice space between cursor and *
 */

But what I get is:

/*
 * Line ... here I press <cr>
 *| <- No space between cursor and *
 */

• I tried to fix the missing indentation problem by adding and deleting a space character:

inoremap <silent> <cr> <esc>:set comments-=://<cr>:set comments+=f://<cr>o<Space><BS><esc>:set comments-=f://<cr> set comments+=://<cr>:startindent!<cr>

This does fix the indentation issue, but now I have a new problem: typing / does not close the comment bock, but instead inserts an / character. This is what I want:

/*
 * ...
 * | <- cursor is here. I type '/', and the line becomes:
 */  <- Notice no space between * and /

But I get:

/*
 * ...
 * | <- cursor is here. I type '/', and the line becomes:
 * / <- Notice there is a space between * and /, comment block is not closed.

At this point I am out of ideas. Any help is much appreciated.

Answer 1

IMO, it's easier to do simply:

inoremap <expr><CR> getline('.') =~# '^\s*//' ? '<C-O>o' : '<CR>'

Perhaps, this mapping (with <buffer> modifier, of course) should be added to after/ftplugin/c.vim and not to the global .vimrc

Answer 2

The idea of removing :// from 'comments' can be made to work, but instead of leaving Insert mode with <Esc>, use <C-\><C-O> to run a single Normal command (in this case, the : command to run the Ex :set command), returning to Insert mode as soon as that command is completed.

Using <C-\><C-O> here instead of <C-O> is important to preserve the cursor position and not lose the whitespace after the * as you were experiencing. The <C-\><C-O> command works "like <C-O> but don't move the cursor." (is what its help says.)

This mapping works:

inoremap <silent> <CR> <C-\><C-o>:set comments-=://<CR><CR><C-\><C-o>:set comments+=://<CR>

Additionally, you might want to keep this mapping only for file type cpp and maybe also c, so I'd recommend adding an additional <buffer> flag to the inoremap command, and storing it in a file ~/.vim/after/ftplugin/cpp_comments.vim so it's loaded for the cpp filetype.

If you also want it for c (or other filetypes with the same comment style), you can have file ~/.vim/after/ftplugin/c_comments.vim load it through a command such as:

source <sfile>:h/cpp_comments.vim

To load the file by that name from the same directory.

For completeness, you might also want to update the b:undo_ftplugin variable, to undo the mapping if you change 'filetype' of the current buffer. In order to do that, add the following snippet to the cpp_comments.vim file with the mapping:

if !exists("b:undo_ftplugin")
  let b:undo_ftplugin = ""
endif
let b:undo_ftplugin = b:undo_ftplugin . "|sil! exe 'iunmap <buffer> <CR>'"

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NOTE: This answer doesn't fix the issue of closing the /* ... */ comment block with a single /, so @Matt's mapping is much better than this one in that sense.

Still refer to this answer for the ftplugin part (which @Matt also mentions) and the undo_ftplugin part.