[TypeErasure] efficiency problem
Hello! In following code TypeErasure creates two copies of s, while Boost.Any creates no copies at all. any x{ s{} }; any y{ std::move(x) }; any z; z = std::move(y); There is move constructor for type_erasure::any, but it does allocate memory and calls copy constructor for s. Why? And why there is no move assignment operator in type_erasure::any? Thanks. Full source: https://wandbox.org/permlink/iHs5a0bjzXsBtpwC
Am 23.03.2017 um 06:55 schrieb Mikhail Strelnikov via Boost-users:
Hello!
In following code TypeErasure creates two copies of s, while Boost.Any creates no copies at all.
any x{ s{} }; any y{ std::move(x) }; any z; z = std::move(y);
There is move constructor for type_erasure::any, but it does allocate memory and calls copy constructor for s. Why?
I am not really sure, but I investigated this over a year ago and I think the problem was either that no real move-constructor was implemented or the wrong constructor overload was taken. (I think, taking the wrong overload happened at least in some of my personal test-cases back then. The constructor with the "universal reference" [1] very often better matched the inputs than some of the overloads.)
And why there is no move assignment operator in type_erasure::any?
A pull-request to Boost.TypeErasure exists [2] which adds support for move-assignment. (I wrote this about a year ago and we have been using it successfully in production-code at our company for over half a year now.) However, this pull-request seems to have been overlooked by the author of Boost.TypeErasure. If you do not mind using a patched version of Boost.TypeErasure you could apply this pull-request to your local version of Boost and use that. Of course, I would prefer if this pull-request (or some other pull-request adding move-assignment support) would make it into the official version of Boost.TypeErasure. Hope that helps, Deniz [1] https://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers [2] https://github.com/boostorg/type_erasure/pull/12
Thanks.
Full source: https://wandbox.org/permlink/iHs5a0bjzXsBtpwC _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users
I had to change the declaration of my any to:
using any = boost::type_erasure::any
;
And then I've tried your patch, it works, thanks.
But it solves only half of the problem - there is still memory
allocation happening every time type_erasure::any is move-constructed
or move-assigned (again, there are no extra memory allocations in this
case when Boost.Any is used).
2017-03-23 15:10 GMT+03:00 Deniz Bahadir via Boost-users
Am 23.03.2017 um 06:55 schrieb Mikhail Strelnikov via Boost-users:
Hello!
In following code TypeErasure creates two copies of s, while Boost.Any creates no copies at all.
any x{ s{} }; any y{ std::move(x) }; any z; z = std::move(y);
There is move constructor for type_erasure::any, but it does allocate memory and calls copy constructor for s. Why?
I am not really sure, but I investigated this over a year ago and I think the problem was either that no real move-constructor was implemented or the wrong constructor overload was taken. (I think, taking the wrong overload happened at least in some of my personal test-cases back then. The constructor with the "universal reference" [1] very often better matched the inputs than some of the overloads.)
And why there is no move assignment operator in type_erasure::any?
A pull-request to Boost.TypeErasure exists [2] which adds support for move-assignment. (I wrote this about a year ago and we have been using it successfully in production-code at our company for over half a year now.) However, this pull-request seems to have been overlooked by the author of Boost.TypeErasure.
If you do not mind using a patched version of Boost.TypeErasure you could apply this pull-request to your local version of Boost and use that. Of course, I would prefer if this pull-request (or some other pull-request adding move-assignment support) would make it into the official version of Boost.TypeErasure.
Hope that helps, Deniz
[1] https://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers [2] https://github.com/boostorg/type_erasure/pull/12
Thanks.
Full source: https://wandbox.org/permlink/iHs5a0bjzXsBtpwC _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users
_______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users
AMDG On 03/22/2017 11:55 PM, Mikhail Strelnikov via Boost-users wrote:
In following code TypeErasure creates two copies of s, while Boost.Any creates no copies at all.
any x{ s{} }; any y{ std::move(x) };
Boost.TypeErasure tries to be as faithful as possible
to normal overload resolution rules. As a result,
the move constructor should resolve as follows:
If constructible
any z; z = std::move(y);
There is move constructor for type_erasure::any, but it does allocate memory and calls copy constructor for s. Why?
And why there is no move assignment operator in type_erasure::any?
There will be once I get around to merging Deniz' PR. In Christ, Steven Watanabe
participants (3)
-
Deniz Bahadir -
Mikhail Strelnikov -
Steven Watanabe