Re: [boost] [review] Convert library

Subject: Re: [boost] [review] Convert library
From: Nelson, Erik - 2 (erik.l.nelson_at_[hidden])
Date: 2014-05-24 22:21:24

Vladimir Batov wrote on Saturday, May 24, 2014 9:40 PM

>> Habraken wrote
>> The most trivial use of boost::convert, and boost::lexical_cast and 
>> boost::coerce simply don't compare in my opinion.
>> 
>>     boost::cstringstream_converter cnv;
>>     int i = boost::convert <int>::from("23", cnv).value();
>> 
>> versus
>> 
>>     int i = boost::lexical_cast<int>("23");
>>     int j = boost::coerce::as<int>("23");
>
>Here we go again... :-) lexical_cast will be haunting me to my grave. :-) 
>
>For starters, please stop using that silly lexical_cast deployment example as our benchmark. Such deployment does not exist. >lexical_cast cannot possibly be deployed like that in any real application. The correct and responsible deployment is
>
>int i = -1;
>int j = -1;
>
>try { i = boost::lexical_cast<int>(str); } catch (...) {} try { j = boost::lexical_cast<int>(str); } catch (...) {}
>
>which is not exactly the two-liner that you sited! Compared to that the "convert" usage is
>
>    boost::cstringstream_converter cnv;
>    int i = boost::convert<int>(str, cnv).value_or(-1);
>    int j = boost::convert<int>(str, cnv).value_or(-1);
>
>does look simple and intuitive enough IMO, configurable and potentially more efficient. (In the post_review branch the >convert::from API has been retired). 

I guess I'm silly, but such deployment does exist in my code base, to good effect.  Convert "something" to "something else".  If it doesn't work, throw an exception.  That's a typical C++ idiom.  Any implementation that requires some cryptic object-creation-before-I-call-a-function doesn't make sense to me.  Your example adds default-value functionality, which is usually (in my use cases) inappropriate.

>> When the convert library makes it into boost I'd want to mentally 
>> deprecate lexical_cast as a library I should use, and always use 
>> convert instead.
>> This means convert should have a default converter, which may well be 
>> backed by lexical_cast with an interface such as:
>> 
>>     int i = boost::convert<int>::from("23").value();
>> 
>> This still doesn't completely satisfy me as I'd like to write
>> 
>>     int i = boost::convert<int>::from("23");
>
>Again, I can't possibly stress more that the (deceivingly simple) usage examples you insist on do not exist in real >applications -- they are functionally-incomplete (as they do not handle all possible use-cases).
>Still, with the post_review branch one can responsibly(!) write
>
>    int i = boost::convert<int>(str, cnv).value_or(-1);
>

I can't possibly stress more that those cases do exist.  I know that in my application, I almost never know some default value_or, so if the conversion doesn't work, I *want* an exception.  Forcing the programmer to decide appropriate default values for every conversion isn't silly- I don't know, I just want a valid conversion or an exception.

At any rate, I largely agree with Jeroen's points, and I find the Convert interface excessively complex.  Specifically, if I have to create a conversion object that I then immediately pass as an argument to a conversion function, that seems excessively complex to me.

Erik

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