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Subject: Re: [boost] [review] string convert
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-04 15:33:20

Matthew Chambers wrote:
> On 5/4/2011 2:01 PM, Stewart, Robert wrote:
> >>> The target type should be required in all cases, I suspect,
> >>> to make things clearer:
> >>>
> >>> try_convert_to<int>(s, i) clearly indicates that int is the
> >>> target type.
> >>
> >> Yes, but thanks to type inference users are free to write
> >> try_convert_to(s, i) instead and that's even worse than
> >> try_convert(s, i). :)
> >
> > You can make the second argument non-deducible thus requiring
> > the target type.
> I didn't know that was possible. How? If the target is non-
> deducible then I agree the _to suffix for optional_convert
> and try_convert is good.

template <class T>
struct identity
   typedef T type;

template <class T, class S>
try_convert_to(S, identity<T>::type);

identity<T>::type is T, but the compiler cannot deduce T from identity<T>::type. Therefore, T must be given specifically.

> Back to something you said earlier:
> >>> If others agree, I could also go with Vincente's version of
> >>> try_convert. It doesn't bother me that passing in the
> >>> variable by reference makes it a two-liner, since the
> >>> expression itself can go inside the if statement and it
> >>> implicitly supports non-default-constructable types.
> >
> > It is certainly non-surprising.
> >
> > Using Vicente's default_value customization point still makes
> > convert_cast<optional<T>> a viable candidate:
> >
> > auto const c(convert_cast<optional<int>>(s));
> > if (c)
> > {
> > // use c.get()
> > }
> What about UDT specializations for casting to and from
> optional<T> (i.e. string<->optional<int> where 'n/a'
> could be the unset string)?

I don't understand your question, I'm afraid.

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

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