4.6 Electric Keys and Keywords

Most punctuation keys provide electric behavior: as well as inserting themselves they perform some other action, such as reindenting the line. This reindentation saves you from having to reindent a line manually after typing, say, a ‘}’. A few keywords, such as else, also trigger electric action.

You can inhibit the electric behavior described here by disabling electric minor mode (see Minor Modes).

Common to all these keys is that they only behave electrically when used in normal code (as contrasted with getting typed in a string literal or comment). Those which cause re-indentation do so only when c-syntactic-indentation has a non-nil value (which it does by default).

These keys and keywords are:

# ¶

Pound (bound to c-electric-pound) is electric when typed as the first non-whitespace character on a line and not within a macro definition. In this case, the variable c-electric-pound-behavior is consulted for the electric behavior. This variable takes a list value, although the only element currently defined is alignleft, which tells this command to force the ‘#’ character into column zero. This is useful for entering preprocessor macro definitions.

Pound is not electric in AWK buffers, where ‘#’ starts a comment, and is bound to self-insert-command like any typical printable character.

* ¶

/

A star (bound to c-electric-star) or a slash (c-electric-slash) causes reindentation when you type it as the second component of a C style block comment opener (‘/*’) or a C++ line comment opener (‘//’) respectively, but only if the comment opener is the first thing on the line (i.e., there’s only whitespace before it).

Additionally, you can configure CC Mode so that typing a slash at the start of a line within a block comment will terminate the comment. You don’t need to have electric minor mode enabled to get this behavior. See Clean-ups.

In AWK mode, ‘*’ and ‘/’ do not delimit comments and are not electric.

< ¶

>

A less-than or greater-than sign (bound to c-electric-lt-gt) is electric in two circumstances: when it is an angle bracket in a C++ ‘template’ declaration (and similar constructs in other languages) and when it is the second of two < or > characters in a C++ style stream operator. In either case, the line is reindented. Angle brackets in C ‘#include’ directives are not electric.

( ¶

)

The normal parenthesis characters ‘(’ and ‘)’ (bound to c-electric-paren) reindent the current line. This is useful for getting the closing parenthesis of an argument list aligned automatically.

You can also configure CC Mode to insert a space automatically between a function name and the ‘(’ you’ve just typed, and to remove it automatically after typing ‘)’, should the argument list be empty. You don’t need to have electric minor mode enabled to get these actions. See Clean-ups.

{ ¶

}

Typing a brace (bound to c-electric-brace) reindents the current line. Also, one or more newlines might be inserted if auto-newline minor mode is enabled. See Auto-newline Insertion. Additionally, you can configure CC Mode to compact excess whitespace inserted by auto-newline mode in certain circumstances. See Clean-ups.

: ¶

Typing a colon (bound to c-electric-colon) reindents the current line. Additionally, one or more newlines might be inserted if auto-newline minor mode is enabled. See Auto-newline Insertion. If you type a second colon immediately after such an auto-newline, by default the whitespace between the two colons is removed, leaving a C++ scope operator. See Clean-ups.

If you prefer, you can insert ‘::’ in a single operation, avoiding all these spurious reindentations, newlines, and clean-ups. See Other Commands.

; ¶

,

Typing a semicolon or comma (bound to c-electric-semi&comma) reindents the current line. Also, a newline might be inserted if auto-newline minor mode is enabled. See Auto-newline Insertion. Additionally, you can configure CC Mode so that when auto-newline has inserted whitespace after a ‘}’, it will be removed again when you type a semicolon or comma just after it. See Clean-ups.

Command: c-electric-continued-statement ¶

Certain keywords are electric, causing reindentation when they are preceded only by whitespace on the line. The keywords are those that continue an earlier statement instead of starting a new one: else, while, catch (only in C++ and Java) and finally (only in Java).

An example:

for (i = 0; i < 17; i++)
  if (a[i])
    res += a[i]->offset;
else

Here, the else should be indented like the preceding if, since it continues that statement. CC Mode will automatically reindent it after the else has been typed in full, since only then is it possible to decide whether it’s a new statement or a continuation of the preceding if.

CC Mode uses Abbrev mode (see Abbrevs in GNU Emacs Manual) to accomplish this. It’s therefore turned on by default in all language modes except IDL mode, since CORBA IDL doesn’t have any statements.