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().

Examples

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));