... it doesn't matter if the change is made inside the modify function as long as modify is called.

(So, this statement from above does not appear strictly true: "Key modification has to be done via modify() so as to give the container a chance to reindex elements.")

Because (in my case) the change is made elsewhere, I am calling modify with

a functor that does nothing.

I guess more generally, why is this paradigm implictly disallowed? index_itor->value++; index.update_indices( index_itor ); While I can see the benefit of tying the modification to the multi-index object via modify, there is no explicit reason that I can see why the above is invalid (as per my perfectly working code that uses this paradigm). And "forcing" the user to use a functor to modify can introduce annoying code overhead. Don't get me wrong, I think that using modify with a functor is very clear and helps prevent invalid indices. But, at the same time, it can be a pain to write a functor. (Here's to C++0x lambda's). But in my case, where the code that modifies the object is unaware of multi-indices on that object, it is not feasible to use modify() to make the change. Neil