There isn't anything necessarily wrong with this. The
flush base class interface can simply define that ambiguity as part of its contract.
We can clearly specify it in the documented requirements that
produce is invoked, it is required to prepare the output in a buffer. The
flush method passes on the buffered output, causing the buffer to be empty. Moreover, implementations of produce may pass on the output, as if they implicitly called flush before returning, in which case the implementation of
flush can be a null operation.
The software which invokes this functionality has to always call
produce is not required to call it.
The key ideas here are:
The specification of an object behavior can be given in terms of an interface contract that spans multiple methods; in effect a protocol. The substitution principle is obeyed if every implementation of the object satisfies the protocol.
From the point of view of the object system itself, it's enough that the object has a compatible flush method. That is to say, we would instantly have an OOP-level substitutability problem if the object whose
flush is a no-op simply omitted implementing that function. Or, say, erring in the other direction: suppose some object has a
flush but one that requires an argument, contrary to the base class specification which specifies a no-argument function. You've satisfied the most basic requirement for substituability by ensuring that all the derived objects have all the required methods, in a way that is call compatible.
It boils down to supporting all the required methods (basic substitutability at the OOP system level so the program builds and runs at all), and furthermore that they behave according to their documented protocol (rules of the interface contract spanning multiple calls to multiple methods). The protocol can specify deliberate latitudes about which method takes care of certain responsibilities, and those choices will be visible to a client which deviates from the protocol. (E.g. expecting that
produce generates output instantly without calling
flush, because it worked in one case with one kind of object.)
If we step back to the design though, it is probably best to avoid too many latitudes like this; there has to be a good reason for allowing certain flexibilities, such as that it is significantly easier for certain variations of the object to be implemented, or there is some useful efficiency gain.
If there is leeway about which methods do what, it creates challenges for unit testing. In this specific case, we cannot have a generic test of the
flush function that works for objects of all types. We would like our test to call
produce and then have our test case verify that no output has taken place, and then call
flush, and check that the output was produced, and it would be nice if that worked the same way for every object. If it doesn't, then either we cannot have that kind of test, or the test has to "just know" that certain objects flush inside
produce, and others don't, and kind of work outside of the official protocol. Once the test works outside of the protocol, then it's becoming coupled to the implementation details.