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Since there's a lot of normal text in comments and string literals, CC Mode provides features to edit these like in text mode. The goal is to do it as seamlessly as possible, i.e. you can use auto fill mode, sentence and paragraph movement, paragraph filling, adaptive filling etc wherever there's a piece of normal text without having to think much about it. CC Mode should keep the indentation, fix the comment line decorations, and so on, for you. It does that by hooking in on the different line breaking functions and tuning relevant variables as necessary.
To make Emacs recognize comments and treat text in them as normal
paragraphs, CC Mode makes several standard
variables(15) buffer local and modifies them
according to the language syntax and the style of line decoration that
starts every line in a comment. The style variable
c-comment-prefix-regexp contains the regexp used to recognize
this comment line prefix. The default is `//+\\|\\**', which
matches C++ style line comments like
// blah blah |
with two or more slashes in front of them, and C style block comments like
/* * blah blah */ |
with zero or more stars at the beginning of every line. If you change
that variable, please make sure it still matches the comment starter
(i.e. //) of line comments and the line prefix inside
block comments. Also note that since CC Mode uses the value of
c-comment-prefix-regexp to set up several other variables at mode
initialization, you need to reinitialize the program mode if you change
it inside a CC Mode buffer.
Line breaks are by default handled (almost) the same regardless whether
they are made by auto fill mode (see section `Auto Fill' in The Emacs Editor), paragraph filling (e.g. with M-q), or explicitly with
M-j or similar methods. In string literals, the new line gets the
same indentation as the previous nonempty line (may be changed with the
string syntactic symbol). In comments, CC Mode uses
c-comment-prefix-regexp to adapt the line prefix from the other
lines in the comment.
CC Mode uses adaptive fill mode (see section `Adaptive Fill' in The Emacs Editor) to make Emacs correctly keep the line prefix when filling paragraphs. That also makes Emacs preserve the text indentation inside the comment line prefix. E.g. in the following comment, both paragraphs will be filled with the left margins kept intact:
/* Make a balanced b-tree of the nodes in the incoming * stream. But, to quote the famous words of Donald E. * Knuth, * * Beware of bugs in the above code; I have only * proved it correct, not tried it. */ |
It's also possible to use other adaptive filling packages, notably Kyle
E. Jones' Filladapt package(16),
which handles things like bulleted lists nicely. There's a convenience
function c-setup-filladapt that tunes the relevant variables in
Filladapt for use in CC Mode. Call it from a mode hook, e.g. with
something like this in your `.emacs':
(defun my-c-mode-common-hook () (c-setup-filladapt) (filladapt-mode 1)) (add-hook 'c-mode-common-hook 'my-c-mode-common-hook) |
Normally the comment line prefix inserted for a new line inside a
comment is deduced from other lines in it. However there's one
situation when there's no clue about how the prefix should look, namely
when a block comment is broken for the first time. The string in the
style variable c-block-comment-prefix(17) is used in that case. It defaults to `* ', which
makes a comment
/* Got O(n^2) here, which is a Bad Thing. */ |
break into
/* Got O(n^2) here, * which is a Bad Thing. */ |
Note that it won't work to justify the indentation by putting leading
spaces in the c-block-comment-prefix string, since CC Mode
still uses the normal indentation engine to indent the line. Thus, the
right way to fix the indentation is by setting the c syntactic
symbol. It defaults to c-lineup-C-comments, which handles the
indentation of most common comment styles, see 9. Indentation Functions.
When auto fill mode is enabled, CC Mode can selectively ignore it
depending on the context the line break would occur in, e.g. to never
break a line automatically inside a string literal. This behavior can
be controlled with the c-ignore-auto-fill variable. It takes a
list of symbols for the different contexts where auto-filling never
should occur:
string -- Inside a string or character literal.
c -- Inside a C style block comment.
c++ -- Inside a C++ style line comment.
cpp -- Inside a preprocessor directive.
code -- Anywhere else, i.e. in normal code.
By default, c-ignore-auto-fill is set to '(string cpp
code), which means that auto-filling only occurs in comments when
auto-fill mode is activated. In literals, it's often desirable to have
explicit control over newlines. In preprocessor directives, the
necessary `\' escape character before the newline is not
automatically inserted, so an automatic line break would produce invalid
code. In normal code, line breaks are normally dictated by some logical
structure in the code rather than the last whitespace character, so
automatic line breaks there will produce poor results in the current
implementation.
The commands that does the actual work follows.
c-fill-paragraph)
fill-paragraph in CC Mode
buffers. It's used to fill multiline string literals and both block and
line style comments. In Java buffers, the Javadoc markup words are
recognized as paragraph starters. The line oriented Pike autodoc markup
words are recognized in the same way in Pike mode.
The function keeps the comment starters and enders of block comments as they were before the filling. This means that a comment ender on the same line as the paragraph being filled will be filled with the paragraph, and one on a line by itself will stay as it is. The comment starter is handled similarly(18).
c-indent-new-comment-line)
indent-new-comment-line. It breaks
the line at point and indents the new line like the current one.
If inside a comment and comment-multi-line is non-nil, the
indentation and line prefix are preserved. If inside a comment and
comment-multi-line is nil, a new comment of the same type
is started on the next line and indented as appropriate for comments.
indent-new-comment-line in
comments and newline-and-indent elsewhere, thus combining those
two in a way that uses each one in the context it's best suited for.
I.e. in comments the comment line prefix and indentation is kept for the
new line, and in normal code it's indented according to context by the
indentation engine.
It's not bound to a key by default, but it's intended to be used on the
RET key. If you like the behavior of newline-and-indent on
RET, you might consider switching to this function.
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