Re: [boost] [review] string convert

Subject: Re: [boost] [review] string convert
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-05 13:32:11

Jeff Flinn wrote:
> Stewart, Robert wrote:
> > Vicente BOTET wrote:
> >> De : "Vladimir Batov"
> >>> "Stewart, Robert"  wrote
>
> >>> Maybe to avoid the temptation of pairing up we could
> >>> simplify (at least from the user perspectives) down to:
> >>>
> >>> 1. T convert_cast(S)
> >>> 2. T convert_cast(S, T, throw_t =nothrow)
> >> I don't see why (2) the user could want to throw when is
> >>> giving a fallback. So the throw_t =nothrow is not needed.
> >
> > That's the non-DefaultConstructible, no-sentinel use case:
> >
> > struct example
> > {
> >    example(int);
> > };
> >
> > If one wants an exception to indicate conversion failure,
> > an initial value is needed and the desire for an exception
> > must be noted:
> >
> > example const e(convert_cast<example>("1", 3, throw_));
>
> Sorry if I'm just being dense, but if the above throws, the
> default value is never used, so why supply it?

Think about how convert_cast() might be implemented:

template <class T, class S, formatting-stuff>
T
convert_cast(S _source, formatting-stuff _formatting)
{
   T result;
   std::stringstream s;
   s << _source;
   s >> _formatting >> result;
   if (!s)
   {
      throw something;
   }
   return result;
}

Since example cannot be default constructed, you'd need:

template <class T, class S, class U, formatting-stuff>
T
convert_cast(S _source, U value, formatting-stuff _formatting, throw_t)
{
   std::stringstream s;
   s << _source;
   s >> _formatting >> value;
   if (!s)
   {
      throw something;
   }
   return value;
}

> > Obviously, with your default_value CP, the second argument
> > isn't needed for the last use case:
> >
> > template <>
> > default_value<example>
> > {
> >    static example
> >    apply()
> >    {
> >       return example(3);
> >    }
> > };
> >
> > example const e(convert_cast<example>("1")); // exception
>
> So it's really just the no-default function that needs to be
> told to throw or return an optional.

Yes.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer                     using std::disclaimer;
Dev Tools & Components
Susquehanna International Group, LLP  http://www.sig.com

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