OOP Design Player and AI Controlled Unit. dynamic_cast? - c++

Lets say I have a Actor type, which can be any placeable object on my Game Level.
I also have Unit which is a child class of Actor. A Unit can be either a player or a AI controlled hero.
And Unit also got 2 child classes: Player (the player) and Hero (AI controlled unit).
In the Unit class, there will be movement info, rotation, and other general settings which will be needed for both Player and Hero. In the child classes AI will have different and more functions than the player.
Right now I'm facing the following problem:
A function only accepts Actor as parameter (e.g. OnOverlap(Actor a*))
but OnOverlap() should only do something if it is a Unit class (hero or player).
Therefore I would need something like instanceof() from Java in C++.
A workaround would be either to use dynamic_cast, but I'm not sure if thats a good idea for performance. Or use virtual, but this won't work when Hero has more functions than Player.
Or should I attempt a whole new OOP design?


I'd say that a dynamic_cast is code smell. Not necessary bad per se, but a sign that something might be going wrong with your design.
One of the important concepts in OOP is polymorphism: the idea that objects behave differently depending on their type, and that this behavior is encapsulated, hidden behind an interface. If you explicitely check the type of an object to change the logic you want to apply to it, then you're violating polymorphism.
Now virtual methods aren't that great either, they indeed incur runtime costs, and can sometimes bring too much complexity to the table. C++ makes virtual methods the exception, not the default, I believe for these very reasons.
One thing you should know is that virtual methods are only one kind of polymorphism, called dynamic polymorphism. There's another way to get different behaviors depending on a type that is less impeding for the runtime: static polymorphism, AKA template metaprogramming. But this isn't really helping on the complexity side.
Now what you really should do in this case is treat separate things separately. You probably want to have this OnOverlap() method on Units for a reason: let's say you're doing collision check on Units only, not all Actors. Then maintain a separate list of Units, and have that OnOverlap() method non-virtual on the Unit class. Thinking this way is often key.


This technique is called Run-Time Type Information (RTTI), and checking if the output of a dynamic_cast is NULL is a valid implementation.
Another method is to #include <typeinfo> and use typeid(*a).name() to get the string name of a's type.


The need to use dynamic_cast is often (though not always) a sign that you have a design flaw. In this case, I think you do.
Without further comment on the overall design of the architecture you've described, I would say using virtual methods is the right approach. Have OnOverlap takes it's actor and call an OnOverlapped method on the actor, perhaps giving it a pointer to the thing executing the initial OnOverlap method (it's not clear from your question what that is(*)).
void X::OnOverlap (Actor * actor) {
actor->OnOverlapped(this);
}
OnOverlapped is itself virtual. This allows you to "do the right thing" in the case of an actor unit, but do nothing or some other default behavior everywhere else. You are correct that in this case that means you can only work with the public API of Actor. You'd have to move anything requiring Player's additional methods into the Player implementation of OnOverlapped. If you really think you cannot do that for some reason, you should perhaps consider an alternative design at a higher level (and may need to edit your question to provide more details about your architecture).
(*) My initial read of your question made me think OnOverlap is not a method of Actor already; that it was part of something unrelated. If it is part of Actor, you may be instead interested in a pattern called double dispatch, which can be used for this sort of "collision handling" type of problem, and is also a technique using dynamic dispatch (virtual)).


These function is predefined by my Engine, where the parameter must
but the parent function "Actor". So if any Actor collides with the
Mesh, the function is called. In this case for me i see no other
solution, than getting the instance of the Actor object to know if its
a player of just something else
A simple virtual function in the Actor base class can be a noop. Override it in Unit class, and specialize it even more in Hero and Player if needed. Non-Unit Actor's will call an empty method. Cleaner than RTTI and the instanceof idiom.
It is ok to have a default base implementation that expects to be overridden by specialized descendants. That is the point of abstract base classes, except in this case you are providing a default implementation that does nothing. The idea is that you are dispatching the "DoCollision" message (a virtual function in C++). You don't care which classes handle it or how. Just that the object has the DoCollision compile-time "interface" and that you dispatched it correctly.

Related

C++ Inheritence vs Using parent class as a field in children classes [duplicate]

Why prefer composition over inheritance? What trade-offs are there for each approach? When should you choose inheritance over composition?

Prefer composition over inheritance as it is more malleable / easy to modify later, but do not use a compose-always approach. With composition, it's easy to change behavior on the fly with Dependency Injection / Setters. Inheritance is more rigid as most languages do not allow you to derive from more than one type. So the goose is more or less cooked once you derive from TypeA.
My acid test for the above is:
Does TypeB want to expose the complete interface (all public methods no less) of TypeA such that TypeB can be used where TypeA is expected? Indicates Inheritance.
e.g. A Cessna biplane will expose the complete interface of an airplane, if not more. So that makes it fit to derive from Airplane.
Does TypeB want only some/part of the behavior exposed by TypeA? Indicates need for Composition.
e.g. A Bird may need only the fly behavior of an Airplane. In this case, it makes sense to extract it out as an interface / class / both and make it a member of both classes.
Update: Just came back to my answer and it seems now that it is incomplete without a specific mention of Barbara Liskov's Liskov Substitution Principle as a test for 'Should I be inheriting from this type?'

