Re: [boost] Boost.Preprocessor: Alternatives to defined(x)

From: Vesa Karvonen (vesa_karvonen_at_[hidden])
Date: 2003-01-05 12:39:31

David Abrahams:
>Vesa Karvonen:
> > to the Boost Preprocessor library to help detect whether flag like
> > macros are defined. These macros would make it easier to move some
> > logic from unusable #if blocks into usable macros.

>Can you show an example of such a transformation?

I got this idea while reading the following thread on the comp.programming 
newsgroup:
http://groups.google.com/groups?dq=&hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm=Xns92FA369D1BE1newspubwuggyorg%40217.32.252.50&prev=/groups%3Fq%3Dcomp.programming%26ie%3DUTF-8%26oe%3DUTF-8%26hl%3Den

I thought about suggesting use of macros (with help from 
Boost.Preprocessor), which would allow using code like this:

  #pragma SECTION(RXFILTERS_C,rxdata,ADC_output)
  // ...

While writing the answer, I got bored writing the "BOOL" versions of the 
macros, so I started thinking about how to detect whether a macro was 
defined. The BOOST_PP_IS_EMPTY(), etc... macros that I came up with are not 
a complete replacement for defined(), but they can help (especially in 
probably the most common case when a macro simply serves as a flag and is 
defined to be either empty or 1 (this excludes __cplusplus and many other 
compiler defined flags in the general case, btw)).

I think that repeating #if blocks, often using defined(), could be extracted 
from quite a few programs written in C or C++ that have been ported to 
multiple platforms. I don't have more specific examples in mind, however.

-Vesa Karvonen

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