How to Create Calculated Internal Properties?

From: Phillip Ratzloff (pratzl_at_[hidden])
Date: 2002-10-09 23:58:23

I wish to create an internal calculated property that is derived from
existing properties in a vertex or edge.  I’d also like to be able to access
it like any other internal property.

For instance, I want a vertex property called weight_sum that would sum the
weights for all the out edges and return the answer.  I want to be able to
use a statement "get(vertex_weight_sum,G,v)" to get the value.  Later on, I
may decide that I want to cache the value on the vertex (like a normal
property) and existing code should work without change.

I have been unable to figure out any template class specializations to do
this.  Additionally, if I try to define the property using the property<>
template in the graph declaration it reserves storage for the property,
which is undesired.

Am I missing something?  Is there a way to define a calculated internal
property that doesn’t reserve storage?