digraph g
{
  graph
    [
      nodesep="0.6"
    ];

  edge
    [
      fontname="Helvetica",
      fontsize=10,
      labelfontname="Helvetica",
      labelfontsize=10
    ];

  node
    [
      fontname="Helvetica",
      fontsize=10,
      shape=box
    ];

  edge
    [
      arrowhead="open"
    ]

  // Program elements.
  {
    operating_system [ label="Operating System", shape=ellipse ];
    io_service [ label="io_service" ];
    io_object [ label="I/O Object\ne.g. socket" ];
    your_program [ label="Your Program" ];
  }

  // Owning relationships.
  {
    edge [ arrowtail="diamond" ];
    your_program:w -> io_object:nw;
    your_program:se -> io_service:ne;
  }

  // Non-owning relationships;
  {
    io_object:sw -> io_service:w;
  }

  // Actions.
  {
    edge [ style="dashed", color="#808080" ];

    // Forward actions.
    {
      your_program:sw -> io_object:n [ label="1" ];
      io_object:s -> io_service:nw [ label="2" ];
      io_service:sw -> operating_system:nw [ label="3" ];
    }

    // Reverse actions.
    {
      edge [ arrowhead="none", arrowtail="open" ];
      io_service:se -> operating_system:ne [ label="4" ];
      io_object:se -> io_service:n [ label="5" ];
      your_program:s -> io_object:ne [ label="6" ];
    }
  }
}