Example:
Ostrich. If class
Bird defines the virtual function
Fly and you derive a new class
Ostrich (a notoriously flightless bird) from
Bird , how do you implement
Ostrich::Fly ? The answer is, "It depends." If
Bird::Fly guarantees success (i.e., provides the no-fail guarantee; see Item 71) because flying is an essential part of the
Bird model, then
Ostrich is not an adequate implementation of that model.