GNU Info

(ccmode)Syntactic Analysis

---

Next: Indentation Calculation Up: New Indentation Engine

---

Enter node , (file) or (file)node

Syntactic Analysis
==================

   The first thing CC Mode does when indenting a line of code, is to
analyze the line, determining the "syntactic component list" of the
construct on that line.  A syntactic component consists of a pair of
information (in lisp parlance, a _cons cell_), where the first part is
a "syntactic symbol", and the second part is a "relative buffer
position".  Syntactic symbols describe elements of C code (1), e.g.
`statement', `substatement', `class-open', `class-close', etc.  Note:
Syntactic Symbols, for a complete list of currently recognized
syntactic symbols and their semantics.  The style variable
`c-offsets-alist' also contains the list of currently supported
syntactic symbols.

   Conceptually, a line of C code is always indented relative to the
indentation of some line higher up in the buffer.  This is represented
by the relative buffer position in the syntactic component.

   Here is an example.  Suppose we had the following code as the only
thing in a C++ buffer (2):

       1: void swap( int& a, int& b )
       2: {
       3:     int tmp = a;
       4:     a = b;
       5:     b = tmp;
       6: }

   We can use the command `C-c C-s' (`c-show-syntactic-information') to
simply report what the syntactic analysis is for the current line.
Running this command on line 4 of this example, we'd see in the echo
area(3):

     ((statement . 35))

   This tells us that the line is a statement and it is indented
relative to buffer position 35, which happens to be the `i' in `int' on
line 3.  If you were to move point to line 3 and hit `C-c C-s', you
would see:

     ((defun-block-intro . 29))

   This indicates that the `int' line is the first statement in a top
level function block, and is indented relative to buffer position 29,
which is the brace just after the function header.

   Here's another example:

       1: int add( int val, int incr, int doit )
       2: {
       3:     if( doit )
       4:         {
       5:             return( val + incr );
       6:         }
       7:     return( val );
       8: }

Hitting `C-c C-s' on line 4 gives us:

     ((substatement-open . 46))

which tells us that this is a brace that _opens_ a substatement block.
(4)

   Syntactic component lists can contain more than one component, and
individual syntactic components need not have relative buffer positions.
The most common example of this is a line that contains a "comment only
line".

       1: void draw_list( List<Drawables>& drawables )
       2: {
       3:         // call the virtual draw() method on each element in list
       4:     for( int i=0; i < drawables.count(), ++i )
       5:     {
       6:         drawables[i].draw();
       7:     }
       8: }

Hitting `C-c C-s' on line 3 of this example gives:

     ((comment-intro) (defun-block-intro . 46))

and you can see that the syntactic component list contains two syntactic
components.  Also notice that the first component, `(comment-intro)'
has no relative buffer position.

   ---------- Footnotes ----------

   (1) Unless otherwise noted, the term "C code" to refers to all the
C-like languages.

   (2) The line numbers in this and future examples don't actually
appear in the buffer, of course!

   (3) With a universal argument (i.e. `C-u C-c C-s') the analysis is
inserted into the buffer as a comment on the current line.

   (4) A "substatement" is the line after a conditional statement, such
as `if', `else', `while', `do', `switch', etc.  A "substatement block"
is a brace block following one of these conditional statements.