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@wanadoo.fr> 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