On Wed, Dec 2, 2009 at 10:33 AM, Howard Hinnant <howard.hinnant@gmail.com> wrote:
On Dec 1, 2009, at 1:15 PM, Michael Fawcett wrote:
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
I'm replying here, but thank you, Vicente, for your explanation as well. I'm trying to figure out what the best tool for the job is and it appears I have 3 choices, Typeof, common_type, and promote_args. Suppose I have a mixed type vector2 class with a magnitude_squared function defined as: template <typename X, typename Y> BOOST_TYPEOF_TPL(X() * X() + Y() * Y()) magnitude_squared(const vector2<X, Y> &v) { return v.x * v.x + v.y * v.y; } Typeof works, but I'm wondering if it's overkill and that perhaps common_type is much more lightweight and appropriate. promote_args doesn't do quite what I need here, but thank you for pointing that facility out to me. Regardless, the original post was about how to proceed, and I definitely see reason to break those components out of Boost.Chrono. I didn't mean to hijack the thread ;) --Michael Fawcett