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Subject: Re: [boost] [chrono] type_traits/common_type and integer/ratio
From: Vicente Botet Escriba (vicente.botet_at_[hidden])
Date: 2009-12-01 22:20:32

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?
> Thanks,
> --Michael Fawcett
> _______________________________________________
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Happy to hear you could find a use for this class.

Even if in a first look they sound to be close, common_type and typeof have
different purposes. You use typeof to get the type of an expression, while
you use common_type to set explicitly the type returned of a template
function. Both are complementary. For example with the preceding declaration
and the needed Typeof registrations, you can do

  complex<int> ic; complex<double> id;

  BOOST_AUTO(icd, ic+id);

Surely others would give a better explanation,


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