15.5 Add subclass

The trivial base class ErrorModel does not do anything interesting, but it provides a useful base class interface (Corrupt () and Reset ()), forwarded to virtual functions that can be subclassed. Let's next consider what we call a BasicErrorModel which is based on the ns-2 ErrorModel class (in ns-2/queue/errmodel.{cc,h}).

What properties do we want this to have, from a user interface perspective? We would like for the user to be able to trivially swap out the type of ErrorModel used in the NetDevice. We would also like the capability to set configurable parameters.

Here are a few simple requirements we will consider:

• Ability to set the random variable that governs the losses (default is UniformVariable)
• Ability to set the unit (bit, byte, packet, time) of granularity over which errors are applied.
• Ability to set the rate of errors (e.g. 10^-3) corresponding to the above unit of granularity.
• Ability to enable/disable (default is enabled)

15.5.1 How to subclass

We declare BasicErrorModel to be a subclass of ErrorModel as follows,

class BasicErrorModel : public ErrorModel
{
public:
  static TypeId GetTypeId (void);
  ...
private:
  // Implement base class pure virtual functions
  virtual bool DoCorrupt (Ptr<Packet> p);
  virtual bool DoReset (void);
  ...
}

and configure the subclass GetTypeId function by setting a unique TypeId string and setting the Parent to ErrorModel:

TypeId RateErrorModel::GetTypeId (void)
{
  static TypeId tid = TypeId ("ns3::RateErrorModel")
    .SetParent<ErrorModel> ()
    .AddConstructor<RateErrorModel> ()
  ...

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