Re: [boost] [lexical_cast] A suggestion

Subject: Re: [boost] [lexical_cast] A suggestion
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2009-02-07 05:19:56

Vladimir Batov wrote:
>> ... I like the lexical_cast< int >("x", -1); syntax much more.
>>
>> If the syntax is considered unacceptable, I can suggest another 
>> alternative:
>>
>>   int x = lex_cast< int >("x").with_default(10);
>>   int y = lex_cast< int >("y").or_default_constructed();
> 
> 
> I sympathize with the idealistic desire for lexical_cast to look like 
> the dynamic_cast et al. The problem is that no matter how you make 
> lexical_cast look it does not do what other language-supported casts do 
> -- dynamic_cast el al work with *types*, lexical_cast works with 
> *values*. Making lexical_cast look like what it is not (the "real" 
> casts) has no practical value and is just misleading.

I disagree. Making lexical_cast behave similar to other casts makes 
perfect sense. It does change the type of its argument just as well as 
its physical representation.

> Now seemingly out of desperation people are venturing into
> 
>    lexical_cast<optional>
>    lex_cast< int >("x").with_default(10);
>    lex_cast< int >("y").or_default_constructed();
> 
> It's just crazy from the usability point of view. Are you sure users 
> will appreciate all that mounting complexity on top of lexical_cast? All 
> that is needed
> 
>    lexical_cast(Source const&, Target const& =Target())

No, that kind of implementation seems unacceptable to me. I want 
lexical_cast to throw exceptions just as often as I don't want it to. 
And, of course, I'm against introducing a breaking change.

> or most likely a completely independent overload
> 
>    lexical_cast(Source const&, Target const&)

That would be my preferred way to implement this extension.