On 13/03/2020 09:53, MM wrote:

I want a collection to store a relationship N to M like so:

Item of type A                 Item of type B A1                                    B1,B2,B3 A2                                    B4 A3                                    B1

and I would like a convenient way to get from a A instance to the list of B instances, and vice versa, from a given B instance to the list of A instances.

At runtime, the A objects are created first, then a list of B objects is created. Later, B objects get modified asynchronously and that modification need to update the relevant A instances

what container to use?

boost::bimap< unordered_set ,  unordered_set >

?

I don't think that'd work unless a B had some other means to find the set that includes it. A relatively simple method would be to have each B store a weak_ptr<A> of its "owner". Given that, there's a few different ways to model the relationship: 1. An external unordered_map> 2. An external unordered_set and each A has a unordered_set 3. One of the above but substitute vector for unordered_set if you do care about the order. (Which of these makes more sense depends on your intended ownership relations and other factors. Other things are also possible, of course.) Given something like this, you can explore the whole graph from any one node easily enough.