Indenting #defines

Question

I know that #defines, etc. are normally never indented. Why?

I\'m working in some code at the moment which has a horrible mixture of #define

Answer 1

Regarding the parsing of preprocessor directives, the C99 standard (and the C89 standard before it) were clear about the sequence of operations performed logically by the compiler. In particular, I believe it means that this code:

/* */ # /* */ include /* */  /* */

is equivalent to:

#include 

For better or worse, GCC 3.4.4 with '-std=c89 -pedantic' accepts the comment-laden line, at any rate. I'm not advocating that as a style - not for a second (it is ghastly). I just think that it is possible.

ISO/IEC 9899:1999 section 5.1.1.2 Translation phases says:

1. [Character mapping, including trigraphs]

2. [Line splicing - removing backslash newline]

3. The source file is decomposed into preprocessing tokens and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment. Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is implementation-defined.

4. Preprocessing directives are executed, macro invocations are expanded, [...]

Section 6.10 Preprocessing directives says:

A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that (at the start of translation phase 4) is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character, and is ended by the next new-line character.

The only possible dispute is the parenthetical expression '(at the start of translation phase 4)', which could mean that the comments before the hash must be absent since they are not otherwise replaced by spaces until the end of phase 4.

As others have noted, the pre-standard C preprocessors did not behave uniformly in a number of ways, and spaces before and in preprocessor directives was one of the areas where different compilers did different things, including not recognizing preprocessor directives with spaces ahead of them.

It is noteworthy that backslash-newline removal occurs before comments are analyzed. Consequently, you should not end // comments with a backslash.