void reserve( size_type new_cap );
Increase the capacity of the string to a value that's greater or equal to
new_cap is greater than the current capacity(), new storage is allocated, otherwise the method does nothing.
new_cap is greater than capacity(), all iterators and references, including the past-the-end iterator, are invalidated. Otherwise, no iterators or references are invalidated.
|new_cap||-||new capacity of the string|
 Return value
At most linear in the size() of the string
reserve() cannot be used to reduce the capacity of the container; to that end shrink_to_fit() is provided.
 See also
| returns the number of characters that can be held in currently allocated storage |
(public member function)