Get number of weak_ptr objects that point to resource

I am trying to create a custom cashing mechanism where I am returning a weak_ptr to the cache created. Internally, I hold a shared_ptr to control the lifetime of the object.

When the maximum cache pre-set is consumed, the disposer looks for those cache objects that are not accessed for a long time and will clean them up.

Unfortunately this may not be ideal. If it was possible to check how many cache objects can be accessed through the weak_ptr, then this can be a criteria for making the decision to clean up or not.

Turns out there is no way to check how many weak_ptr(s) have handle to the resource.

But when I look at the shared_ptr documentation and implementation notes

=> the number of weak_ptrs that refer to the managed object

is part of the implementation. Why is this not exposed through an API ?