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Hashed Container Capacity
As elements are inserted into the hashed container, the internal hash table automatically expands when it becomes full (defined as capacity = length). The standard does not specify what the load factor is. It says only that capacity is the maximum length before which no automatic rehashing will occur. You need to care about rehashing, because its expensive. If you know the total number of elements prior to insertion, use Reserve_Capacity to preallocate the buckets array, and thus avoid rehashing.