Re: [boost] Re: [string algo review] overloading on const

From: Pavol Droba (droba_at_[hidden])
Date: 2003-10-23 13:41:48

On Thu, Oct 23, 2003 at 12:51:41PM -0400, David Abrahams wrote:
> Beman Dawes <bdawes_at_[hidden]> writes:
> 
> > At 10:47 AM 10/23/2003, Pavol Droba wrote:
> >  >On Thu, Oct 23, 2003 at 11:28:38PM +1000, Thorsten Ottosen wrote:
> >  >> I would say we have to provide
> >  >>
> >  >> template<typename InputContainerT, typename SearchContainerT>
> >  >>     iterator_range< typename string_algo::container_const_iterator<
> >  >> InputContainerT >::type >
> >  >>     find_first(const InputContainerT &, const SearchContainerT &);
> >  >>...
> >  >
> >  >Yes you have overlooked one important point. Your second proposed variant
> >  >does not make sense and what worse, it is even very dangerous.
> >  >
> >  >find_xxx algorithms return a reference into the input container by the
> >  >means
> >  >of iterators. Using a temporary as an input, you would get an invalid
> >  >reference as a result.
> >  >
> >  >Returning a reference from a temporary is quite a big error in my
> >   opinion.
> >
> > What is the fix? There really needs to be some convenient way to do a
> > find_first on a const container. Provide a version that takes begin
> > and end iterators? Or is that already done by one of the algorithms in
> > detail that you are going to move into boost::algorithm::string?
> 
> I'm not sure a fix is needed.  The first variant will already match
> const containers, as long as they're not temporaries.
> 
>       
>       template <class T>
>       int f(T& x);
> 
>       int const x;
>       int y = f(x); // T deduced as const int.
> 

You are right, there is no restriction on const, rather ther is a restriction,
that the input must be valid R-value i.e. not a temporary.

I think this is absolutely correct behaviour.

Pavol