Re: [boost] Boost review of the Convert library is ongoing

Subject: Re: [boost] Boost review of the Convert library is ongoing
From: Vladimir Batov (vb.mail.247_at_[hidden])
Date: 2014-05-23 03:13:42

Krzysztof Czainski wrote
> 2014-05-23 2:25 GMT+02:00 Vladimir Batov <

> Vladimir.Batov@

> >:
> [...]
> 
> template
> <class TypeOut, class TypeIn>
>> boost::optional
> <TypeOut>
>  operator()(TypeIn const&); //#3
>>
>> surely seems attractive and with that interface converters can be easily
>> used on their own (if needed/preferred) bypassing "convert" api
>> altogether.
>>
>> Let's take a simple case of, say, the "direction" class from the docs
>> (which has no default constructor) and try converting it to std::string.
>> Say, we want that conversion possible using a few converters with
>> std::sstream-based being one of them. In that setting I say #1 has the
>> following advantages (no matter how ugly I personally find it):
> 
> [...]
> 
>> template&lt;TypeIn, TypeOut&gt;
>>
>  boost::optional
> <TypeOut>
>> basic_stringstream_converter::operator()(const TypeIn& in)
>> {
>>     boost::optional
> <TypeOut>
>  out = boost::make_optional(boost::
>> convert_default_maker
> <TypeOut>
> ::make());
>>
>>     stream_.str(std::basic_string< char_type>());
>>     stream_ << in; stream_ >> *out;
>>
>>     return out = stream_.fail() ? boost::optional
> <TypeOut>
>  : out;
>> }
>>
>> template&lt;TypeIn, TypeOut&gt;
>> boost::optional
> <TypeOut>
>> lexical_cast_converter::operator()(const TypeIn& in)
>> {
>>
>>     try
>>     {
>>         boost::optional
> <TypeOut>
>  out_value = boost::make_optional(boost::
>> convert_default_maker
> <TypeOut>
> ::make());
>>         *out_value = boost::lexical_cast
> <TypeOut>
> (in);
>>         return out;
>>     }
>>     catch (...)
>>     {
>>         return out = boost::optional
> <TypeOut>
> ;
>>     }
>> }
>>
> 
> Why would you make a default TypeOut to then assign to it, instead of:
> 
> template&lt;TypeIn, TypeOut&gt;
> boost::optional
> <TypeOut>
> lexical_cast_converter::operator()(const TypeIn& in)
> {
> 
>     try
>     {
>         return boost::lexical_cast
> <TypeOut>
> (in);
>     }
>     catch (...)
>     {
>         return boost::none;
>     }
> }

Indeed, typed in haste... embarrassing.

As for the

optional<TypeOut> operator()(TypeIn const&);

converter signature, then I am slowly warming up to it. If we declare
something like

namespace boost
{
    template<class T> create_default() { return T(); 
}

outside "convert", then, say, lexical_cast might use it as well and get rid
of DefaultConstructibily requirement as well. And, indeed, as Alex pointed
out sometimes we'll be able to avoid using it altogether

template<class TypeOut, class TypeIn>
boost::optional<TypeOut>
lexical_cast_converter::operator()(TypeIn const& in)
{
    try
    {
        return boost::optional<TypeOut>(boost::lexical_cast<TypeOut>(in));
    }
    catch (...)
    {
        return boost::optional<TypeOut>();
    }
}

Alex, does it make you happy? Could anyone comment on that please?

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