* \brief holds a vector of std::pair of Ptr<Ipv4> and interface index.

 * Typically ns-3 Ipv4Interfaces are installed on devices using an Ipv4 address

 * helper.  The helper's Assign() method takes a NetDeviceContainer which holds 

 * some number of Ptr<NetDevice>.  For each of the NetDevices in the 

 * NetDeviceContainer the helper will find the associated Ptr<Node> and

 * Ptr<Ipv4>.  It makes sure that an interface exists on the node for the 

 * device and then adds an Ipv4Address according to the address helper settings

 * (incrementing the Ipv4Address somehow as it goes).  The helper then converts

 * the Ptr<Ipv4> and the interface index to a std::pair and adds them to a 

 * container -- a container of this type.

 *

 * The point is then to be able to implicitly associate an index into the 

 * original NetDeviceContainer (that identifies a particular net device) with

 * an identical index into the Ipv4InterfaceContainer that has a std::pair with

 * the Ptr<Ipv4> and interface index you need to play with the interface.

 *

 * @see Ipv4AddressHelper

 * @see Ipv4

   * \returns the number of Ptr<Ipv4> and interface pairs stored in this 

   * Ipv4InterfaceContainer.

   * Manually add an entry to the container consisting of the individual parts

   * of an entry std::pair.

   *

   * \param ipv4 pointer to Ipv4 object

   * \param interface interface index of the Ipv4Interface to add to the container

   *

   * @see Ipv4InterfaceContainer

   */

  void Add (Ptr<Ipv4> ipv4, uint32_t interface);

  /**

   * Manually add an entry to the container consisting of a previously composed 

   * entry std::pair.

   *

   *

   * @see Ipv4InterfaceContainer

  /**

   * Manually add an entry to the container consisting of the individual parts

   * of an entry std::pair.

   *

   * \param ipv4Name std:string referring to the saved name of an Ipv4 Object that

   *        has been previously named using the Object Name Service.

   * \param interface interface index of the Ipv4Interface to add to the container

   *

   * @see Ipv4InterfaceContainer

   */

  /**

   * Get the std::pair of an Ptr<Ipv4> and interface stored at the location 

   * specified by the index.

   *

   * \param i the index of the entery to retrieve.

   *

   * @see Ipv4InterfaceContainer

   */

  std::pair<Ptr<Ipv4>, uint32_t> Get (uint32_t i) const;