Boost C++ Libraries: Ticket #5096: display_expr is noncopyable

status changed; resolution set

Eric Niebler — Thu, 20 Jan 2011 03:09:24 GMT

status new → closed
resolution → invalid

It's noncopyable by design. This function object maintains state. Using copies will give erratic results.

resolution changed

Eric Niebler — Thu, 20 Jan 2011 04:06:24 GMT

resolution invalid → fixed

(In [68314]) display_expr is copyable, fixes #5096

Christopher Schmidt — Sat, 22 Jan 2011 21:13:56 GMT

Strictly speaking, [68314] defers the problem to detail::display_expr_impl, which is passed to fusion::for_each in display_expr_impl::impl although it is noncopyable. This is fine for now as Fusion passes around functors by const reference although the documentation states the opposite.

https://github.com/ryppl/boost-svn/blob/fad63fe4389e8a46e8ea9183eae46e401ea4bb6f/boost/fusion/algorithm/iteration/for_each.hpp http://www.boost.org/doc/libs/1_45_0/libs/fusion/doc/html/fusion/algorithm/iteration/functions/for_each.html

AFAIK Fusion's current implementation is plain wrong. Functors should never be passed around by const reference. IIRC that's due to a defect in C++98.

Anyway, wontfix/invalid is fine for me for now. I will try to propose a patch for fusion that removes const/reference qualifiers on functors and adds support for reference wrappers.

Eric Niebler — Mon, 24 Jan 2011 04:34:13 GMT

Ah, I see. I overlooked that fact that display_expr_impl was invoking fusion::for_each. Well, that's an internal detail of display_expr and can be easily changed. At least my change benefits others who want to pass disply_expr around by value.