Boost C++ Libraries: Ticket #12136: Propose lexical_cast type traits

Antony Polukhin — Tue, 19 Apr 2016 05:40:14 GMT

LexicalCast requires default construction and istreamable/ostreamable from operator:

template <class T>
struct is_lcast_istreamable: mpl::bool_<
    (is_default_constructible<T>::value &&
        (boost::has_right_shift<std::basic_istream<wchar_t>, T>::value ||
         boost::has_right_shift<std::basic_istream<char>, T>::value)
    )
>{};

You can make the is_lcast_ostreamable similarly. Or you could just use the detection idiom.

Anyway, such trait seems to have limited usage. I'd rather not add it and keep the interface of the LexicalCast library as simple as possible.

Tue, 19 Apr 2016 14:38:47 GMT

Thanks for detection idiom pointer - look interesting.

Thanks for suggested implementation.

Thoughts:

Though people technically can implement their own trait code (like your suggested implementation), they probably shouldn't because, AFAIA, the library doesn't present this expression as part of it's interface, ie using it means depending on the library's internals. Unless the library's public documentation can guarantee that this expression is guaranteed to remain correct, this functionality is best provided by the library so it can insulate users' code from changes to the internals.
I wholeheartedly agree that lexical_cast's simple interface is one of its strengths. That said, I think the proposed traits would add very little complexity and could be kept to a corner of the documentation so they don't confuse new users.

Thanks very much.

status changed; resolution set

Antony Polukhin — Sun, 09 Sep 2018 20:11:32 GMT

status new → closed
resolution → wontfix

After 2 years of thought I came to the decision to not add the trait. People who need it could invent it on their own, others probably should not know about it at all.