Is there any point in any programmer's lifetime to restrict a class to extending this class?
Example is that I don't want Package to extend Box.
Yes. In Java they call this a final class. In C# they call it a sealed class.
It is done because you cannot always predict the many ways a consumer might extend your class, so to prevent you from having to design your class to account for all possible ways someone might extend it, you simply seal it and require the consumer to use composition instead of inheritance.
Further Reading
Why Are So Many Of The Framework Classes Sealed?
Related
A theoretical question. I'm reading Gof's Design Patterns, section Abstract Factory. The book mentions the possibility of implementing this pattern like a Prototype or, if the language permits it, with a Prototype which stores classes instead of objects.
I have understood this; e.g. in Java or Smalltalk, classes are also objects (in Java they are in fact instances of the class Class). Hence, we can store them inside a class and, when needed, invoke the creation of instances of these classes.
In C++, classes are not first class objects; hence, we shouldn't be able to follow this approach. However, couldn't we declare nested classes inside inside a Concrete Factory, with methods which invoke their constructors (and return their instances)? The final result would be the same of other languages such as Java or Smalltalk. Am I right?
Thank you for your attention.
I am using bada and refer to the tutorial here, which begins:
class MainForm:
public Osp::Ui::Controls::Form,
public Osp::Ui::IActionEventListener,
public Osp::Ui::ITouchEventListener
{
I am running code where I recently removed the public specifier to cut down on my public API. You'll see that the functions implementing those interfaces where all also declared publicly, for which I saw no need and made private. I would do this without hesitation when implementing my own interfaces when those interfaces may provide more access than I would wish regular clients of my concrete class to receive.
What is the reason for making them public, what am I missing?
I guess it is advocated to aid extensibility, but for a dev making apps not libraries I would challenge this wisdom.
If Form, IActionEventListener and ITouchEventListener already support many usable methods, in most cases why hide them? On the contrary: if you hide them and in the future someone will need them, it will be harder for you to maintain the class because you'll need to provide them again.
If you need to hide the parent's methods, there's another way to do this: instead of inheriting, enclose the "parent" as a field in your new class.
In some languages such as C#, public inheritance is the only option.
For me private inheritance of "interfaces" is a non sens.
The interface of an object is its set of public methods. As llya said, if you want to use the functionalities provided by a class internally, use object composition. If you want to provide a subset of the interface, then either compose or simply declare a more restrictive interface.
If the "interface" and the functions taking object from this interface are in a third party library then its means that the developers wanted to force you to implement every methods, so you have to provide them.
I'm making a game GUI API where each widget inherits from the Widget class. I was thinking, when others make there own widgets, they might not be fully satisfied with the base class. They might want to add getTheme() for example. Would it be a good idea to make all my widgets virtually inherit from Widget then so that this is possible?
Thanks
Just because the user would add their own methods to a child class doesn't mean you need to use virtual inheritance. You would use it if, in your library, you have a single base class with multiple children, and people could inherit from multiple child classes at once (for example mixin rather than substitution).
To resolve a diamond-shaped inheritance problem. (B and C both inherit from A. What happens to A's attributes in D that itself inherits from B and C?)
A client of your library could see a RedWidget and a FlyingWidget, and might want to combine them into a RedFlyingWidget.
User would have to specify one of the base classes to be virtual when inheriting. But that is not responsibility of a library maker.
OOP flows better with single-implementation inheritance, so that's what I'd use throughout a library.
There are also "upside-down inheritance" trees, as described by Alexandrescu's excellent "Modern C++ Design." They allow clients to pull in more functionality in a form of mix-ins that are called policies.
Programming with policies allows for greater ability to combine functionality, at the expense of syntactical cleanliness. Think STL implementation, for example.
When is virtual inheritance a good idea?
That's a design question.
For your Widgets, I would say Yes, multi-derived classes should have the option to be just 1 Widget.
