Re: This can't be done in Ada...or?

[Peter Amey <peter.amey@praxis-cs.co.uk>]

Per Lindquist wrote:
> Hi, this question has already sort of been discussed in topics about
> multiple target platforms, conditional compilation, preprocessors and
> build strategies. Before giving up I thought I'd give it a try one last
> time...
> 
> My impression from reading some threads is that Ada DOES NOT offer a
> decent way to implement trace printouts (log messages), at least not
> with the features stated below.
> 
> The problem
> -----------
> 
> We want to be able to output log messages, or traces as we call it,
> from an Ada program. There will be a few trace categories like Error,
> Warning, Note etc. The messages (strings) can be sent to a file or
> stdout or whatever (not important).
> 
> A call might look something like this:
> 
> Trace.Error(Package_Name, Subprogram_Name, "Woops, V=" &
> Some_Type'Image(V));
> 
> How do we write a tracing utility in Ada that:
> 1. has different behaviour on different target platforms
> 2. can disable some categories depending on target platform at
> compile-time.
> 3. does *not* cause any execution overhead if disabled.
> 4. does not need yucky "if Trace_Wanted then ... end if;" around all
> calls
> 5. does not need preprocessing (some of you say it's never needed in
> Ada...)
> 
> I say it can't be done in Ada. Please prove me wrong!
> 

One useful approach I have used is to make us of subunits.  Each of the 
reporting routines in your package Trace can be made a subunit of Trace. 
  The subunit itself, e.g. Error,  can have two implementations, one 
that reports the error and one which is just a null body.  The compiler 
"make" system and library manager can be used to build different 
variants with different levels of reporting.  If the stub is marked as 
in_line then there is no overhead when the null version is called.

Peter