Re: [boost] [Conversion] Practical applications

Subject: Re: [boost] [Conversion] Practical applications
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2011-08-23 14:07:38

Le 23/08/11 19:05, Phil Endecott a écrit :
> Hi Vicente,
>
> Vicente J. Botet Escriba wrote:
>> Le 22/08/11 22:50, Phil Endecott a ?crit :
>>> Vicente J. Botet Escriba wrote:
>>>> If we can construct implicitly a pair from another pair of 
>>>> implicitly convertible types, why we couldn't allow the conversion 
>>>> from a pair of type that are extrinsically implicitly convertible 
>>>> to them?
>
>>> Right.  But does your library make this possible?
>
>> The specialization of the customization point implicit_convert_cp 
>> allows to convert 'implicitly' pairs of extrinsicaly convertible 
>> types using the implicit_convert_to function. Now the user ca use the 
>> explicit_convert_to function to convert these types as for example
>>
>>      B1 b1;
>>      B2 b2;
>>      std::pair<B1,B2> b;
>>      std::pair<A1,A2> 
>> a1(boost::conversion::explicit_convert_to<std::pair<A1,A2> >(b));
>>
>> Note that implicit convertible implies explicit convertible.
>>
>> I wrote implicitly with coutes because we need to use a function 
>> which makes the call explicit.
>
>> Resuming, the user wanting extrinsically implicit conversions, must 
>> use explicitly the 'implicitly' function.
>
> I was really hoping that I had misunderstood something and that 
> actually your library could make it possible to have extrinsic 
> implicit conversions.  But it doesn't; the conversions still require 
> additional syntax.  So they aren't implicit conversions.
>
Strinctly speaking not. But implicitly(s) is extrinsicaly implicitly 
convertible. It is something similar as how ref(x) is not a reference to 
x, but can be used to transport a reference, which is enough useful.

> Since you can't add this additional syntax to e.g. the std:: 
> algorithms, I am still struggling to see how the library is useful.
>
We can add them whereever we consider is useful to Boost.Containers (of 
course the last word is to Ion). Note that we can define also conversion 
between containers extrinsically.

Best,
Vicente