void reserve(size_type n);
If n > capacity() will attempt to acquire a capacity() greater to or equal to n, an exception is thrown on failure. The precise type of the exception thrown is dictated by the allocator. The default allocator will throw a std::bad_alloc on failure. If an exception is thrown, there are no effects. If n <= capacity() then there are no effects.
Postcondition: capacity() >= n.