Think of containment as a has a relationship. A car "has an" engine, a person "has a" name, etc.
Think of inheritance as an is a relationship. A car "is a" vehicle, a person "is a" mammal, etc.
I take no credit for this approach. I took it straight from the Second Edition of Code Complete by Steve McConnell, Section 6.3.

If you understand the difference, it's easier to explain.
Procedural Code
An example of this is PHP without the use of classes (particularly before PHP5). All logic is encoded in a set of functions. You may include other files containing helper functions and so on and conduct your business logic by passing data around in functions. This can be very hard to manage as the application grows. PHP5 tries to remedy this by offering a more object-oriented design.
Inheritance
This encourages the use of classes. Inheritance is one of the three tenets of OO design (inheritance, polymorphism, encapsulation).
class Person {
String Title;
String Name;
Int Age
}
class Employee : Person {
Int Salary;
String Title;
}
This is inheritance at work. The Employee "is a" Person or inherits from Person. All inheritance relationships are "is-a" relationships. Employee also shadows the Title property from Person, meaning Employee.Title will return the Title for the Employee and not the Person.
Composition
Composition is favoured over inheritance. To put it very simply you would have:
class Person {
String Title;
String Name;
Int Age;
public Person(String title, String name, String age) {
this.Title = title;
this.Name = name;
this.Age = age;
}
}
class Employee {
Int Salary;
private Person person;
public Employee(Person p, Int salary) {
this.person = p;
this.Salary = salary;
}
}
Person johnny = new Person ("Mr.", "John", 25);
Employee john = new Employee (johnny, 50000);
Composition is typically "has a" or "uses a" relationship. Here the Employee class has a Person. It does not inherit from Person but instead gets the Person object passed to it, which is why it "has a" Person.
Composition over Inheritance
Now say you want to create a Manager type so you end up with:
class Manager : Person, Employee {
...
}
This example will work fine, however, what if Person and Employee both declared Title? Should Manager.Title return "Manager of Operations" or "Mr."? Under composition this ambiguity is better handled:
Class Manager {
public string Title;
public Manager(Person p, Employee e)
{
this.Title = e.Title;
}
}
The Manager object is composed of an Employee and a Person. The Title behaviour is taken from Employee. This explicit composition removes ambiguity among other things and you'll encounter fewer bugs.

With all the undeniable benefits provided by inheritance, here's some of its disadvantages.
Disadvantages of Inheritance:
You can't change the implementation inherited from super classes at runtime (obviously because inheritance is defined at compile time).
Inheritance exposes a subclass to details of its parent class implementation, that's why it's often said that inheritance breaks encapsulation (in a sense that you really need to focus on interfaces only not implementation, so reusing by sub classing is not always preferred).
The tight coupling provided by inheritance makes the implementation of a subclass very bound up with the implementation of a super class that any change in the parent implementation will force the sub class to change.
Excessive reusing by sub-classing can make the inheritance stack very deep and very confusing too.
On the other hand Object composition is defined at runtime through objects acquiring references to other objects. In such a case these objects will never be able to reach each-other's protected data (no encapsulation break) and will be forced to respect each other's interface. And in this case also, implementation dependencies will be a lot less than in case of inheritance.

Another, very pragmatic reason, to prefer composition over inheritance has to do with your domain model, and mapping it to a relational database. It's really hard to map inheritance to the SQL model (you end up with all sorts of hacky workarounds, like creating columns that aren't always used, using views, etc). Some ORMLs try to deal with this, but it always gets complicated quickly. Composition can be easily modeled through a foreign-key relationship between two tables, but inheritance is much harder.

While in short words I would agree with "Prefer composition over inheritance", very often for me it sounds like "prefer potatoes over coca-cola". There are places for inheritance and places for composition. You need to understand difference, then this question will disappear. What it really means for me is "if you are going to use inheritance - think again, chances are you need composition".
You should prefer potatoes over coca cola when you want to eat, and coca cola over potatoes when you want to drink.
Creating a subclass should mean more than just a convenient way to call superclass methods. You should use inheritance when subclass "is-a" super class both structurally and functionally, when it can be used as superclass and you are going to use that. If it is not the case - it is not inheritance, but something else. Composition is when your objects consists of another, or has some relationship to them.
So for me it looks like if someone does not know if he needs inheritance or composition, the real problem is that he does not know if he want to drink or to eat. Think about your problem domain more, understand it better.

