On Sep 12, 2012, at 11:16 AM, Andrey Semashev wrote:
On Wednesday 12 September 2012 07:31:34 Jeffrey Lee Hellrung, Jr. wrote:
Regarding the signature one instantiates result_of with, the present Boost.ResultOf documentation states that result_of< F ( T ) >::type is the result of f(t) where f is an *lvalue* of type F and t is an *lvalue* of type T. I've personally always found this needlessly limiting (precludes deducing strictly correct result types for rvalue arguments), and it appears to contradict what is expressed in this patch. I understand this patch as strictly an addition to the present documentation, so I wonder if it is appropriate to alter the existing Boost.ResultOf documentation text to reflect these "new" usage guidelines (specifically, distinguishing between lvalue and rvalue parameters).
Good catch, I missed that limitation in the formal result_of description. Indeed, with the current wording the meaning of result_of<f(int)>::type is unclear.
std::result_of<F(T)> is defined in terms of std::declval<T>() which effectively means that when T is not a lvalue reference, the argument is assumed to be rvalue. This is outlined in the guidelines and I think, the formal description should be updated accordingly, unless there are some background reasons not to.
The statement regarding the objects being lvalues is from the original documentation and should really only serve as an example these days. I changed that part of the documentation to refer to it as such; i.e. an example not a restrictive definition. I also added a reference to the more formal/explicit decltype(declval<f>()(declval<T>(),...)) expression. - Daniel