Re: Limitations of Ada

[kst@thomsoft.com (Keith Thompson)]

In <4tsvs7$ctk@portal.gmu.edu> jscheibl@mason2.gmu.edu (Jack W Scheible) writes:
> There is no reason at all that the C pre-processor cannot be
> run on Ada source before sending it to the compiler.  Therefore,
> one _can_ use macros, but it is neither standard nor advisable - 
> macros can be a source of very hard-to-find bugs.

Actually, there is a reason.  The C preprocessor chokes on some Ada
constructs.  The ISO C standard requires the input to the preprocessing
phase to be a valid token stream.  I don't remember the exact details,
but one consequence is that an apostrophe that's not part of a character
literal (e.g., in an Ada attribute) is illegal.

Here's an example:

package Foo is
#ifdef FIRST
   X : Integer := Integer'First;
#else
   X : Integer := Integer'Last;
#endif
end Foo;

The "cc -E" command under Solaris complains:

"foo.ads", line 5: invalid input token: 'Last;

Of course, there's nothing in the ISO C standard that requires the
preprocessor to exist as a separate program, or "cc -E" to behave in any
particular fashion.  Similarly, there's nothing preventing someone from
implementing a C-like preprocessor with fewer restrictions.  However,
using the C preprocessor on Ada code is likely to cause problems, as
the example above shows.

-- 
Keith Thompson (The_Other_Keith) kst@thomsoft.com <*>
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