Re: Re: Re: boost::signals build and C++Builder 6 Pro in 1.29.0

From: Paul Mensonides (yg-boost-users_at_[hidden])
Date: 2003-03-06 01:41:25

Edward Diener wrote:

>> It is implementation-defined whether or not the contents of a #pragma
>> directive are macro expanded--I think that VC allows it.  C99
>> explicitly allows it, but says that any #pragma that begins with STDC
>> is explicitly _not_ macro expanded. C99, of course, has the _Pragma
>> operator which can be generated by a macro expansion (not that that
>> is relevent here).
>
> The opposite way was what I was wondering about. Can a #define create
> a #pragma which then is picked up as a preprocessor line ? As an
> example will this work:
>
> #define CREATE_PRAGMA #pragma somepragma
>
> // Code
> CREATE_PRAGMA
> // Code
>
> Will a compiler correctly pick up the #pragma as a preprocessor line
> or is the preprocessor stage finished once the macro is fully
> expanded ? I believe the latter is true, although I could be wrong.

It is not technically legal, but it might work anyway, as #pragmas usually
aren't picked up by the preprocessor.

> If I am right, then creating a Boost macro for these data layout
> #pragmas for BCB and VC++, and any other compiler which lets one fix
> data layout details such as packing from within each header file,
> will not work. In that case the alternative of a easily remembered
> header file name, which contains the appropriate #pragma, is a good
> alternative for adding such #pragmas in the correct place of header
> files which need such data layout reliability.

Well, #pragmas that the compiler doesn't understand are supposed to be ignored.
However, using a header for it is also a good option.  You can even use a macro
to introduce the header if you like:

#include CREATE_PRAGMA()

Paul Mensonides