34 C and C++ Support

You can also put any file in C mode manually by typing M-x c-mode Enter. Similarly, you can use c++-mode to put a buffer into C++ mode.

Motion Commands

Advanced C motion commands

Keystrokes	Command name	Action
M-a	c-beginning-of-statement	Move to the beginning of the current statement.
M-e	c-end-of-statement	Move to the end of the current statement.
M-q	c-fill-paragraph	If in comment, fill the paragraph, preserving indentations and decorations.
C-M-a	beginning-of-defun	Move to the beginning of the body of the function surrounding the point.
C-M-e	end-of-defun	Move to the end of the function.
C-M-h	c-mark-function	Put the cursor at the beginning of the function, the mark at the end.
C-c C-q	c-indent-defun	Indent the entire function according to indentation style.
C-c C-u	c-up-conditional	Move to the beginning of the current preprocessor conditional.
C-c C-p	c-backward-conditional	Move to the previous preprocessor conditional.
C-c C-n	c-forward-conditional	Move to the next preprocessor conditional.

Customizing Code Indentation Style

M-x c-set-style

Style	Description
bsd	Style used in code for BSD-derived versions of Unix.
cc-mode	The default coding style, from which all others are derived .
ellemtel	Style used in C++ documentation from Ellemtel Telecommunication Systems Laboratories in Sweden .
gnu	Style used in C code for Emacs itself and other GNU-related programs .
java	Style used in Java code (the default for Java mode).
k&r	Style of the classic text on C, Kernighan and Ritchie’sThe C Programming Language .
linux	Style used in C code that is part of the Linux kernel.
python	Style used in python extensions.
stroustrup	C++ coding style of the standard reference work, Bjarne Stroustrup’sThe C++ Programming Language .
user	Customizations you make to.emacs or via Custom (see Chapter 10). All other styles inherit these customizations if you set them.
whitesmith	Style used in Whitesmith Ltd.’s documentation for their C and C++ compilers .

Once you decide on a coding style, you can set it up permanently by putting a line in your .emacs file that looks like this:

(add-hook 'c-mode-hook
       '(lambda ( )
         (c-set-style "stylename")))

Additional C and C++ Mode Features

Auto-newline is off by default. To turn it on, type C-c C-a for c-toggle-auto-state. (Repeat the same command to turn it off again.) You will see the (C) in the mode line change to (C/a) as an indication.

The other optional electric feature, hungry-delete-key, is also off by default. To toggle it on, type C-c C-d (for c-toggle-hungry-state). You will see the (C) on the mode line change to (C/h), or if you have auto-newline turned on, from (C/a) to (C/ah).

You can toggle the states of both auto-newline and hungry-delete-key with the command C-c C-t (for c-toggle-auto-hungry-state).

If you want either of these features on by default when you invoke Emacs, you can put lines like the following in your .emacs file:

(add-hook 'c-mode-hook
      '(lambda ( )
         (c-toggle-auto-state)))

If you want to combine this customization with another C mode customization, such as the indentation style in the previous example, you need to combine the lines of Emacs Lisp code as follows:

(add-hook 'c-mode-hook
      '(lambda ( )
         (c-set-style "stylename")
          (c-toggle-auto-state)))

You can customize M-j (for indent-new-comment-line) so that Emacs continues the same comment on the next line instead of creating a new pair of delimiters. The variable comment-multi-line controls this feature: if it is set to nil (the default), Emacs generates a new comment on the next line, as in the example from earlier in the chapter:

This outcome is the result of typing M-j after multiplicand, and it shows that the cursor is positioned so that you can type the text of the second comment line. However, if you set comment-multi-line to t (or any value other than nil), you get this outcome instead:

The final feature we’ll cover is C-c C-e, (for c-macro-expand). Like the conditional compilation motion commands (e.g., C-c C-u for c-up-conditional), c-macro-expand helps you answer the often difficult question, “What code actually gets compiled?” when your source code contains a morass of preprocessor directives.

To use c-macro-expand, you must first define a region. Then, when you type C-c C-e, it takes the code within the region, passes it through the actual C preprocessor, and places the output in a window called *Macroexpansion*.

If you want to use c-macro-expand with a different C preprocessor command, instead of the default /lib/cpp -C (the -C option means “preserve comments in the output”), you can set the variable c-macro-preprocessor. For example, if you want to use an experimental preprocessor whose filename is /usr/local/lib/cpp, put the following line in your .emacs file:

(setq c-macro-preprocessor "/usr/local/lib/cpp -C")

C++ Mode Differences

If you want C++ mode’s indentation style set to Stroustrup with automatic newlines instead of the default style, put the following in your .emacs file:

(add-hook 'c++-mode-hook
      '(lambda ( )
         (c-set-style "Stroustrup")
         (c-toggle-auto-state)))

C++ mode provides an additional command: C-c **: (for **c-scope-operator). This command inserts the C++ double colon (::) scope operator.

Finally, both C and C++ mode contain the commands c-forward-into-nomenclature and c-backward-into-nomenclature, which aren’t bound to any keystrokes by default. These are like forward-word and backward-word, respectively, but they treat capital letters in the middle of words as if they were starting new words. For example, they treat ThisVariableName as if it were three separate words while the standard forward-word and backward-word commands treat it as one word. ThisTypeOfVariableName is a style used by C++ programmers, as opposed to this_type_of_variable_name, which is somehow more endemic to old-school C code.