std::vector::reserve
From cppreference.com
                    
                                        
                    
                    
                                                            
                    
|   void reserve( size_type size ); 
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Sets the capacity of the container to at least size. New storage is allocated if necessary. 
If size() is greater than capacity(), all iterators and references, including the past-the-end iterator, are invalidated. Otherwise, no iterators or references are invalidated.
Contents | 
[edit] Parameters
| size | - | new capacity of the container | 
[edit] Return value
(none)
[edit] Exceptions
std::length_error if size > max_size().
[edit] Complexity
Linear in the size of the container
[edit] See also
|    returns the number of elements that can be held in currently allocated storage   (public member function)  | 
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