id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
11132,Boost.Variant's boost::recursive_wrapper missing noexcept specifier on move constructor,peterhuene@…,Antony Polukhin,"Using a sequence container of recursive variant, like std::vector<recursive_variant_>, results in the variant type not being movable.

Example:
{{{
#include <boost/variant.hpp>
#include <vector>

using namespace std;
using namespace boost;

struct foo
{
	foo() = default;
	foo(foo&&) noexcept = default;
	foo(foo const&) = delete;
};

int main() {
	typedef make_recursive_variant<
		foo,
		vector<recursive_variant_>
	>::type variant_type;

	vector<variant_type> value;
	value.emplace_back();
	variant_type other = std::move(value);
}
}}}

This errors at the move assignment because variant_type is not movable.  The reason it is not movable is because boost::recursive_wrapper's move constructor is not marked as noexcept, which is a requirement for vector to enable move semantics.

Currently boost::recursive_wrapper's move constructor is not marked noexcept because it allocates a new T when being moved.

I propose that boost::recursive_wrapper be changed to store a shared_ptr<T> instead of T* so that, when moved, we move the shared_ptr, which is a noexcept operation.  For the overload that constructs by T&&, we can leave as not specifying noexcept and construct with:

{{{
_p(new T(detail::variant::move(operand)))
}}}

The copy semantics of boost::recursive_wrapper can be left as-is.",Bugs,closed,To Be Determined,variant,Boost 1.57.0,Problem,wontfix,,