Boost C++ Application Development Cookbook - Second Edition

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The std::vector allocates bigger chunks of memory if a user inserts an element and it is impossible to fit the new value into the already allocated memory. In that case, std::vector moves elements from the old location to the new one if the elements are nothrow move constructible. Otherwise, std::vector copies elements to a new location and after that calls destructor for each element in the old location.

Because of that, behavior std::vector has amortized constant complexity for many member functions. The static_vector never allocates memory so that it does not have to move or copy elements from an old location to a new one. Because of that, operations that have amortized O(1) complexity for std::vector have true O(1) complexity ...