[boost] Re: Two way maps/multimaps

From: Andy Glew (glew_at_[hidden])
Date: 1999-10-08 01:15:02

Oh, I forgot to mention: in my own work, 
my relations are usually not between scalar keys,
but are between objects, as in 

    multi_map<Producer*,Consumer*> forward_map;
    map<Consumer*,Producer*> reverse_map;

You can grossly encapsulate this, if you make the fairly
universal but nonetheless ugly assumption that
    map<void*,void*>
is interconvertible with
    map<Consumer*,Producer*>
with suitable casts
thereby avoiding the mutual recursion in the definition.

===

Also, I wasted a lot of time trying to make

    Left.rightptr = &Right

automatically update Right.leftptr, and vice versa.

In terms of your problem, that would be like saying
    
    map2way.forward_map[key] = value

automatically adding pair<ValueType,KeyType>(value,key)
to the reverse_map.

I eventually figured out how to do it, but it is kind of gross,
requiring what amounts to subtracting the offset of a member
from the member's address, to arrive at a pointer to the object
containing the member.  Works everywhere I have tried it,
but I don't think that it is guaranteed to be portable.

I think that it is better to define a linking operation

    reln_1_to_N::link( KeyType k, ValueType v )

and just use the array [ ] notation for lookup.