Didn't find a satisfactory answer here, so I wrote a new one.
To understand why "prefer composition over inheritance", we need first get back the assumption omitted in this shortened idiom.
There are two benefits of inheritance: subtyping and subclassing
Subtyping means conforming to a type (interface) signature, i.e. a set of APIs, and one can override part of the signature to achieve subtyping polymorphism.
Subclassing means implicit reuse of method implementations.
With the two benefits comes two different purposes for doing inheritance: subtyping oriented and code reuse oriented.
If code reuse is the sole purpose, subclassing may give one more than what he needs, i.e. some public methods of the parent class don't make much sense for the child class. In this case, instead of favoring composition over inheritance, composition is demanded. This is also where the "is-a" vs. "has-a" notion comes from.
So only when subtyping is purposed, i.e. to use the new class later in a polymorphic manner, do we face the problem of choosing inheritance or composition. This is the assumption that gets omitted in the shortened idiom under discussion.
To subtype is to conform to a type signature, this means composition has always to expose no less amount of APIs of the type. Now the trade offs kick in:
Inheritance provides straightforward code reuse if not overridden, while composition has to re-code every API, even if it's just a simple job of delegation.
Inheritance provides straightforward open recursion via the internal polymorphic site this, i.e. invoking overriding method (or even type) in another member function, either public or private (though discouraged). Open recursion can be simulated via composition, but it requires extra effort and may not always viable(?). This answer to a duplicated question talks something similar.
Inheritance exposes protected members. This breaks encapsulation of the parent class, and if used by subclass, another dependency between the child and its parent is introduced.
Composition has the befit of inversion of control, and its dependency can be injected dynamically, as is shown in decorator pattern and proxy pattern.
Composition has the benefit of combinator-oriented programming, i.e. working in a way like the composite pattern.
Composition immediately follows programming to an interface.
Composition has the benefit of easy multiple inheritance.
With the above trade offs in mind, we hence prefer composition over inheritance. Yet for tightly related classes, i.e. when implicit code reuse really make benefits, or the magic power of open recursion is desired, inheritance shall be the choice.

Inheritance is pretty enticing especially coming from procedural-land and it often looks deceptively elegant. I mean all I need to do is add this one bit of functionality to some other class, right? Well, one of the problems is that inheritance is probably the worst form of coupling you can have
Your base class breaks encapsulation by exposing implementation details to subclasses in the form of protected members. This makes your system rigid and fragile. The more tragic flaw however is the new subclass brings with it all the baggage and opinion of the inheritance chain.
The article, Inheritance is Evil: The Epic Fail of the DataAnnotationsModelBinder, walks through an example of this in C#. It shows the use of inheritance when composition should have been used and how it could be refactored.

When can you use composition?
You can always use composition. In some cases, inheritance is also possible and may lead to a more powerful and/or intuitive API, but composition is always an option.
When can you use inheritance?
It is often said that if "a bar is a foo", then the class Bar can inherit the class Foo. Unfortunately, this test alone is not reliable, use the following instead:
a bar is a foo, AND
bars can do everything that foos can do.
The first test ensures that all getters of Foo make sense in Bar (= shared properties), while the second test makes sure that all setters of Foo make sense in Bar (= shared functionality).
Example: Dog/Animal
A dog is an animal AND dogs can do everything that animals can do (such as breathing, moving, etc.). Therefore, the class Dog can inherit the class Animal.
Counter-example: Circle/Ellipse
A circle is an ellipse BUT circles can't do everything that ellipses can do. For example, circles can't stretch, while ellipses can. Therefore, the class Circle cannot inherit the class Ellipse.
This is called the Circle-Ellipse problem, which isn't really a problem, but more an indication that "a bar is a foo" isn't a reliable test by itself. In particular, this example highlights that derived classes should extend the functionality of base classes, never restrict it. Otherwise, the base class couldn't be used polymorphically. Adding the test "bars can do everything that foos can do" ensures that polymorphic use is possible, and is equivalent to the Liskov Substitution Principle:
Functions that use pointers or references to base classes must be able to use objects of derived classes without knowing it
When should you use inheritance?
Even if you can use inheritance doesn't mean you should: using composition is always an option. Inheritance is a powerful tool allowing implicit code reuse and dynamic dispatch, but it does come with a few disadvantages, which is why composition is often preferred. The trade-offs between inheritance and composition aren't obvious, and in my opinion are best explained in lcn's answer.
As a rule of thumb, I tend to choose inheritance over composition when polymorphic use is expected to be very common, in which case the power of dynamic dispatch can lead to a much more readable and elegant API. For example, having a polymorphic class Widget in GUI frameworks, or a polymorphic class Node in XML libraries allows to have an API which is much more readable and intuitive to use than what you would have with a solution purely based on composition.

In Java or C#, an object cannot change its type once it has been instantiated.
So, if your object need to appear as a different object or behave differently depending on an object state or conditions, then use Composition: Refer to State and Strategy Design Patterns.
If the object need to be of the same type, then use Inheritance or implement interfaces.

