[Boost-users] Re: swapping keys in multiindex

David Gruener writes:

Hello,

how can one swap keys in an *ordered* index of a multi index container? Well, you cannot say something like:

blah.modify_key(blah.find(foo1), _1 = foo2); // from now index contains one element fewer, since // keys must be uniq blah.modify_key(blah.find(foo2), _1 = foo1);

I dind't found a way in the manual to swap directly.

There's no particular provision for swapping keys. You could do something like: blah.modify_key(blah.find(foo1), _1 = unused_value); blah.modify_key(blah.find(foo2), _1 = foo1); blah.modify_key(blah.find(unused_value), _1 = foo2); Providing a general key_swap facility is not as simple as it might seem: swapping keys in an index can, in general, have side effects on other indices --for instance, on those indices that also depend on that particular key. Example: multi_index_container< int, indexed_by< ordered_unique, ordered_unique

Swapping keys in index #0 has also an impact on index #1. Best regards, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo