conditional includes

[joe at modcomp.UUCP]

Hi, fellow netlanders,

Several of you responded to my query about what C compilers, if any, provide
their users with a convenient conditional include facility.  As promised,
I'm summarizing (read: liberally plagiarizing) your letters in this response.

1.  The Objective-C language (used on the NeXT) has exactly the feature
    described.  The keyword used is "#import".

2.  The "High C" compiler for the IBM PC/RT supports once-only includes.  It 
    prevents multiple includes based on filename, so referencing a file by 
    different names will result in multiple inclusion.

3.  One compiler (unnamed) has a cpp pragma "#pragma idempotent" that is
    inserted into the header file.  Any subsequent include of that filename
    is skipped.

4.  A combination of (3) and the original solution results in an effective
    solution which does not require any language extensions:  replace
    "#pragma idempotent" with the lines:

	#ifdef <whatever>
	#define <whatever>

    and add a "#endif" to the end of the include file.  Controlling multiple
    inclusion from within the file itself solves the problem mentioned in (2)
    as well as the excessive wordiness of the original solution.

I have assumed that all respondents prefer to remain anonymous.  My
apologies if that assumption is wrong.

joe korty
uunet!modcomp!joe