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Subject: Re: [boost] [review] string convert
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2011-05-06 07:51:45

Vladimir Batov wrote:
> > Stewart, Robert <Robert.Stewart <at>> writes:
> > ...
> > OK, that means we're down to the following now:
> >
> > 1 - default_value<T> customization point
> > 2 - converter<T,S> customization point, main logic, functor
> > 3 - T convert_cast<T,S>(S, formatting = none); can throw
> > 4 - T convert_cast<T,S>(S, T, formatting = none)
> > 5 - bool try_convert_to<T,S>(S, T &, formatting = none)
> > ...
> #3, #4. I do not think these can cause any controversy
> (although people who refuse reading documentation might be
> surprised by #2 throwing or #3 not throwing ;-) ).

I don't care too much if they can't read the documentation far enough to learn that much.

> #5 IMO can. It deploys the Pascal-style parameter passing and
> modifications. I remember reading Stroustrup (I think) long
> time ago advising against passing non-const references and I
> personally agree. That's due to potential confusion and wrong
> expectations. I am not aware of any function in std and boost
> doing that. Introducing such a precedent might be a hard-sell.
> Especially confusing might be the fact that #4 does not modify
> the second parameter when #5 does.

I know some folks don't care for output parameters but they are useful. As for #4 not modifying and #5 modifying the second parameter, the names are distinct, so distinct behavior should not be surprising.

> std::pair<T, bool> try_convert_to<T,S>(S, T, formatting = none)
> is far less-controversial and even familiar

That cannot be used as straightforwardly:

int i(3);
if (!try_convert_to<int>("1", i))
   // report failure
// i == 3


std::pair<int,bool> maybe(attempt_to_convert_to<int>("1", i));
if (!maybe.second)
   // report failure
   // what's i's value?

Note that I changed the name. The "try" prefix, as noted by Vicente, implies returning a bool, not a pair.

> (even though I personally dislike std::pair due to
> unreadability of the resulting code sprinkled with faceless
> first and second).

I agree that a dedicated type, with names for the two fields, would be better. Furthermore, that avoids the chance for reversing the parameterizing types:

std::pair<bool,int> bad(attempt_to_convert_to...); // Mismatch!

> Could I suggest convert_result as a more palatable
> alternative? :-)

That would have to be convert_result<int>.

> #1. I am personally not thrilled with default_value because it
> looks to me like additional/avoidable/arbitrary piece of
> machinery that I have to implement to incorporate my class into
> the framework. I've been managing without it. I'd expect a new
> design not to force me to work harder.

That's based upon the idea that default constructors and zero-initialization are available for most types one would use this library to convert. However, I note that you replied elsewhere that relatively few classes in your environment have default constructors. That very clearly is a sore spot in this design.

Vicente's desire for uniformity, through a generic interface, is laudable. Using a CP like default_value permits that. Having to specialize default_value for most of your classes would be annoying, I agree, but consider that the alternative is to explicitly create an initial value for every conversion call you make.

> Additional note. While you are on the interface, it probably
> needs to be kept in mind that the current (as I understand it
> anyway) Boost policy is to avoid introducing content directly
> in the boost namespace.

That's an excellent point.

> Therefore, I presume fully qualified it'll be something like
> int i = boost::conversion::convert_cast<int>(str);

A namespace alias or using declaration or directive may be used to overcome such a verbose name, of course.

> So, maybe you might consider shortening it to something like
> int i = boost::convert::cast<int>(str);

With a using directive, that would end up just being cast<int>(str), which is unhelpful.

> int i = boost::convert::to<int>(str);

That's good.

> int i = boost::convert<int>::from(str); // Just kidding.

I've seen that somewhere before. Now, where was that?

> because otherwise the user will be implicitly forced to
> namespace cvt = boost::conversion;
> or worse
> using namespace boost::conversion;

A user may prefer to avoid the verboseness of "boost::conversion::convert_cast," but there's no implicit force. Given that "convert_cast" is a nice, descriptive name, I'd be likely to use a using declaration.

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

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