Re: Implementing C/C++ style #include...

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

"Marin David Condic" <marin.condic.auntie.spam@pacemicro.com> writes:

> I guess I've just seen it done enough different ways that work as well or
> better that I don't really see any necessity for a preprocessor to do this
> job. Managing different configurations can be handled with package body
> substitution, static elimination of code via normal "if" statements, pragma
> Debug, available configuration management tools, etc. 

Not this one:

   function Ada_Size_Of_Input is new 
      Interfaces_C_More.Storage_Unit_Size_Of (Input_Type);
#if GNAT_Compiler
   pragma Export (C, Ada_Size_Of_Input,
                  "ball_ct633_ada_size_of_input",  
                  "ball_ct633_ada_size_of_input");
#elsif ObjectAda_Compiler
   pragma Export (DLL_STDCALL, Ada_Size_Of_Input, 
                  "ball_ct633_ada_size_of_input", 
                  "ball_ct633_ada_size_of_input");
#end if;

The difference is in the name of the Convention. Preprocessing is, in
this case, the best way to handle it. I believe the only alternative
is to reproduce the entire package spec, and change this one value.
Surely you do not claim that is better from a maintenance point of
view? 

I can only assume you have not personally encountered a situation like
this. Until you have, you need to be a little more open :).

There are similar examples of needing to support a mix of Ada 83/95,
compiler vendors, and targets, where limited, structured,
well-designed use of pre-processing is the best way to go.

> As a result, I think that trying to include special pre-processor
> directives in the source and going through an extra pre-compilation
> step (fraught with peril!) is unnecessary and potentially dangerous.

"potentially dangerous" yes, just like Unchecked_Conversion and
Unchecked_Deallocation. "unecessary" no, just like
Unchecked_Conversion and Unchecked_Deallocation!

> 
> (Try looking at some older C code that may have very convoluted
> "#include" chains, etc. if you need an example of how dangerous this
> can get.) I've never once seen a case where you could accomplish
> some goal with a preprocessor that you couldn't accomplish with a
> less error prone technique. 

Have you seen the above case?

> So why ask for the trouble?

I'm not asking for "trouble", I'm asking for "expressive power".
"Trouble" I can handle with coding guidelines and project discipline.

Note that the gnat preprocessor does _not_ define "include", only
"if". And it does _not_ define macros.

-- 
-- Stephe