Whenever there is a possibility that the users of your library are going to use several classes from your library as a base class (ie derive from them), you have to use virtual inheritance. In other words, it is a good idea to use it in your case.
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
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.
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.
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.
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.
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.
I am working on a collection of classes used for video playback and recording. I have one main class which acts like the public interface, with methods like play(), stop(), pause(), record() etc... Then I have workhorse classes which do the video decoding and video encoding.
I just learned about the existence of nested classes in C++, and I'm curious to know what programmers think about using them. I am a little wary and not really sure what the benefits/drawbacks are, but they seem (according to the book I'm reading) to be used in cases such as mine.
The book suggests that in a scenario like mine, a good solution would be to nest the workhorse classes inside the interface class, so there are no separate files for classes the client is not meant to use, and to avoid any possible naming conflicts? I don't know about these justifications. Nested classes are a new concept to me. Just want to see what programmers think about the issue.
I would be a bit reluctant to use nested classes here. What if you created an abstract base class for a "multimedia driver" to handle the back-end stuff (workhorse), and a separate class for the front-end work? The front-end class could take a pointer/reference to an implemented driver class (for the appropriate media type and situation) and perform the abstract operations on the workhorse structure.
My philosophy would be to go ahead and make both structures accessible to the client in a polished way, just under the assumption they would be used in tandem.
I would reference something like a QTextDocument in Qt. You provide a direct interface to the bare metal data handling, but pass the authority along to an object like a QTextEdit to do the manipulation.
You would use a nested class to create a (small) helper class that's required to implement the main class. Or for example, to define an interface (a class with abstract methods).
In this case, the main disadvantage of nested classes is that this makes it harder to re-use them. Perhaps you'd like to use your VideoDecoder class in another project. If you make it a nested class of VideoPlayer, you can't do this in an elegant way.
Instead, put the other classes in separate .h/.cpp files, which you can then use in your VideoPlayer class. The client of VideoPlayer now only needs to include the file that declares VideoPlayer, and still doesn't need to know about how you implemented it.
One way of deciding whether or not to use nested classes is to think whether or not this class plays a supporting role or it's own part.
If it exists solely for the purpose of helping another class then I generally make it a nested class. There are a whole load of caveats to that, some of which seem contradictory but it all comes down to experience and gut-feeling.
sounds like a case where you could use the strategy pattern
Sometimes it's appropriate to hide the implementation classes from the user -- in these cases it's better to put them in an foo_internal.h than inside the public class definition. That way, readers of your foo.h will not see what you'd prefer they not be troubled with, but you can still write tests against each of the concrete implementations of your interface.
We hit an issue with a semi-old Sun C++ compiler and visibility of nested classes which behavior changed in the standard. This is not a reason to not do your nested class, of course, just something to be aware of if you plan on compiling your software on lots of platforms including old compilers.
Well, if you use pointers to your workhorse classes in your Interface class and don't expose them as parameters or return types in your interface methods, you will not need to include the definitions for those work horses in your interface header file (you just forward declare them instead). That way, users of your interface will not need to know about the classes in the background.
You definitely don't need to nest classes for this. In fact, separate class files will actually make your code a lot more readable and easier to manage as your project grows. it will also help you later on if you need to subclass (say for different content/codec types).
Here's more information on the PIMPL pattern (section 3.1.1).
You should use an inner class only when you cannot implement it as a separate class using the would-be outer class' public interface. Inner classes increase the size, complexity, and responsibility of a class so they should be used sparingly.
Your encoder/decoder class sounds like it better fits the Strategy Pattern
One reason to avoid nested classes is if you ever intend to wrap the code with swig (http://www.swig.org) for use with other languages. Swig currently has problems with nested classes, so interfacing with libraries that expose any nested classes becomes a real pain.
Another thing to keep in mind is whether you ever envision different implementations of your work functions (such as decoding and encoding). In that case, you would definitely want an abstract base class with different concrete classes which implement the functions. It would not really be appropriate to nest a separate subclass for each type of implementation.