Personally I learned to always prefer composition over inheritance. There is no programmatic problem you can solve with inheritance which you cannot solve with composition; though you may have to use Interfaces(Java) or Protocols(Obj-C) in some cases. Since C++ doesn't know any such thing, you'll have to use abstract base classes, which means you cannot get entirely rid of inheritance in C++.
Composition is often more logical, it provides better abstraction, better encapsulation, better code reuse (especially in very large projects) and is less likely to break anything at a distance just because you made an isolated change anywhere in your code. It also makes it easier to uphold the "Single Responsibility Principle", which is often summarized as "There should never be more than one reason for a class to change.", and it means that every class exists for a specific purpose and it should only have methods that are directly related to its purpose. Also having a very shallow inheritance tree makes it much easier to keep the overview even when your project starts to get really large. Many people think that inheritance represents our real world pretty well, but that isn't the truth. The real world uses much more composition than inheritance. Pretty much every real world object you can hold in your hand has been composed out of other, smaller real world objects.
There are downsides of composition, though. If you skip inheritance altogether and only focus on composition, you will notice that you often have to write a couple of extra code lines that weren't necessary if you had used inheritance. You are also sometimes forced to repeat yourself and this violates the DRY Principle (DRY = Don't Repeat Yourself). Also composition often requires delegation, and a method is just calling another method of another object with no other code surrounding this call. Such "double method calls" (which may easily extend to triple or quadruple method calls and even farther than that) have much worse performance than inheritance, where you simply inherit a method of your parent. Calling an inherited method may be equally fast as calling a non-inherited one, or it may be slightly slower, but is usually still faster than two consecutive method calls.
You may have noticed that most OO languages don't allow multiple inheritance. While there are a couple of cases where multiple inheritance can really buy you something, but those are rather exceptions than the rule. Whenever you run into a situation where you think "multiple inheritance would be a really cool feature to solve this problem", you are usually at a point where you should re-think inheritance altogether, since even it may require a couple of extra code lines, a solution based on composition will usually turn out to be much more elegant, flexible and future proof.
Inheritance is really a cool feature, but I'm afraid it has been overused the last couple of years. People treated inheritance as the one hammer that can nail it all, regardless if it was actually a nail, a screw, or maybe a something completely different.

My general rule of thumb: Before using inheritance, consider if composition makes more sense.
Reason: Subclassing usually means more complexity and connectedness, i.e. harder to change, maintain, and scale without making mistakes.
A much more complete and concrete answer from Tim Boudreau of Sun:
Common problems to the use of inheritance as I see it are:
Innocent acts can have unexpected results - The classic example of this is calls to overridable methods from the superclass
constructor, before the subclasses instance fields have been
initialized. In a perfect world, nobody would ever do that. This is
not a perfect world.
It offers perverse temptations for subclassers to make assumptions about order of method calls and such - such assumptions tend not to
be stable if the superclass may evolve over time. See also my toaster
and coffee pot analogy.
Classes get heavier - you don't necessarily know what work your superclass is doing in its constructor, or how much memory it's going
to use. So constructing some innocent would-be lightweight object can
be far more expensive than you think, and this may change over time if
the superclass evolves
It encourages an explosion of subclasses. Classloading costs time, more classes costs memory. This may be a non-issue until you're
dealing with an app on the scale of NetBeans, but there, we had real
issues with, for example, menus being slow because the first display
of a menu triggered massive class loading. We fixed this by moving to
more declarative syntax and other techniques, but that cost time to
fix as well.
It makes it harder to change things later - if you've made a class public, swapping the superclass is going to break subclasses -
it's a choice which, once you've made the code public, you're married
to. So if you're not altering the real functionality to your
superclass, you get much more freedom to change things later if you
use, rather than extend the thing you need. Take, for example,
subclassing JPanel - this is usually wrong; and if the subclass is
public somewhere, you never get a chance to revisit that decision. If
it's accessed as JComponent getThePanel() , you can still do it (hint:
expose models for the components within as your API).
Object hierarchies don't scale (or making them scale later is much harder than planning ahead) - this is the classic "too many layers"
problem. I'll go into this below, and how the AskTheOracle pattern can
solve it (though it may offend OOP purists).
...
My take on what to do, if you do allow for inheritance, which you may
take with a grain of salt is:
Expose no fields, ever, except constants
Methods shall be either abstract or final
Call no methods from the superclass constructor
...
all of this applies less to small projects than large ones, and less
to private classes than public ones

Inheritance is very powerful, but you can't force it (see: the circle-ellipse problem). If you really can't be completely sure of a true "is-a" subtype relationship, then it's best to go with composition.

Inheritance creates a strong relationship between a subclass and super class; subclass must be aware of super class'es implementation details. Creating the super class is much harder, when you have to think about how it can be extended. You have to document class invariants carefully, and state what other methods overridable methods use internally.
Inheritance is sometimes useful, if the hierarchy really represents a is-a-relationship. It relates to Open-Closed Principle, which states that classes should be closed for modification but open to extension. That way you can have polymorphism; to have a generic method that deals with super type and its methods, but via dynamic dispatch the method of subclass is invoked. This is flexible, and helps to create indirection, which is essential in software (to know less about implementation details).
Inheritance is easily overused, though, and creates additional complexity, with hard dependencies between classes. Also understanding what happens during execution of a program gets pretty hard due to layers and dynamic selection of method calls.
I would suggest using composing as the default. It is more modular, and gives the benefit of late binding (you can change the component dynamically). Also it's easier to test the things separately. And if you need to use a method from a class, you are not forced to be of certain form (Liskov Substitution Principle).

