Unit testing large non-member functions

Question

Hey all,

I have a function, which I'd like to get under test. However, it's very large and creates a finite state machine. The function looks roughly like:

std::auto_ptr<IFiniteStateMachine> CreateFSM() 
{
    std::auto_ptr<IFiniteStateMachine> = apFSM( new CFiniteStateMachine() );

    std::list<IState*> listStates;

    listState.push_back( new CStateFoo( /*params*/ ) );
    listState.push_back( new CStateBar( /*params*/ ) );

    // etc.

    apFSM->AddState( listStates );

    // Define the transition table
    // 1st param: event received
    // 2nd param: current state
    // 3rd param: next state
    apFSM->AddTransition( "EventA", "StateFoo", "StateBar" );

    // etc.

    return apFSM;
}

Basically, this function just creates a FSM, states, and transition tables. Each of the individual components (the IFiniteStateMachine and IState classes) are testable.

So my question is: can a function like this be put under test? I'm not sure how you would even verify that this function worked as expected. I could probably divide it up into smaller functions that create states and transitions. However, a function that just created these states also seems rather complicated and hard to test.

Thanks.

Answer 1

Ask yourself what should be true after the function has run? Sounds like in this case a certain state machine should be in place - so that's what you should check for so you can do something like:

FSM expectedFSM = Factory.blankFSM().addState().addTransition().... etc
FSM actualFSM = CreateFSM();
ASSERT(expectedFSM == actualFSM); //assuming such == exists

or sometimes an easier way of doing this could be using string represntation/serialization:

String expectedFSMStr = loadFSMFromFile(expectedOutput1);
FSM actualFSM = CreateFSM();
ASSERT_EQUALS(expectedFSMStr, actualFSM.toString());

This can be easier to debug and is very useful if your classes have a reliable toStrinb/serialization method.