Re: [boost] [chrono] [dependence of common_type on Boost.TypeOf]

Subject: Re: [boost] [chrono] [dependence of common_type on Boost.TypeOf]
From: vicente.botet (vicente.botet_at_[hidden])
Date: 2010-08-16 03:29:11

----- Original Message ----- 
From: "Adam Merz" <adammerz_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Monday, August 16, 2010 7:23 AM
Subject: Re: [boost][chrono] [dependence of common_type on Boost.TypeOf]

> 
> vicente.botet <vicente.botet <at> wanadoo.fr> writes:
>> ----- Original Message ----- 
>> From: "Adam Merz" <adammerz <at> hotmail.com>
>> To: <boost <at> lists.boost.org>
>> Sent: Sunday, August 15, 2010 9:44 AM
>> Subject: Re: [boost][chrono] [dependence of common_type on Boost.TypeOf]
>> 
>> > 
>> > vicente.botet <vicente.botet <at> wanadoo.fr> writes:
>> >> Hi again,
>> >> I have reached to manage with the ambiguous overloads and all the tests
>> >> works as before.
>> >> I have do it as follows:
>> > <snip code>
>> > 
>> > That doesn't seem very robust in the case that there is no common type (in
>> > terms of the compiler error anyway). 
>> 
>> Could you send the compiler error so we can talk of concrete things?
>> 
>> Thanks,
>> Vicente
> 
> Really? You were just speaking of ambiguity errors yourself...
> 
> Okay then, two issues I see with a sizeof-based approach as you've shown it:
> First, it requires that both types be default constructable; otherwise, you get
> errors like the following (using MSVC10):
> 
> 1>error C2512: 'Bar::Bar' : no appropriate default constructor available
> 1>  see reference to class template instantiation 'common_type2<T1,T2>' being
> 1>      compiled
> 1>  with
> 1>  [
> 1>    T1=Foo,
> 1>    T2=Bar
> 1>  ]
> 
> This can of course be worked around by working in terms of pointers to the
> provided types rather than the types themselves. Second, if there is no
> common type, you get potentially confusing ambiguity errors:

I will take care of this.

> Or, if you make the aforementioned pointer changes, you get the even more
> confusing:
> 
> 1>error C2446: ':' : no conversion from 'Baz *' to 'Foo *'
> 1>  Types pointed to are unrelated; conversion requires reinterpret_cast,
> 1>      C-style cast or function-style cast
> 1>  see reference to class template instantiation 'common_type2<T1,T2>' being
> 1>      compiled
> 1>  with
> 1>  [
> 1>    T1=Foo,
> 1>    T2=Baz
> 1>  ]
> 1>error C2070: ''unknown-type'': illegal sizeof operand

I find the  message no so confusing:  "Types pointed to are unrelated; conversion requires reinterpret_cast,  C-style cast or function-style cast" and common_type2<T1,T2> appears also in the message. 

What others think?

Best,
Viecnte