We've talked about this before. Most of the OpenVDB points tools currently assume that the Descriptor is shared and we don't even check to make sure.

I propose we improve this by introducing points::isDescriptorShared() and points::checkDescriptorIsShared() with the latter looking something like this:

template <typename TreeT>
void checkDescriptorIsShared(const TreeT& tree)
{
    // checks descriptorPtr.use_count() >= tree.leafCount()
    if (!points::isDescriptorShared(tree, /*strict=*/false)) {
        OPENVDB_THROW(RuntimeError, "This method requires a shared Descriptor.");
    }
    // checks leaf node pointers when asserts enabled       
    assert(points::isDescriptorShared(tree, /*strict=*/true));
}

The strict check will make sure that all the pointers are the same and the fast check will simply test the reference count.