The font locking for most of the CC Mode languages were provided directly by the Font Lock package prior to version 5.30 of CC Mode. In the transition to CC Mode the patterns have been reworked completely and are applied uniformly across all the languages except AWK mode, just like the indentation rules (although each language still has some peculiarities of its own, of course). Since the languages previously had completely separate font locking patterns, this means that it’s a bit different in most languages now.
The main goal for the font locking in CC Mode is accuracy, to provide
a dependable aid in recognizing the various constructs. Some, like
strings and comments, are easy to recognize while others, like
declarations and types, can be very tricky. CC Mode can go to great
lengths to recognize declarations and casts correctly, especially when
the types aren’t recognized by standard patterns. This is a fairly
demanding analysis which can be slow on older hardware, and it can
therefore be disabled by choosing a lower decoration level with the
variable font-lock-maximum-decoration (see Font Lock in GNU Emacs Manual).
The decoration levels are used as follows:
*-font-lock-extra-types (where ‘*’ is the name of the
language) are used to recognize types (see below). Documentation
comments like Javadoc are fontified according to
c-doc-comment-style (see Documentation Comments).
Use this if you think the font locking is too slow. It’s the closest corresponding level to level 3 in the old font lock patterns.
*-font-lock-extra-types variables are still used, but user
defined types are recognized correctly anyway in most cases. Therefore
those variables should be fairly restrictive and not contain patterns
that are uncertain.
This level is designed for fairly modern hardware and a font lock support mode like Lazy Lock or Just-in-time Lock mode that only fontifies the parts that are actually shown. Fontifying the whole buffer at once can easily get bothersomely slow even on contemporary hardware. See Font Lock in GNU Emacs Manual.
Since user defined types are hard to recognize you can provide additional regexps to match those you use:
For each language there’s a variable *-font-lock-extra-types,
where ‘*’ stands for the language in question. It contains a list
of regexps that matches identifiers that should be recognized as types,
e.g., ‘\\sw+_t’ to recognize all identifiers ending with ‘_t’
as is customary in C code. Each regexp should not match more than a
single identifier.
The default values contain regexps for many types in standard runtime libraries that are otherwise difficult to recognize, and patterns for standard type naming conventions like the ‘_t’ suffix in C and C++. Java, Objective-C and Pike have as a convention to start class names with capitals, so there are patterns for that in those languages.
Despite the names of these variables, they are not only used for fontification but in other places as well where CC Mode needs to recognize types.