Suppose an aircraft has only two parts: an engine and wings.
Then there are two ways to design an aircraft class.
Class Aircraft extends Engine{
var wings;
}
Now your aircraft can start with having fixed wings
and change them to rotary wings on the fly. It's essentially
an engine with wings. But what if I wanted to change
the engine on the fly as well?
Either the base class Engine exposes a mutator to change its
properties, or I redesign Aircraft as:
Class Aircraft {
var wings;
var engine;
}
Now, I can replace my engine on the fly as well.

If you want the canonical, textbook answer people have been giving since the rise of OOP (which you see many people giving in these answers), then apply the following rule: "if you have an is-a relationship, use inheritance. If you have a has-a relationship, use composition".
This is the traditional advice, and if that satisfies you, you can stop reading here and go on your merry way. For everyone else...
is-a/has-a comparisons have problems
For example:
A square is-a rectangle, but if your rectangle class has setWidth()/setHeight() methods, then there's no reasonable way to make a Square inherit from Rectangle without breaking Liskov's substitution principle.
An is-a relationship can often be rephrased to sound like a has-a relationship. For example, an employee is-a person, but a person also has-an employment status of "employed".
is-a relationships can lead to nasty multiple inheritance hierarchies if you're not careful. After all, there's no rule in English that states that an object is exactly one thing.
People are quick to pass this "rule" around, but has anyone ever tried to back it up, or explain why it's a good heuristic to follow? Sure, it fits nicely into the idea that OOP is supposed to model the real world, but that's not in-and-of-itself a reason to adopt a principle.
See this StackOverflow question for more reading on this subject.
To know when to use inheritance vs composition, we first need to understand the pros and cons of each.
The problems with implementation inheritance
Other answers have done a wonderful job at explaining the issues with inheritance, so I'll try to not delve into too many details here. But, here's a brief list:
It can be difficult to follow a logic that weaves between base and sub-class methods.
Carelessly implementing one method in your class by calling another overridable method will cause you to leak implementation details and break encapsulation, as the end-user could override your method and detect when you internally call it. (See "Effective Java" item 18).
The fragile base problem, which simply states that your end-user's code will break if they happen to depend on the leakage of implementation details when you attempt to change them. To make matters worse, most OOP languages allow inheritance by default - API designers who aren't proactively preventing people from inheriting from their public classes need to be extra cautious whenever they refactor their base classes. Unfortunately, the fragile base problem is often misunderstood, causing many to not understand what it takes to maintain a class that anyone can inherit from.
The deadly diamond of death
The problems with composition
It can sometimes be a little verbose.
That's it. I'm serious. This is still a real issue and can sometimes create conflict with the DRY principle, but it's generally not that bad, at least compared to the myriad of pitfalls associated with inheritance.
When should inheritance be used?
Next time you're drawing out your fancy UML diagrams for a project (if you do that), and you're thinking about adding in some inheritance, please adhere to the following advice: don't.
At least, not yet.
Inheritance is sold as a tool to achieve polymorphism, but bundled with it is this powerful code-reuse system, that frankly, most code doesn't need. The problem is, as soon as you publicly expose your inheritance hierarchy, you're locked into this particular style of code-reuse, even if it's overkill to solve your particular problem.
To avoid this, my two cents would be to never expose your base classes publicly.
If you need polymorphism, use an interface.
If you need to allow people to customize the behavior of your class, provide explicit hook-in points via the strategy pattern, it's a more readable way to accomplish this, plus, it's easier to keep this sort of API stable as you're in full control over what behaviors they can and can not change.
If you're trying to follow the open-closed principle by using inheritance to avoid adding a much-needed update to a class, just don't. Update the class. Your codebase will be much cleaner if you actually take ownership of the code you're hired to maintain instead of trying to tack stuff onto the side of it. If you're scared about introducing bugs, then get the existing code under test.
If you need to reuse code, start out by trying to use composition or helper functions.
Finally, if you've decided that there's no other good option, and you must use inheritance to achieve the code-reuse that you need, then you can use it, but, follow these four P.A.I.L. rules of restricted inheritance to keep it sane.
Use inheritance as a private implementation detail. Don't expose your base class publicly, use interfaces for that. This lets you freely add or remove inheritance as you see fit without making a breaking change.
Keep your base class abstract. It makes it easier to divide out the logic that needs to be shared from the logic that doesn't.
Isolate your base and child classes. Don't let your subclass override base class methods (use the strategy pattern for that), and avoid having them expect properties/methods to exist on each other, use other forms of code-sharing to achieve that. Use appropriate language features to force all methods on the base class to be non-overridable ("final" in Java, or non-virtual in C#).
Inheritance is a last resort.
The Isolate rule in particular may sound a little rough to follow, but if you discipline yourself, you'll get some pretty nice benefits. In particular, it gives you the freedom to avoid all of the main nasty pitfalls associated with the inheritance that were mentioned above.
It's much easier to follow the code because it doesn't weave in and out of base/sub classes.
You can not accidentally leak when your methods are internally calling other overridable methods if you never make any of your methods overridable. In other words, you won't accidentally break encapsulation.
The fragile base class problem stems from the ability to depend on accidentally leaked implementation details. Since the base class is now isolated, it will be no more fragile than a class depending on another via composition.
The deadly diamond of death isn't an issue anymore, since there's simply no need to have multiple layers of inheritance. If you have the abstract base classes B and C, which both share a lot of functionality, just move that functionality out of B and C and into a new abstract base class, class D. Anyone who inherited from B should update to inherit from both B and D, and anyone who inherited from C should inherit from C and D. Since your base classes are all private implementation details, it shouldn't be too difficult to figure out who's inheriting from what, to make these changes.
Conclusion
My primary suggestion would be to use your brain on this matter. What's far more important than a list of dos and don'ts about when to use inheritance is an intuitive understanding of inheritance and its associated pros and cons, along with a good understanding of the other tools out there that can be used instead of inheritance (composition isn't the only alternative. For example, the strategy pattern is an amazing tool that's forgotten far too often). Perhaps when you have a good, solid understanding of all of these tools, you'll choose to use inheritance more often than I would recommend, and that's completely fine. At least, you're making an informed decision, and aren't just using inheritance because that's the only way you know how to do it.
Further reading:
An article I wrote on this subject, that dives even deeper and provides examples.
A webpage talking about three different jobs that inheritance does, and how those jobs can be done via other means in the Go language.
A list of reasons why it can be good to declare your class as non-inheritable (e.g. "final" in Java).
The "Effective Java" book by Joshua Bloch, item 18, which discusses composition over inheritance, and some of the dangers of inheritance.

