Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c++ > #5258 > unrolled thread

Using sizeof in the preprocessor

Back to article view | Back to comp.lang.c++

Contents

  Using sizeof in the preprocessor Johannes Schaub <schaub.johannes@googlemail.com> - 2011-05-21 15:35 +0200
    Re: Using sizeof in the preprocessor Pete Becker <pete@versatilecoding.com> - 2011-05-21 09:42 -0400
    Re: Using sizeof in the preprocessor Huibert Bol <huibert.bol@quicknet.nl> - 2011-05-21 16:08 +0200
      Re: Using sizeof in the preprocessor Pete Becker <pete@versatilecoding.com> - 2011-05-21 10:37 -0400
      Re: Using sizeof in the preprocessor Juha Nieminen <nospam@thanks.invalid> - 2011-05-21 18:51 +0000
        Re: Using sizeof in the preprocessor Huibert Bol <huibert.bol@quicknet.nl> - 2011-05-21 21:52 +0200
        Re: Using sizeof in the preprocessor Johannes Schaub <schaub.johannes@googlemail.com> - 2011-05-21 21:54 +0200
        Re: Using sizeof in the preprocessor Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2011-05-21 21:54 +0200
          Re: Using sizeof in the preprocessor Juha Nieminen <nospam@thanks.invalid> - 2011-05-22 06:02 +0000
    Re: Using sizeof in the preprocessor Johannes Schaub <schaub.johannes@googlemail.com> - 2011-05-21 22:19 +0200

#5258 — Using sizeof in the preprocessor

I have this code in my pet project

    #if sizeof -1 > 4
    #error "This doesn't work on 64bit"
    #endif

My friend was telling me that this shouldn't work because "sizeof is not 
evaluated in the preprocessor". Yet the compiler accepts it and my program 
works. 

Can anybody please explain this? Is it a compiler bug? Thanks to all!

[toc] | [next] | [standalone]

#5259

On 2011-05-21 09:35:53 -0400, Johannes Schaub said:

> I have this code in my pet project
> 
>     #if sizeof -1 > 4
>     #error "This doesn't work on 64bit"
>     #endif
> 
> My friend was telling me that this shouldn't work because "sizeof is not
> evaluated in the preprocessor". Yet the compiler accepts it and my program
> works.
> 
> Can anybody please explain this? Is it a compiler bug? Thanks to all!

Bug or extension. The preprocessor does text manipulation; it doesn't 
know anything about types (although it has its own set of rules for 
arithmetic).

-- 
  Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The 
Standard C++ Library Extensions: a Tutorial and Reference 
(www.petebecker.com/tr1book)

[toc] | [prev] | [next] | [standalone]

#5260

Johannes Schaub wrote:

> I have this code in my pet project
>
>     #if sizeof -1 > 4
>     #error "This doesn't work on 64bit"
>     #endif
>
> My friend was telling me that this shouldn't work because "sizeof is not 
> evaluated in the preprocessor". Yet the compiler accepts it and my program 
> works. 

Identifiers in #if expressions are evaluated as zero, so the expression
is equivalent to

  #if 0 -1 > 4

which is never true.

-- 
Huibert
"Okay... really not something I needed to see."  --Raven

[toc] | [prev] | [next] | [standalone]

#5271

On 2011-05-21 10:08:58 -0400, Huibert Bol said:

> Johannes Schaub wrote:
> 
>> I have this code in my pet project
>> 
>> #if sizeof -1 > 4
>> #error "This doesn't work on 64bit"
>> #endif
>> 
>> My friend was telling me that this shouldn't work because "sizeof is not
>> evaluated in the preprocessor". Yet the compiler accepts it and my program
>> works.
> 
> Identifiers in #if expressions are evaluated as zero,

Good point.

-- 
  Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The 
Standard C++ Library Extensions: a Tutorial and Reference 
(www.petebecker.com/tr1book)

[toc] | [prev] | [next] | [standalone]

#5285

Huibert Bol <huibert.bol@quicknet.nl> wrote:
> Identifiers in #if expressions are evaluated as zero

  It seems that at least with gcc "#if true" and "#if false" work as one
would expect. Is this standard?

[toc] | [prev] | [next] | [standalone]

#5290

Juha Nieminen wrote:

> Huibert Bol <huibert.bol@quicknet.nl> wrote:
>> Identifiers in #if expressions are evaluated as zero
>
>   It seems that at least with gcc "#if true" and "#if false" work as one
> would expect. Is this standard?

Yes, "except true and false".  Also the alternative tokens (and, andeq,
bitand, etc, ...) are replaced as they are not considered identifiers.

-- 
Huibert
"Okay... really not something I needed to see."  --Raven

[toc] | [prev] | [next] | [standalone]

#5291

Juha Nieminen wrote:

> Huibert Bol <huibert.bol@quicknet.nl> wrote:
>> Identifiers in #if expressions are evaluated as zero
> 
>   It seems that at least with gcc "#if true" and "#if false" work as one
> would expect. Is this standard?

There are explicit exceptions for true and false. The spec says

After all replacements due to macro expansion and the defined unary operator
have been performed, all remaining identifiers and keywords, except for true 
and false, are replaced with the pp-number 0, and then each preprocessing 
token is converted into a token. The resulting tokens comprise the 
controlling constant expression which is evaluated according to the rules of 
5.19 ...

[toc] | [prev] | [next] | [standalone]

#5292

Juha Nieminen <nospam@thanks.invalid> writes:

> Huibert Bol <huibert.bol@quicknet.nl> wrote:
>> Identifiers in #if expressions are evaluated as zero
>
>   It seems that at least with gcc "#if true" and "#if false" work as one
> would expect. Is this standard?

Funny idea. Here (gcc/g++ 4.4.4), gcc and g++ both with -E give
different results on the following input:

#if true
#error "then"
#else
#error "else"
#endif

-- Alain.

[toc] | [prev] | [next] | [standalone]

#5319

Alain Ketterlin <alain@dpt-info.u-strasbg.fr> wrote:
> Funny idea. Here (gcc/g++ 4.4.4), gcc and g++ both with -E give
> different results on the following input:
> 
> #if true
> #error "then"
> #else
> #error "else"
> #endif

#if true
std::cout << "You rock!\n"
#else
#error "Your language is too old. Please upgrade."
#endif

[toc] | [prev] | [next] | [standalone]

#5294

Johannes Schaub wrote:

> I have this code in my pet project
> 
>     #if sizeof -1 > 4
>     #error "This doesn't work on 64bit"
>     #endif
> 
> My friend was telling me that this shouldn't work because "sizeof is not
> evaluated in the preprocessor". Yet the compiler accepts it and my program
> works.
> 
> Can anybody please explain this? Is it a compiler bug? Thanks to all!

I want to emphasize that I do *not* intent to troll. I wanted to raise a 
common issue and put people in alert mode when they see such code and don't 
get a compiler error. 

See http://chat.stackoverflow.com/transcript/message/752965#752965 and the 
following rage of "FredNurk" for the discussion and why I need to defend 
this question like that.

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.c++

csiph-web