Using non-abstract class as base

Question

Hi,

I need to finish others developer work but problem is that he started in different way... So now I found in situation to use existing code where he chooses to inherit a non-abstract class (very big class, without any virtual functions) that already implements bunch of interfaces or to dismiss that code (which shouldn't be to much work) and to write another class that implements interfaces I need.

What are the pros and cons that would help me to choose the better approach.

p.s. please note that I don't have to much experience

Many Thanks

Answer 1

From you question it is not too clear what the problem is - looking at the title (Using non-abstract class as base) I can tell you that using an abstract class (non pure virtual - when you talk about interfaces in C++ I am assuming pure virtual abstract classes) as base makes sense only if there is common functionality you can share between subclasses - meaning that a number of classes extend the same abstract class inheriting the common implementation. If that's not the case (and you're pretty confident it's never gonna happen) then it doesn't make sense to use an abstract class.

If you can extract out some of the functionality in you big class in such a way that leads to (even potential) code reuse then it could make sense - otherwise I wouldn't see the point.

Answer 2

Although it is very tempting to say write it from scratch again, don't do it! The existing code may be ugly, but it looks like it does work. Since the class is big, I assume there is fair bit of history behind it as well. It might have solutions for some very obscure cases which you might not have imagined till now. What I suggest is, if possible first talk to the person who developed that class, understand how it works, then derive from it (after making its destructor virtual of course) and complete your work. Then as and when time permits slowly refactor the parts of the class into smaller more manageable classes. Also, don't forget to write a good unit-tester before you start so that you can validate the new behavior against the existing class's behavior. One more thing, there is nothing wrong in inheriting from a non-abstract base class as long as it makes sense and the base class destructor is virtual.

Answer 3

Is there anything at all you want to use from the base class or would you end up overriding everything?

Does it define some sort of type that you want to use for an "is-a" relationship? (for example, base class is "animal" and you want to make "cat", but if it doesn't add any behavior to its interface, that doesn't seem likely)

Is the base class used in other interfaces you need to use? (like if someone is passing objects through a reference/pointer to the base class)

If not, I'd say there's no advantage in inheriting from that class over implementing the interface(s) yourself.

Answer 4

If the other developer has written a base-class with no virtual functions, then those functions do not need to be overridden, and it is correct to define them in a non-abstract base class.

If those functions define functionality that all the child-classes require then it would be a mistake to get rid of the base class, as you would then need to implement those functions individually in each of the child classes.

I've seen a lot of developers go 'interface-mad' in the last couple of years, but base classes still serve a function over interfaces - to provide a concrete implementation that is common to all child classes. It would be a mistake to get rid of the base class and have seperate implementations of these functions in each of the child classes.

HOWEVER, if the child classes are inheriting functionality that they do not require, or require a separate implementation of, then the Base class is a mistake and interfaces would seem like the better option to divide the functionality between the child classes.

Despite this, I would agree with Naveen that its probably not worth the extra work this will give you, it may seem simple, but if this is a big class with a lot of inheritors then it could turn out to be a nightmare. Quite often in Software Engineering you have to deal with another developer's code that you might have implemented differently. If you re-implemented it ever time you will be a very unproductive developer. I say work with what you've got and get the project finished on time.

Answer 5

What are the pros and cons that would help me to choose the better approach.

It's legal to derive from a class with no virtual functions, but that doesn't make it a good idea. When you derive from a class with virtual functions, you often use that class through pointers (eg., a class Derived that inherits from Base is often manipulated through Base*s). That doesn't work when you don't use virtual functions. Also, if you have a pointer to the base class, delete-ing it can lead to a memory leak.

However, it sounds more like these classes aren't being used through pointers-to-the-base. Instead the base class is simply used to get a lot of built in functionality, although the classes aren't related in the normal sense. Inversion of control (and has-a relationships) is a more common way to do that nowadays (split the functionality of the base class into a number of interfaces -- pure virtual base classes -- and then have the objects that currently derive from the base class instead have member variables of those interfaces).

At the very least, you'll want to split the big base class into well-defined smaller classes and use those (like mixins), which sounds like your second option.

However, that doesn't mean rewrite all the other code that uses the blob base class all in one go. That's a big undertaking and you're likely to make small typos and similar mistakes. Instead, buy yourself copies of Working Effectively With Legacy Code and Large-Scale C++ Software Design, and do the work piecemeal.