Re: [boost] [chrono] type_traits/common_type and integer/ratio

Subject: Re: [boost] [chrono] type_traits/common_type and integer/ratio
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2009-12-02 10:33:57

On Dec 1, 2009, at 1:15 PM, Michael Fawcett wrote:

> On Mon, Nov 30, 2009 at 7:25 AM, Vicente Botet Escriba
> <vicente.botet_at_[hidden]> wrote:
>> 
>> "In a nutshell, `common_type` is a trait that takes 1 or more types, and
>> returns a type which all of the types will convert to. The default
>> definition demands this conversion be implicit. However the trait can be
>> specialized for user-defined types which want to limit their inter-type
>> conversions to explicit, and yet still want to interoperate with the
>> `common_type` facility.
>> 
>> Example:
>> 
>>    template <class T, class U>
>>    typename common_type<complex<T>, complex<U> >::type
>>    operator+(complex<T>, complex<U>);
>> 
>> In the above example, "mixed-mode" complex arithmetic is allowed. The return
>> type is described by `common_type`. For example the resulting type of adding
>> a `complex<int>` and `complex<double>` might be a `complex<double>`."
> 
> I've needed this feature many times.  Can you explain the pros/cons of
> common_type against Boost.Typeof?

common_type is closer in nature to promote_args<class ...T> in boost/math/tools/promotion.hpp than it is to Boost.Typeof, though it is not exactly the same as promote_args either.  common_type<T1, T2>::type simply represents the result of some operation on T1 and T2, and defaults to the type obtained by putting T1 and T2 into a conditional statement.

It is meant to be customizable (via specialization) if this default is not appropriate.  Here is a link to the D version of the same tool:

http://www.digitalmars.com/d/2.0/phobos/std_traits.html#CommonType

-Howard