Memory usage for each instance

[rmolinari]

Thanks so much for this fascinating code!

I am suggesting a function that looks like this:

size_t memory_usage( <any container type> *cntr );

It would return the amount of (heap) memory associated with cntr.

Use case: when I am writing extension code for Ruby in C, the Ruby runtime likes to know how much memory is associated with a given object (although it doesn't insist on this knowledge). This helps with profiling and (I suspect) the Ruby garbage collector.