If I initialize an io_service object and a socket object in my header file, how do I pass io_service to the socket as a parameter in my implementation file?

Take this header file as an example:

// foo.h

#include <boost/asio.hpp>

class foo
{
public:

    foo(); // Constructor.

private:

    boost::asio::io_service ios;

    boost::asio::ip::udp::socket sock;
};

And the corresponding implementation file:

// foo.cc

#include "foo.h"

foo::foo()
{
sock(ios); // <-- This throws a compiler error.
}

Why doing sock(ios); doesn't work?

What is the proper way to do it?

Thank you.

Álvaro