You need to have a look at The Liskov Substitution Principle in Uncle Bob's SOLID principles of class design. :)

To address this question from a different perspective for newer programmers:
Inheritance is often taught early when we learn object-oriented programming, so it's seen as an easy solution to a common problem.
I have three classes that all need some common functionality. So if I
write a base class and have them all inherit from it, then they will
all have that functionality and I'll only need to maintain it in once
place.
It sounds great, but in practice it almost never, ever works, for one of several reasons:
We discover that there are some other functions that we want our classes to have. If the way that we add functionality to classes is through inheritance, we have to decide - do we add it to the existing base class, even though not every class that inherits from it needs that functionality? Do we create another base class? But what about classes that already inherit from the other base class?
We discover that for just one of the classes that inherits from our base class we want the base class to behave a little differently. So now we go back and tinker with our base class, maybe adding some virtual methods, or even worse, some code that says, "If I'm inherited type A, do this, but if I'm inherited type B, do that." That's bad for lots of reasons. One is that every time we change the base class, we're effectively changing every inherited class. So we're really changing class A, B, C, and D because we need a slightly different behavior in class A. As careful as we think we are, we might break one of those classes for reasons that have nothing to do with those classes.
We might know why we decided to make all of these classes inherit from each other, but it might not (probably won't) make sense to someone else who has to maintain our code. We might force them into a difficult choice - do I do something really ugly and messy to make the change I need (see the previous bullet point) or do I just rewrite a bunch of this.
In the end, we tie our code in some difficult knots and get no benefit whatsoever from it except that we get to say, "Cool, I learned about inheritance and now I used it." That's not meant to be condescending because we've all done it. But we all did it because no one told us not to.
As soon as someone explained "favor composition over inheritance" to me, I thought back over every time I tried to share functionality between classes using inheritance and realized that most of the time it didn't really work well.
The antidote is the Single Responsibility Principle. Think of it as a constraint. My class must do one thing. I must be able to give my class a name that somehow describes that one thing it does. (There are exceptions to everything, but absolute rules are sometimes better when we're learning.) It follows that I cannot write a base class called ObjectBaseThatContainsVariousFunctionsNeededByDifferentClasses. Whatever distinct functionality I need must be in its own class, and then other classes that need that functionality can depend on that class, not inherit from it.
At the risk of oversimplifying, that's composition - composing multiple classes to work together. And once we form that habit we find that it's much more flexible, maintainable, and testable than using inheritance.

When you want to "copy"/Expose the base class' API, you use inheritance. When you only want to "copy" functionality, use delegation.
One example of this: You want to create a Stack out of a List. Stack only has pop, push and peek. You shouldn't use inheritance given that you don't want push_back, push_front, removeAt, et al.-kind of functionality in a Stack.

