Re: [boost] [review] string convert

Subject: Re: [boost] [review] string convert
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-03 11:36:12

Jeff Flinn wrote:
> Stewart, Robert wrote:
> > Gordon Woodhull wrote:

[snip massively overquoted context]

> >>> convert<int>::result res = convert<int>::from("blah", -1);
> >>> if (!res) message("conversion failed");
> >>> int value = res.value(); // proceed with the fallback
> >>> anyway.
> >
> >> It's just that the next line might read
> >>
> >> int res2 = convert<int>::from("blah");
> >>
> >> and throw, and there's no indication why one version throws
> >> and the other does not.
> >
> > Of course there is.  One was given a fallback value and the
> > other wasn't.
>
> dynamic_cast sets a precedent for how to handle throwing/non-
> throw variations:
>
>    dynamic_cast<X&>(y); // throws if cast fails
>
>    X* x = dynamic_cast<X*>(y); // returns 0
>
>
> So if boost::optional is the analog of pointer, and we go to
> some xxx_cast naming convention we could have:
>
>    // throws if fails
>    int i = xxx_cast<int>(s);
>
>    // returns empty optional if cast fails
>    boost::optional<int> oi = xxx_cast<boost::optional<int>>(s);
>
>    // never fails
>    int i = xxx_cast<int>(s, 123);
>
> This just reuses boost::optional rather than the (superfluous
> to me) convert<T>::result type. I for one see no reason that
> _cast need be limited to a single argument.

I really dislike the repetition of "boost::optional<int>" in that.  What about going a little farther in the dynamic_cast direction (actually, following the boost::get precedent):

   int const i(xxx_cast<int>(s)); // throws on failure
   boost::optional<int> const oi(xxx_cast<int>(&s));

Whereas dynamic_cast requires casting from a pointer type to a pointer type, here the suggestion is that casting from a pointer type implies casting to an optional.  Note that the cast is "xxx_cast<int>" in both cases; it’s the source argument's being a pointer or not that dictates the overload and, thus, the return type.

> If others find multiple arguments too disgusting, perhaps
> boost::make<int>(s, 123) is more palatable.

That name could work, but does it allow for formatting?  IOW, "convert," like "translate," leaves room to consider formatting, including locales.  "Make" suggests something more fixed to me.

_____
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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