value_initialized should have its own swap functions

[niels_dekker]

For a given type T, the current (1.36.0) version of boost::value_initialized<T> is only Swappable if it is both CopyConstructible and CopyAssignable. IMO, value_initialized<T> should be Swappable whenever T is Swappable. Moreover, when swapping value_initialized<T> objects, the custom swap function of T should be used, whenever available.

I'd like to propose resolving this issue by adding a swap member function to value_initialized<T>, and a non-member swap that calls the member. (As recommended by Scott Meyers, Effective C++ Third Edition, Item 25: Consider support for a non-throwing swap.)

I would like to have value_initialized<T>::swap calling boost::swap to swap its data, using the ​boost::swap utility by Joseph Gauterin et al, that was recently added to the trunk. The boost::swap utility internally uses argument-dependent lookup (ADL) to find the custom swap function of T, if available. Thereby it works around various compiler version specific bugs regarding ADL, as tested at utility-swap_.html. Moreover, it supports swapping arrays (as requested by ticket #2056), so value_initialized<T>::swap would also support having T as a built-in array type.

Please take a look at the attached patch.

[niels_dekker]

Assigned to Fernando Cacciola. Fernando, I wouldn't mind committing the patch myself, but only if you think it's okay :-)

[niels_dekker]

I've just committed the patch to the trunk: changeset [48424] and [48425]. Please note that before merging this to the release branch, ​boost/utility/swap.hpp should be added to the release branch.

[Daniel James]

THis was merged by niels_dekker to release in [49967].