Re: Config and "unused variables"

From: williamkempf_at_[hidden]
Date: 2001-09-24 17:20:47

--- In boost_at_y..., "Fernando Cacciola" <fcacciola_at_g...> wrote:
> 
> ----- Original Message -----
> From: <williamkempf_at_h...>
> To: <boost_at_y...>
> Sent: Monday, September 24, 2001 5:30 PM
> Subject: [boost] Config and "unused variables"
> 
> 
> > It's fairly common to have unused variables in Boost code for 
various
> > reasons.  For instance, a common work around for a VC++ bug is to
> > have code such as:
> >
> > template <typename T>
> > void foo(T* dummy=0)
> > {
> > }
> >
> > The dummy parameter is never used, and is there solely to help the
> > VC++ compiler figure out the template argument types.
> >
> I think you are referring to the lack of explicit template 
instantation in
> VC++. In that case you don't really need a name for the dummy 
argument, so
> the usual workaround looks rather like this:
> 
>  template <typename T>
>  void foo(T const* =0)
>  {
>  }
> 
> Anyway, I know there are places when you do have named unused 
variables.

Even in this case I'd much prefer leaving the name, as I think it 
documents the idiom better.  The documentation is even better when 
you include the unused_parameter() call.

> > Currently I don't
> > see any mechanism for eliminating this stuff in the config files,
> 
> With Borland 5.5.1, I'm used to do the following:
> 
> void foo(MyClass const& a)
> {
>    int b;
>    float c ;
>   (a,b,c); // ==> Removes the 'ununsed' warning while generating no 
code at
> all.
> }
>
> Unfortunately, I don't know how to analyze this idiom in terms of 
Standard
> behavior, so I don't know how portable it is or what side effects 
can it
> present.

I think this is standards compliant, and is the underlying method 
usually used.  However, it does a poor job of documenting anything, 
so in C one usually uses a macro that reduces to the above code.  We 
all know to avoid macros, though, so the template technique does the 
same job in a safer manner.

> > but
> > Jeremy Siek has a simple solution in concept_check.hpp:
> >
> > template <class T> inline void ignore_unused_variable_warning
(const
> > T&) { }
> >
> > I think it would be beneficial to have this in a more reusable
> > fashion for all Boost libraries and think the config files would 
be a
> > handy place for it (though I'd be fine with it in any other 
location
> > as long as it was a small header).  I might reduce the name to 
just
> > unused_variable(), though other's might not care for the less 
verbose
> > name.
> >
> Sounds good. (I like unused_variable()).
> I would have add the possibility of having multiple arguments:
> 
> template<class T0> void unused_variable( T0 const& ) {}
> template<class T0, class T1> void unused_variable( T0 const&, T1 
const& ) {}
> template<class T0, class T1, class T2> void unused_variable( T0 
const&, T1
> const&, T2 const& ) {}
> etc...

This may or may not be overkill for a simple concept.

Bill Kempf