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Resolve handler requirements

A resolve handler must meet the requirements for a handler. A value h of a resolve handler class should work correctly in the expression h(ec, r), where ec is an lvalue of type const error_code and r is an lvalue of type const ip::basic_resolver_results<InternetProtocol>. InternetProtocol is the template parameter of the ip::basic_resolver<> which is used to initiate the asynchronous operation.

Examples

A free function as a resolve handler:

void resolve_handler(
    const boost::system::error_code& ec,
    boost::asio::ip::tcp::resolver::results_type results)
{
  ...
}

A resolve handler function object:

struct resolve_handler
{
  ...
  void operator()(
      const boost::system::error_code& ec,
      boost::asio::ip::tcp::resolver::results_type results)
  {
    ...
  }
  ...
};

A lambda as a resolve handler:

resolver.async_resolve(...,
    [](const boost::system::error_code& ec,
      boost::asio::ip::tcp::resolver::results_type results)
    {
      ...
    });

A non-static class member function adapted to a resolve handler using std::bind():

void my_class::resolve_handler(
    const boost::system::error_code& ec,
    boost::asio::ip::tcp::resolver::results_type results)
{
  ...
}
...
resolver.async_resolve(...,
    std::bind(&my_class::resolve_handler,
      this, std::placeholders::_1,
      std::placeholders::_2));

A non-static class member function adapted to a resolve handler using boost::bind():

void my_class::resolve_handler(
    const boost::system::error_code& ec,
    boost::asio::ip::tcp::resolver::results_type results)
{
  ...
}
...
resolver.async_resolve(...,
    boost::bind(&my_class::resolve_handler,
      this, boost::asio::placeholders::error,
      boost::asio::placeholders::results));

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