# Size and capacity

The following table provides additional information on the members of the `concurrent_unordered_map` and `concurrent_unordered_map` template classes.
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_map and concurrent_unordered_multimap Template Classes