Re: [boost] [review] string convert

Subject: Re: [boost] [review] string convert
From: Vicente BOTET (vicente.botet_at_[hidden])
Date: 2011-05-09 14:09:38

> Message du 09/05/11 16:22
> De : "Stewart, Robert" 
> A : "'boost_at_[hidden]'" 
> Copie à : 
> Objet : Re: [boost] [review] string convert
> 
> Vicente BOTET wrote:
> > De : "Vladimir Batov"
> > > > From: "Stewart, Robert"
> > > > ...
> > > >> int i = boost::convert::to(str);
> > > >
> > > > That's good.
> > >
> > > How about we simplify the API down to just:
> > >
> > > T convert::to(S, boost::parameter-list);
> >
> > As Robert said, the name of a free function must be readable
> > when introducing a using namespace. Or is convert a class?
> 
> I think to("FF") is readable.

Maybe, but the name doesn't scale. So I will prefer to preserve the convert_to prefix.

> > > Then we specialize it for convert::result type so that
> > >
> > > convert::result convert::to>(S, boost::parameter-list);
> >
> > I don't see any inconvenient in introducing a special overload.
> 
> I presume you mean that such an overload could be convenient for someone like Vladimir and would not cause a problem for those preferring other approaches.

Right.

> > I guess a user needs to call it as follows
> >
> > convert::result r = convert::to>(S, boost::parameter-list);
> 
> It's hard to say as that was munged.

convert::result r = convert::to(*convert::result*)(S, boost::parameter-list);

> > > // The predictable. Throws on failure
> > > int i = convert::to("FF", (format_ = std::hex))
> >
> > Would this overload exist only when the source parameter is
> > string? Sorry, but I don't see how it could be used for type-
> > to-type conversions
> 
> If it supports type-to-type conversion via an I/O stream, it would apply, wouldn't it?

It would, but I will prefer to use the stream syntax for that kind of conversion instead of using additional parameters.

> > int i = convert::to(aValue, (format_ = std::hex))
> >
> > Would the std::hex be applied to the output or the input
> > stream?
> >
> > ios << aValue;
> > ios >> std::hex >> i;
> >
> > or
> >
> > ios << std::hex << aValue;
> > ios >> i;
> >
> > Are these equivalent?
> >
> > I think the interface must state clearly when these
> > manipulators are applied.
> 
> I think it should be something like this:
> 
> convert::as(aValue, (in_ = std::hex, out_ = std::hex));

Hmmm, I don't tisk this is enough readable.

> > Vladimir, as I said you my library doesn't pretend to take care
> > of string conversions or conversions via a stream.
> 
> I presume you mean string-to-string conversions when you write "string conversions" ...

No, I meant conversion from string to a type and from a type to a string.
As I tried to explain in this and other posts, to take care of a type-to-type conversion via a streams you needs to state which manipulators are applied to the ostream and which one to the istream. Your in_, out_ is first trial but must be generalized to several manipulators.

> > My library will specialize the convert_to function when one of
> > the parameters is a string and call the best library providing
> > it.
> 
> ...because of that.

The intent of Boost.Conversion is to support generic conversion that can be specialized for specific types. One of the major differences between Boost.Conversion and Boost.Convert, a part from the interface is the customization point. In the case of Boost.Convert it is the class convert and defaults conversion using a iostream. Boost.Conversion is customized by overloading the convert_to function, and by default uses the target conversion operator from a source.

Best,
Vicente