These two ways can live together just fine and actually support each other.
Composition is just playing it modular: you create interface similar to the parent class, create new object and delegate calls to it. If these objects need not to know of each other, it's quite safe and easy to use composition. There are so many possibilites here.
However, if the parent class for some reason needs to access functions provided by the "child class" for inexperienced programmer it may look like it's a great place to use inheritance. The parent class can just call it's own abstract "foo()" which is overwritten by the subclass and then it can give the value to the abstract base.
It looks like a nice idea, but in many cases it's better just give the class an object which implements the foo() (or even set the value provided the foo() manually) than to inherit the new class from some base class which requires the function foo() to be specified.
Why?
Because inheritance is a poor way of moving information.
The composition has a real edge here: the relationship can be reversed: the "parent class" or "abstract worker" can aggregate any specific "child" objects implementing certain interface + any child can be set inside any other type of parent, which accepts it's type. And there can be any number of objects, for example MergeSort or QuickSort could sort any list of objects implementing an abstract Compare -interface. Or to put it another way: any group of objects which implement "foo()" and other group of objects which can make use of objects having "foo()" can play together.
I can think of three real reasons for using inheritance:
You have many classes with same interface and you want to save time writing them
You have to use same Base Class for each object
You need to modify the private variables, which can not be public in any case
If these are true, then it is probably necessary to use inheritance.
There is nothing bad in using reason 1, it is very good thing to have a solid interface on your objects. This can be done using composition or with inheritance, no problem - if this interface is simple and does not change. Usually inheritance is quite effective here.
If the reason is number 2 it gets a bit tricky. Do you really only need to use the same base class? In general, just using the same base class is not good enough, but it may be a requirement of your framework, a design consideration which can not be avoided.
However, if you want to use the private variables, the case 3, then you may be in trouble. If you consider global variables unsafe, then you should consider using inheritance to get access to private variables also unsafe. Mind you, global variables are not all THAT bad - databases are essentially big set of global variables. But if you can handle it, then it's quite fine.

Aside from is a/has a considerations, one must also consider the "depth" of inheritance your object has to go through. Anything beyond five or six levels of inheritance deep might cause unexpected casting and boxing/unboxing problems, and in those cases it might be wise to compose your object instead.

When you have an is-a relation between two classes (example dog is a canine), you go for inheritance.
On the other hand when you have has-a or some adjective relationship between two classes (student has courses) or (teacher studies courses), you chose composition.

A simple way to make sense of this would be that inheritance should be used when you need an object of your class to have the same interface as its parent class, so that it can thereby be treated as an object of the parent class (upcasting). Moreover, function calls on a derived class object would remain the same everywhere in code, but the specific method to call would be determined at runtime (i.e. the low-level implementation differs, the high-level interface remains the same).
Composition should be used when you do not need the new class to have the same interface, i.e. you wish to conceal certain aspects of the class' implementation which the user of that class need not know about. So composition is more in the way of supporting encapsulation (i.e. concealing the implementation) while inheritance is meant to support abstraction (i.e. providing a simplified representation of something, in this case the same interface for a range of types with different internals).

Subtyping is appropriate and more powerful where the invariants can be enumerated, else use function composition for extensibility.

I agree with #Pavel, when he says, there are places for composition and there are places for inheritance.
I think inheritance should be used if your answer is an affirmative to any of these questions.
Is your class part of a structure that benefits from polymorphism ? For example, if you had a Shape class, which declares a method called draw(), then we clearly need Circle and Square classes to be subclasses of Shape, so that their client classes would depend on Shape and not on specific subclasses.
Does your class need to re-use any high level interactions defined in another class ? The template method design pattern would be impossible to implement without inheritance. I believe all extensible frameworks use this pattern.
However, if your intention is purely that of code re-use, then composition most likely is a better design choice.

Inheritance is a very powerfull machanism for code reuse. But needs to be used properly. I would say that inheritance is used correctly if the subclass is also a subtype of the parent class. As mentioned above, the Liskov Substitution Principle is the key point here.
Subclass is not the same as subtype. You might create subclasses that are not subtypes (and this is when you should use composition). To understand what a subtype is, lets start giving an explanation of what a type is.
When we say that the number 5 is of type integer, we are stating that 5 belongs to a set of possible values (as an example, see the possible values for the Java primitive types). We are also stating that there is a valid set of methods I can perform on the value like addition and subtraction. And finally we are stating that there are a set of properties that are always satisfied, for example, if I add the values 3 and 5, I will get 8 as a result.
To give another example, think about the abstract data types, Set of integers and List of integers, the values they can hold are restricted to integers. They both support a set of methods, like add(newValue) and size(). And they both have different properties (class invariant), Sets does not allow duplicates while List does allow duplicates (of course there are other properties that they both satisfy).
Subtype is also a type, which has a relation to another type, called parent type (or supertype). The subtype must satisfy the features (values, methods and properties) of the parent type. The relation means that in any context where the supertype is expected, it can be substitutable by a subtype, without affecting the behaviour of the execution. Let’s go to see some code to exemplify what I’m saying. Suppose I write a List of integers (in some sort of pseudo language):
class List {
data = new Array();
Integer size() {
return data.length;
}
add(Integer anInteger) {
data[data.length] = anInteger;
}
}
Then, I write the Set of integers as a subclass of the List of integers:
class Set, inheriting from: List {
add(Integer anInteger) {
if (data.notContains(anInteger)) {
super.add(anInteger);
}
}
}
Our Set of integers class is a subclass of List of Integers, but is not a subtype, due to it is not satisfying all the features of the List class. The values, and the signature of the methods are satisfied but the properties are not. The behaviour of the add(Integer) method has been clearly changed, not preserving the properties of the parent type. Think from the point of view of the client of your classes. They might receive a Set of integers where a List of integers is expected. The client might want to add a value and get that value added to the List even if that value already exist in the List. But her wont get that behaviour if the value exists. A big suprise for her!
This is a classic example of an improper use of inheritance. Use composition in this case.
(a fragment from: use inheritance properly).

