The cleaner is a functor called when an item is removed from a container.
Note, the cleaner should not delete (deallocate) the value \p val passed in.
However, if the \p value_type type is a structure that contains dynamically allocated
- field(s), the cleaning functor may deallocate it and iniitalize to default value (usually, \p NULL).
+ field(s), the cleaning functor may deallocate it and iniitalize to default value (usually, \p nullptr).
The interface for type \p value_type is:
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