Preprocessing directives

[Ron]

Please, consider the following code:

const int a = 1;
#if a == 1
#define VAR 200
#else
#define VAR 100
#endif

int main() {
cout << VAR << endl;
return 0;
}

Under some compiler it prints 200. So a preprocessing directive can check
constant values. Is it a standard C++ feature?

But how can be possible? Syntax analysis, following the standard, is done
after all preprocessing directive are executed, so how can an #if
be aware about a "const int" construct?

Regards.

[Rolf Magnus]

Re: Preprocessing directives

Ron wrote:
[color=blue]
> Please, consider the following code:
>[/color]

#include <iostream>
using std::cout;
using std::endl;
[color=blue]
> const int a = 1;
> #if a == 1
> #define VAR 200
> #else
> #define VAR 100
> #endif
>
> int main() {
> cout << VAR << endl;
> return 0;
> }
>
> Under some compiler it prints 200. So a preprocessing directive can
> check constant values. Is it a standard C++ feature?[/color]

I doubt that.
[color=blue]
> But how can be possible? Syntax analysis, following the standard, is
> done after all preprocessing directive are executed, so how can an #if
> be aware about a "const int" construct?[/color]

It can't. That's why I doubt it. But I'm not a language guru.

[Dan Cernat]

Re: Preprocessing directives

Rolf Magnus <ramagnus@t-online.de> wrote in message news:<boqh0e$38 5$07$4@news.t-online.com>...[color=blue]
> Ron wrote:
>[color=green]
> > Please, consider the following code:
> >[/color]
>
> #include <iostream>
> using std::cout;
> using std::endl;
>[color=green]
> > const int a = 1;
> > #if a == 1
> > #define VAR 200
> > #else
> > #define VAR 100
> > #endif
> >
> > int main() {
> > cout << VAR << endl;
> > return 0;
> > }
> >
> > Under some compiler it prints 200. So a preprocessing directive can
> > check constant values. Is it a standard C++ feature?[/color]
>
> I doubt that.
>[/color]

FYI

VC 7.1 prints 100

/dan

[Jack Klein]

Re: Preprocessing directives

On Tue, 11 Nov 2003 11:12:31 GMT, Ron <no@spam.com> wrote in
comp.lang.c++:
[color=blue]
> Please, consider the following code:
>
> const int a = 1;
> #if a == 1
> #define VAR 200
> #else
> #define VAR 100
> #endif
>
> int main() {
> cout << VAR << endl;
> return 0;
> }
>
> Under some compiler it prints 200. So a preprocessing directive can check
> constant values. Is it a standard C++ feature?[/color]

Any compiler that outputs 200 is broken. It is not a standard
feature, it is a non-conforming "feature".
[color=blue]
> But how can be possible? Syntax analysis, following the standard, is done
> after all preprocessing directive are executed, so how can an #if
> be aware about a "const int" construct?
>
> Regards.[/color]

There are almost an infinite number of ways one could write a compiler
to create this defect. If you want to know which one occurred in this
particular compiler, you need to either examine the source code if
available, or ask the people who wrote it.

--
Jack Klein
Home: http://JK-Technology.Com
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