Increase the number of buckets of this unordered multimap to a quantity such that the ratio between the specified numElements and the new number of buckets does not exceed max_load_factor. Note that this guarantees that, after the reserve, elements can be inserted to grow the container to size() == numElements without rehashing. Also note that memory allocations may still occur when growing the container to size() == numElements. Also note that this operation has no effect if numElements <= size().