20 Memory management library [mem]
namespace std {
  class bad_weak_ptr : public exception {
  public:
    
    const char* what() const noexcept override;
  };
}
 An exception of type 
bad_weak_ptr is thrown by the 
shared_ptr
constructor taking a 
weak_ptr.const char* what() const noexcept override;
Returns: An 
implementation-defined 
ntbs.