Re: [boost] [lexical_cast] A suggestion

Subject: Re: [boost] [lexical_cast] A suggestion
From: David Abrahams (dave_at_[hidden])
Date: 2009-02-09 15:56:21

on Mon Feb 09 2009, Andrey Semashev <andrey.semashev-AT-gmail.com> wrote:

> My reply may be too late for the discussion, but I'll answer anyway...
>
> David Abrahams wrote:
>
>>>> That's not my concern.  My concern is that I don't think lexical_cast is
>>>> a particularly good interface for its main uses, which could be thought
>>>> of as "to-string" and "from-string."
>>> I disagree. lexical_cast was designed to be simple to use, and I think it does it
> very
>>> well. The "to-string" and "from-string" conversions, in their simple form, are just
>>> corner cases. 
>>
>> Eh?  What other uses has it got?!
>
> To convert objects of different types between each other. 

Yes, I meant what other real-world uses has it got?

> I don't know if conversion between types that are not strings is
> widely used or not.

Then take it from me, it isn't.

> Conversions between custom and fundamental floating point types come
> to mind as an example.

Yes, hypothetically.  In reality, no way.  Nobody who wrote a custom
floating point type would leave its conversion to double up to the
vagaries of iostreams, especially ones that hadn't had the right
manipulators applied.

> I'm quite happy with lexical_cast, except for this default-value
> issue. Why would I write something new?

Because where you want it to go clearly doesn't mesh with the vision of
the original author or the maintainer.

>>>> I just don't think lexical_cast is the interface we need for most
>>>> convenient string conversions.
>>> Do you have a better interface in mind?
>>
>> It would take some thought and discussion to perfect, but just off the
>> top of my head, I'd want something like this:
>>
>> namespace str
>> {
>>   template <class T>
>>   struct value
>>   {
>>       operator T() const;  // throws iff parse unsuccessful
>>       bool parsed() const; // true iff parse succecssful
>>       ...
>>   };
>>
>>   template <class String, class T>
>>   string::value<T>
>>   string::as<T>(String const&);
>>
>>   template <class String, class T>
>>   typename enable_if<is_integral<T>, string::value<T> >::type
>>   string::as<T>(String const&, unsigned base);
>>
>>   template <class T>
>>   std::string
>>   to_str(T const& x);
>>
>>   template <class String, class T>
>>   String
>>   to_str(T const& x);
>> }
>
> This does look pretty much like lexical_cast, a bit extended for
> string conversions, though.
>
> I'm not against a better support for parsing or formatting in
> Boost. However, in case of simple needs, I think the potential of
> lexical_cast is not yet exhausted. Adding another library with so much
> overlapping functionality may confuse users.

Maybe, but lexical_cast clearly isn't targeted at what some large
segment of users needs.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com