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Subject: Re: [boost] Boost.Convert. Take 2.
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2014-02-17 04:10:12

2014-02-16 23:13 GMT+01:00 Vladimir Batov <Vladimir.Batov_at_[hidden]>

> On 02/16/2014 08:35 AM, Andrzej Krzemienski wrote:
>> I have a suggestion regarding the documentation. It is abut the first
>> page.
>> I am expressing my preference, but I am pretty sure I am not the only
>> person here with this expectation. I am a very impatient man and I expect
>> of the first page a number of answers quickly, or otherwise I will be not
>> interested. When I see "Conversion" in the name of the library, I will
>> need
>> the following questions answered quickly (I guess I am not more arrogant
>> than an average stressed programmer):
>> 1. Is it only string to T and T to string conversions, or arbitrary T to U
>> conversion?
>> 2. Will it consider locale?
>> 3. Will it return optional<T> (or some such) so that I can decide myself
>> how to deal with conversion failure?
>> 4. How will I use it. Give me a minimum example.
>> In other words, I need to see from the first page what this library will
>> and will not give me.
> I've extended the Introduction section to address your #1&2 comments. As
> for #3&4 I suspect I am lacking the ability to express myself succinctly
> enough to squeeze those into the Introduction... without turning it into
> "War and Peace"... which would work against me with the impatient kind. ;-)

Now, I can definitely find all my answers in the Introduction. What I would
like to suggest as an option (but please do not treat this as a requirement
or an expression of dissatisfaction -- I can keep giving suggestions
forever, sometimes in a cycle) is to put a very very short code snippet
that will show me how I will be using the library.

> I would also suggest not to start with the comparison with lexical_cast.
>> Your users may not even know lexical_cast. Also, there is something
>> discouraging when I read how your library differs from some other library
>> rather than learning what your library is.
> I see your point. Unfortunately, I feel somewhat paranoid as lexical_cast
> comparisons dogged my V1 proposal all the way through. Back then people's
> reactions were quite natural -- we already have a conversion API which
> seems to be a potentially suitable foundation; why not use it? They were at
> the beginning of the "road" I already went but I had to answer the same
> (not always kind and polite) questions about lexical_cast over and over
> again. And youngsters are usually "quick shooters" -- quick and easy with
> grandiose statements and opinions, slow and reluctant with their own
> research beforehand. So, now I put in (indeed) a lot of stuff related to
> lexical_cast -- design, performance, functionality comparisons... just in
> case... in fact, if we get to the review/discussion stage, I am sure people
> will be asking -- why not lexical_cast, why does not it behave like
> lexical_cast?

Comparison with lexical_cast, and (even more importantly) answering the
questions "why would I use Boost.Convert library instead of lexical_cast"
and "why did you not extend lexical_cast instead" and "why would I need
lexical_cast anymore" are very important for your library's documentation
-- I agree. It is just that I am not sure these explanations belong to the
Introduction page. But I leave it open. I am satisfied with the improved
introduction as it is (except for the one tiny suggestion below).

> Two other suggestions for the initial page. 1. Mention that it works with
>> non-DefaultConstructibel types. It is unusual (in the positive sense) for
>> a
>> conversion library. 2. Since you mentioned that convert can be used
>> without
>> specifying the second (streamer) argument, show this in the initial
>> example: let it be really simple.
> I hope I've addressed your #1 in the Introduction section. Please see if
> you find it satisfactory. As for #2 I have to admit I reconsidered my
> original position. Indeed, I had the converter parameter in
> convert<T>::from(value_in, converter) defaulted to sstream_converter in my
> own code. Because I was lazy and did not care for its performance (I have
> other chunks "eating" so much more).
> Now, as I put it up for everyone to see, it's a different story. My
> original lazy approach had two drawbacks -- convert/api.hpp had
> sstream_converter.hpp included (an unnecessary coupling) and
> convert<T>::from was creating a converter every time it was called. As I
> described in the Performance section it has quite a detrimental effect on
> performance. As I said, *I* am not concerned (for my current
> applications)... but I do not want to give that loophole to people to
> explore, discover that performance sucks in their settings and come back
> swinging... just the same why I took the implicit converter to T from
> convert<T>::result as soon as you mentioned it -- I *personally* find it
> very convenient (no need for "value") but defending it is a loosing battle
> IMO.

Ok, I get it: you do not want to suggest an inefficient use.

> And one other thought (it is not really a suggestion for Boost.Convert,
>> but
>> a general observation regarding string conversions). Your library is
>> mainly
>> about converting string to T. T to string will be less common.
> Hmm, here with all due respect I really have to disagree on various
> levels. :-)
> In my neck of the woods, string-to-T and T-to-string are represented quite
> equally. Say, we consider the management of configuration files. Reading
> values (string-to-T) from cfg files is (to me, anyway) on the same scale as
> writing updated values back (T-to-string). Same goes for a component in a
> processing pipe or a node in a network -- reading/converting a lot of XML,
> converting/writing a lot of XML.
> And I find
>> it hard to imagine that someone would use it as T to U conversions.
> Again, I am far from sure about that. I do currently have to have
> OS-native MBCS (MSWin and Solaris) to UCS-2 and UCS-4 string conversions.
> They are a separate lib. Can't immediately see anything wrong in
> incorporating it into the 'convert' framework. Then I remember Vicente
> having a 'conversion' proposal to address that T-to-U conversion in a
> generic way. So, he must have had a need for it also. I am not saying you
> are wrong. I just do not know. Consider the templates example. Stroustrup
> purposefully designed them in a generic way. Their deployment exploded
> often in surprising ways. I am certainly no Stroustrup but you get the idea.

So, you say, it could be used to convert between two different string
encodings? I didn't think of it but it looks really useful. So useful that
it deserves a bullet in the Introduction. Can it be used to convert from
string to wstring?

For some background, Vicente's library IIRC tried to emulate to some extent
the non-member conversion operator. It was supposed to convert between two
types that were meant to represent the same abstraction but written by two
different programmers/libraries:

time_t t1;
ptime t2;
MyLib::Time t3;

they all represent the same thing and the library should offer a universal
way to convert between them.

> When
>> converting from T to string, you do not really need to return optional<T>,
>> because it is not possible that this conversion can fail. Any T always has
>> a string representation, doesn't it? I wonder (but I do not have a good
>> answer) if conversion in this direction should have the same interface.
> Uhm, I would not be that quick saying that T-to-string conversion can
> never fail. It depends on complexity of the conversion and the used
> character set, etc. What if T is a complex class which in its conversion
> depends on other classes to behave and they don't? What if the conversion
> depends on, say, formatting specification and it is not met or is invalid?

This is an interesting point. If a conversion fails because it required to
acquire some resources and it failed to get them, or because the object was
in a disallowed state (some assertion-like thing failed), do you want to
return an empty result, or just throw?

> On top of it, uniformity (and, therefore, predictability) of API and
> behavior is quite important IMO. Special (and questionable I might say)
> handling of one special case IMO is not worth it (again IMO).



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