Re: [boost] [lexical_cast] A suggestion

Subject: Re: [boost] [lexical_cast] A suggestion
From: Deane Yang (deane_yang_at_[hidden])
Date: 2009-01-13 09:22:09

Neil Groves wrote:
> I would like to add my support for this proposed change:

Me, too.

> 
> On Tue, Jan 13, 2009 at 12:26 PM, Vladimir Batov <batov_at_[hidden]>wrote:
> 
>> 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.
>>
> 
> I have similar use-cases where I require a non-throwing implementation and
> hence I have to, reluctantly avoid lexical_cast. This change appears to be
> very compelling.
> 
> 
>> Best,
>> V.
> 
> 
> Neil Groves
> 
> 
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