I currently intercept calls to malloc/calloc/realloc/... and capture stack traces. This way I know how much memory gets allocated for each allocation site. Since allocations usually go through constructor calls, the presence of a constructor in the stack trace can let you infer how many structures of a given type are been allocated. Knowing how big they are is more tricky since allocation for the whole struct and its parts doesn't have to happen entirely in the constructor (some structures like vectors and hashmaps can grow, some structures can collect data from other sources and then hold onto them, etc)

Furthermore to know how the live memory is broken down between object type and allocation sites, you also need to track freed memory. This is significantly more tricky to do efficiently. I currently take an allocation sample every N bytes being allocated and use a poisson process estimator to scale the total allocated bytes.

The only ways I know to account for in use memory is to track every single allocation or to add some extra space for every allocation where we record whether a block had been sampled and of yes, what was its corresponding allocation event.

Can you please elaborate more on your suggestion?