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Subject: Re: [boost] [Review] Boost.Convert library, last day
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-03 12:50:04

Matthew Chambers wrote:
> On 5/3/2011 10:36 AM, Stewart, Robert wrote:
> > Christopher Jefferson wrote:
> >> Out of interest, what would:
> >>
> >> auto str = boost::convert<std::string>::from(123);
> >>
> >> Do? If boost::convert stores by pointer/reference, it would
> >> be quite dangerous.
> >
> > str would be a boost::convert<std::string>::converter, not a
> > std::string as one would probably expect.
> >
> >> I think making libraries 'auto safe' is a good idea, I
> >> expect to see auto usage shoot up very quickly, and
> >> libraries should do their best to work well with auto.
> >
> > This may be one of the most compelling arguments against the
> > current interface.
> Agreed. I know I'll be jumping on the bandwagon as soon as auto
> becomes mainstream in my two main compilers, MSVC and GCC. So
> this is a compelling argument. But would it even be possible to
> keep the "smart result" (either convert<>::result or optional<>)
> and auto compatibility? If (as I suspect) not, this would seem
> to necessitate different syntax for value/throw and functor.

The above does provide the "smart result," but I think is surprising in the context. Arguably, there would be times a user would want the "smart result," so it isn't certain that this example obviates that part of the interface. Rather, this points to the need for distinct calling conventions -- whether by varying the argument list or using differently named functions (function templates) -- in order that what auto infers is what the user expects.

   auto i(convert_to<int>("123")); // 1
   auto j(convert_to<int>("123", -1)); // 2
   auto t(try_converting_to<int>("123")); // 3
   converter<int> f; // 4

In // 1, i is int and the call can throw an exception on conversion failure.

In // 2, j is int and the call will return -1 on conversion failure.

In // 3, t is boost::optional<int>. Note that using optional rather than Boost.Convert's result type means that accessing the value will not throw an exception; it's undefined behavior, IIRC. "try_converting_to" is rather verbose, so some other name is probably wise.

In // 4, auto isn't helpful, so I didn't show it. I'm thinking f is a function object with a member function template function call operator that forwards to some customization interface that permits target/source-specific customizations.

I think that covers all of the use cases and plays well with auto.

In the first three cases, we could use "xxx_cast" instead of "convert_to" and "try_converting_to," as I'd previously suggested, except that "123" cannot be used to distinguish between the first two and the third cases. Now, given "auto," we could make it work as follows:

   auto i(xxx_cast<int>("123")); // 1
   auto j(xxx_cast<int>("123", -1)); // 2
   auto t(xxx_cast<optional<int>>("123")); // 3

Because of "auto," there's no repetition of "optional<int>" in // 3. Of course there may be those offended by the second argument in a new-style cast as in // 2.

More food for thought.

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

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