Re: [boost] [lexical_cast] A suggestion

Subject: Re: [boost] [lexical_cast] A suggestion
From: Vladimir Batov (batov_at_[hidden])
Date: 2009-02-03 16:35:48

----- Original Message ----- 
From: "Vladimir Batov" <batov_at_[hidden]>
Newsgroups: gmane.comp.lib.boost.devel
To: "Boost Developers' List" <boost_at_[hidden]>
Sent: Tuesday, January 13, 2009 11:26 PM
Subject: [lexical_cast] A suggestion

>I use boost::lexical_cast quite a bit. Possibly due to specifics of my task 
>I constantly stumble upon two things:
>
> 1. It throws if/when conversion fails;
> 2. It requires the 'Target' class to be default-constructible.
>
> I'd like to suggest extending the existing
>
>    template<class Target, class Source>
>    Target
>    lexical_cast(Source const& arg)
>
> interface with
>
>    template<class Target, class Source>
>    Target
>    lexical_cast(Source const& arg, Target const& failure_value)
>
> The behavior of the latter would be to return the 'failure_value' if/when 
> the conversion fails. That communicates the fact of the conversion failure 
> differently (without an exception thrown), i.e. its usage would be as 
> following:
>
>    int value = boost::lexical_cast(some-string, -1);
>    if (value == -1) conversion failed.
>
> In fact, my usage pattern is such that I often do not even need to check 
> the success of the conversion -- if the conversion fails, the supplied 
> failure/default value is returned/used and proceeded with.
>
> Secondly, the proposed lexical_cast requirements would be looser -- no 
> need for the default-constructibility of the Target, only the 
> copy-constructibility (as already required anyway). For that the current 
> lexical_cast implementation
>
>    if(interpreter << arg) {
>        Target result;
>        if (interpreter >> result)
>            return result;
>    }
>    throw_exception(bad_lexical_cast(typeid(Source), typeid(Target)));
>    return Target(); // normally never reached (throw_exception)
>
> would change (for the proposed interface) to something like
>
>    if(interpreter << arg) {
>        Target result(failure_value);
>        if (interpreter >> result)
>            return result;
>    }
>    return failure_value;
>
> I am not sure how much value such an interface extension might have for 
> others but for my usage pattern the benefit would be quite visible as the 
> majority of our classes are with no default constructors and handling 
> conversion-failure exceptions is quite a hassle... not to mention that for 
> whatever guided/misguided reasons exceptions are virtually banned for our 
> mission-critical development.
>
> Best,
> V.
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