Re: [boost] [Review] Boost.Convert library, last day

Subject: Re: [boost] [Review] Boost.Convert library, last day
From: Christopher Jefferson (chris_at_[hidden])
Date: 2011-05-03 11:22:59

On 3 May 2011, at 13:19, Thomas Heller wrote:

> On Tue, May 3, 2011 at 12:41 PM, Vladimir Batov <vbatov_at_[hidden]> wrote:
>>> Hartmut Kaiser <hartmut.kaiser <at> gmail.com> writes:
>>> From reading the docs I came away puzzled. It was highly confusing to me to
>>> have the convert::from<> return different things in different contexts.
>> 
>> I was under impression that from the user perspective convert::from returns
>> either the converted value or convert::result in all contexts. If users "really
>> just want to do the conversion without spending much time reading the docs",
>> then they'll use
>> 
>> int i = convert<int>::from(string)
>> 
>> as described in "Getting Started" without even knowing there is convert::result.
>> Hardly puzzling and confusing.
>> 
>> 
>>> As Thomas Heller pointed out, the existence of several default conversion
>>> paths for the result type will (with high probability) create havoc for
>>> users who really just want to do the conversion without spending much time
>>> reading the docs.
> <snip>
>> Secondly, I am not sure about "existence of several default conversion paths for
>> the result type". Again, the only types convert::from result is converted to are
>> the Target type and the convert::result type. Both types are for different uses
>> and are described in the docs. As for Thomas Heller's doomsday predictions, then
>> it must be so if he says so. Or may be not. I do not see it that way anyway.
> 
> Sorry, this statement is wrong.
> At the very least convert<T>::from returns exactly one thing which is
> this converter thingy,
> which is implicitly converted to either convert<T>::result or T. And
> yes, this means that
> the result is different based on the LHS of from, which is out of your
> control and doomed to fail.
> I wonder how much codes breaks by having this.
> Consider the following example:
> 
> #include <boost/convert.hpp>
> #include <string>
> 
> template <typename Char, typename Allocator>
> void f(std::basic_string<Char, Allocator> string)
> {}
> 
> int main()
> {
>    f(boost::convert<std::string>::from(123));
> }
> 
> Which is not working because of this design flaw I constantly keep
> mocking about.

Out of interest, what would:

auto str = boost::convert<std::string>::from(123);

Do? If boost::convert stores by pointer/reference, it would be quite dangerous.

I think making libraries 'auto safe' is a good idea, I expect to see auto usage shoot up very quickly, and libraries should do their best to work well with auto.

Chris