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        A legacy completion handler must meet the requirements for a handler.
        A legacy completion handler must meet the requirements of CopyConstructible
        types (C++Std [copyconstructible]). A value h
        of a completion handler class should work correctly in the expression h().
      
A free function as a completion handler:
void completion_handler() { ... }
A completion handler function object:
struct completion_handler { ... void operator()() { ... } ... };
A lambda as a completion handler:
my_io_service.post( []() { ... });
        A non-static class member function adapted to a completion handler using
        std::bind():
      
void my_class::completion_handler() { ... } ... my_io_service.post(std::bind(&my_class::completion_handler, this));
        A non-static class member function adapted to a completion handler using
        boost::bind():
      
void my_class::completion_handler() { ... } ... my_io_service.post(boost::bind(&my_class::completion_handler, this));