Subject: Re: [boost] [Review] Boost.Convert library, last day
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2011-05-03 08:19:38
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.
Sorry to be the party pooper here. But your design is just confusing
and does not
really work like you are advertising it.