How to use a C++ class with pure virtual function? - interface

Question

I was given a class with pure virtual function like the following:
class IRecordingHour{
public:
virtual int getData() const = 0;
}
Now, I have another class that uses the IRecordingHour class:
class ProcessRecordingHours {
public:
ProcessRecordingHours (IRecordingHour &);
proteted:
IRecordingHour & recordingHour;
}
I was told that I am not allowed to implement the IRecordingHour class (the one with the pure virtual function).
My question is: without implementing the IRecordingHour clas, how do I use it in the ProcessingRecordingHours? That is, how do I create an instance of the IRecordingHour and pass it to the constructor of the ProcessRecordingHours?

Answer 1

You should create a subclass of IRecordingHour and implement the method getData, like
class ARecordingHour : public IRecordingHour
{
public:
int getData() const override //override is valid from C++11
{
return 42;
}
}
And then you can do:
ARecordingHour arh{};
ProcessRecordingHours prh{arh}; //{}- Modern C++ initialization
You can find similar examples in a good C++ programming book, such as The C++ Programming Language

Answer 2

Though you equate them, your two questions are in fact quite different.
how do I use it in the ProcessingRecordingHours?
There is no particular problem in implementing ProcessingRecordingHours without implementing a subclass of IRecordingHour. You don't need to do anything special. You simply avoid relying on anything not declared by IRecordingHour, which is no different than you ought to do anyway.
how do I create an instance of the IRecordingHour and pass it to the constructor of the ProcessRecordingHours?
You cannot. A class with a pure virtual method cannot be directly instantiated. Your ProcessRecordingHours can be used in conjunction with classes that extend IRecordingHour, but if you are not permitted to create such a class then you cannot exercise those parts of your ProcessRecordingHours class that depend on an IRecordingHour.
Perhaps, however, you have misunderstood the problem. You may be forbidden from implementing IRecordingHour::getData(), but not from implementing a subclass that overrides that method with a concrete one. Such a subclass could be instantiable, and would be usable in conjunction with ProcessRecordingHours.

Answer 3

I think your teacher plan to inject an implementation of IRecordingHour into ProcessRecordingHours.
Also, you can't use that class unless you generate a stub for IRecordingHour or you implement one yourself with some dummy return values.
/// <summary>
/// in C# and interface can only contain virtual methods. so no need to say virtual.
/// </summary>
interface IRecordingHour
{
int getData();
}
class MockRecordingHour : IRecordingHour
{
public int getData()
{
//just return something. This will be enough to get ProcessRecordingHours to work
return 100;
}
}
/// <summary>
/// this class expects a IRecordingHour.
///
/// how we get a IRecordingHour depends on who's implementing it. You, some 3rd party vendor or another developer who's using this class that you've written.
///
/// Oh wait. Since you're building ProcessRecordingHours, you need a IRecordingHour to get against. You can use a mocking tool or create one yourself that returns some dummy data
/// </summary>
class ProcessRecordingHours
{
private IRecordingHour _recording;
public ProcessRecordingHours(IRecordingHour recording)
{
this._recording = recording;
}
public void DoSomething() {
Console.WriteLine("Recording Data: {0}", this._recording.getData());
}
}