const_iterator lower_bound(const key_type &x) const
Find the greatest key less than or equal to x.
iterator lower_bound(const key_type &x,
bool readonly=false)
Find the greatest key less than or equal to x.
The following functions are returning iterators, and they by default return read-write iterators.
If you intend to use the returned iterator only to read, you should call the const version of each function using a const reference to this container. Using const iterators can potentially promote concurrency a lot. You can also set the readonly parameter to each non-const version of the functions to true if you don't use the returned iterator to write, which also promotes concurrency and overall performance.