The problem with Instrument2.cpp can be seen by running the program. The output is Instrument::play. This is clearly not the desired output, because you happen to know that the object is actually a Wind and not just an Instrument. The call should produce Wind::play. For that matter, any object of a class derived from Instrument should have its version of play( ) used, regardless of the situation.

The behavior of Instrument2.cpp is not surprising, given C’s approach to functions. To understand the issues, you need to be aware of the concept of binding.