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Capacity vs. Length
•The function Capacity returns the total amount of internal storage.  A vector automatically increases the capacity during insertion, when the current length (number of elements) equals the current capacity. •Reserve_Capacity tells the vector to preallocate a specified amount of internal storage.  If you know the total number of elements in advance of insertion, it’s more efficient to reserve the necessary capacity, since the expansion of the internal array is done only once.