Re: [boost] RE [Boost-announce] [Review] Boost.Type Traits Extension by Frederic Bron

Subject: Re: [boost] RE [Boost-announce] [Review] Boost.Type Traits Extension by Frederic Bron
From: Vicente Botet (vicente.botet_at_[hidden])
Date: 2011-03-15 16:38:20

Jeffrey Lee Hellrung, Jr. wrote:
> 
> On 3/15/2011 7:28 AM, Vicente Botet wrote:
>>
>> Jeffrey Lee Hellrung, Jr. wrote:
>>>
>>> On 3/15/2011 3:00 AM, Vicente Botet wrote:
>>>>
>>>> Frédéric Bron wrote:
>>>>>
>>>>>
>>>>>> For example:
>>>>>> has_operator_unary_minus<   class RHS, class RET=RHS>
>>>>>> has_operator_minus_assign<   class LHS, class RHS=LHS, class RET=LHS>
>>>>>> This would take care of most of the normal use cases.
>>>>>
>>>>> I think it is best to have the default behavior to forget about the
>>>>> return type. Why would you impose return type to be LHS?
>>>>> It is fine for +, -, ... when RHS=LHS but when you start to deal with
>>>>> different types, I think it just adds confusion.
>>>>> has_operator_plus&lt;double, int&gt; would then check for a return
>>>>> type
>>>>> "double" while has_operator_plus&lt;int, double&gt; for a return type
>>>>> int.
>>>>> You would break the symmetry of these operators.
>>>>>
>>>>
>>>> User wanting symmetry could use common_type&lt;LHS,RHS&gt; in this
>>>> case.
>>>> But
>>>> using it as default is not good idea as common_type is not always
>>>> defined.
>>>> So I agree to change the default behavior to don't check.
>>>
>>> When is common_type not defined?  I see no documentation in
>>> Boost.TypeTraits.CommonType mentioning it not being available for
>>> certain compilers...
>>>
>>
>> I was not talking about compiler restrictions, but of non existence of a
>> common type to two unrelated classes or I'm yet missing something.
>>
>> Given
>>
>> class A{};
>> class B{};
>>
>> isn't common_type&lt;A,B&gt;::type undefined?
> 
> Oh, of course, I should have guessed that's what you meant...my apologies
> :/
> 
> One could, instead of using common_type directly, use a variant, e.g., 
> common_type_or_void, that evaluates to void if A and B have no common 
> type.  I don't know if such a metafunction is possible or worth it, 
> though...
> 
> 

Yes, this could be a good default, but we don't have it yet :(

Vicente

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