std::pmr::memory_resource::memory_resource
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                    < cpp | memory | memory resource
                    
                                                            
                    |   memory_resource() = default;  | 
(1) |  (since C++17)  (implicitly declared)  | 
|   memory_resource( const memory_resource& ) = default;  | 
(2) |  (since C++17)  (implicitly declared)  | 
1) Implicitly declared default constructor.
2) Implicitly declared copy constructor.