# Size and Capacity

The following table provides additional information on the members of this template class.
Member | Description
--- | ---
bool empty() const | Returns: `size()!=0.`
size_type size() const | Returns: Number of items in `*this`. ### Caution Though the current implementation takes time `O(1)`, possible future implementations might take time `O(P)`, where `P` is the number of hardware threads.
size_type max_size() const | Returns: Upper bound on number of items that `*this` can hold. ### Caution The upper bound may be much higher than what the container can actually hold.

Parent topic: concurrent_unordered_set and concurrent_unordered_multiset Template Classes