Even though Composition is preferred, I would like to highlight pros of Inheritance and cons of Composition.
Pros of Inheritance:
It establishes a logical "IS A" relation. If Car and Truck are two types of Vehicle ( base class), child class IS A base class.
i.e.
Car is a Vehicle
Truck is a Vehicle
With inheritance, you can define/modify/extend a capability
Base class provides no implementation and sub-class has to override complete method (abstract) => You can implement a contract
Base class provides default implementation and sub-class can change the behaviour => You can re-define contract
Sub-class adds extension to base class implementation by calling super.methodName() as first statement => You can extend a contract
Base class defines structure of the algorithm and sub-class will override a part of algorithm => You can implement Template_method without change in base class skeleton
Cons of Composition:
In inheritance, subclass can directly invoke base class method even though it's not implementing base class method because of IS A relation. If you use composition, you have to add methods in container class to expose contained class API
e.g. If Car contains Vehicle and if you have to get price of the Car, which has been defined in Vehicle, your code will be like this
class Vehicle{
protected double getPrice(){
// return price
}
}
class Car{
Vehicle vehicle;
protected double getPrice(){
return vehicle.getPrice();
}
}

A rule of thumb I have heard is inheritance should be used when its a "is-a" relationship and composition when its a "has-a". Even with that I feel that you should always lean towards composition because it eliminates a lot of complexity.

As many people told, I will first start with the check - whether there exists an "is-a" relationship. If it exists I usually check the following:
Whether the base class can be instantiated. That is, whether the base class can be non-abstract. If it can be non-abstract I usually prefer composition
E.g 1. Accountant is an Employee. But I will not use inheritance because a Employee object can be instantiated.
E.g 2. Book is a SellingItem. A SellingItem cannot be instantiated - it is abstract concept. Hence I will use inheritacne. The SellingItem is an abstract base class (or interface in C#)
What do you think about this approach?
Also, I support #anon answer in Why use inheritance at all?
The main reason for using inheritance is not as a form of composition - it is so you can get polymorphic behaviour. If you don't need polymorphism, you probably should not be using inheritance.
#MatthieuM. says in https://softwareengineering.stackexchange.com/questions/12439/code-smell-inheritance-abuse/12448#comment303759_12448
The issue with inheritance is that it can be used for two orthogonal purposes:
interface (for polymorphism)
implementation (for code reuse)
REFERENCE
Which class design is better?
Inheritance vs. Aggregation

Composition v/s Inheritance is a wide subject. There is no real answer for what is better as I think it all depends on the design of the system.
Generally type of relationship between object provide better information to choose one of them.
If relation type is "IS-A" relation then Inheritance is better approach.
otherwise relation type is "HAS-A" relation then composition will better approach.
Its totally depend on entity relationship.

Is using an empty base class justified in this example?

I'm writing a Window class which propagates different types of events, listed in
enum Event {WINDOW_ClOSE=0x1, WINDOW_REDRAW=0x2, MOUSE_MOVE=0x4, ...};
to objects which have registered for notification with the window. For each type of event, I have an abstract class which any object must extend in order to allow notification. To react to, say, a MOUSE_MOVE event, my object would inherit from MouseMoveListener, which has a process_mouse_move_event() method which is called by Window. Listening to many events can be combined by extending multiple of these classes, which all inherit from the EventListener base class. To register an object, I would call
void Window::register(EventListener* object, int EventTypes)
{
if(EventTypes&WINDOW_CLOSE)
/* dynamic_cast object to WindowCloseListener*, add to internal list of all
WindowCloseListeners if the cast works, else raise error */
if(EventTypes&MOUSE_MOVE)
/* dynamic_cast object to MouseMoveListener*, add to internal list of all
MouseMoveListeners if the cast works, else raise error */
...
}
This works fine, but my gripe is that EventListener is completely empty and that seems code smelly to me. I know I could avoid this by removing EventListener altogether and having a separate Window::register for each type of event, but I feel that this would blow up my interface needlessly (especially since methods other than register might crop up with the same problem). So I guess I am looking for answers that either say:
"You can keep doing it the way you do, because ..." or
"Introduce the separate Window::register methods anyway, because ..." or of course
"You are doing it all wrong, you should ...".
EDIT:
From the link in Igors comment: What I do above only works if there is at least one virtual member in EventListener for example a virtual destructor, so the class is not technically completely empty.
EDIT 2:
I prematurely accepted n.m.'s solution as one of the type "I'm doing it all wrong". However, it is of the second type. Even if I can call EventListener->register(Window&) polymorphically, Window needs to implement a highly redundant interface (in terms of declared methods) that allows EventListeners to register for selective notification. This is equivalent to my alternative solution described above, only with the additional introduction of the EventListener class for no good reason. In conclusion, the canonical answer seems to be:
Don't do dynamic_cast + empty base class just to avoid declaring many similar functions, it will hurt you when maintaining the code later. Write the many functions.
EDIT 3:
I found a solution (using templates) which is satisfactory for me. It does not use an empty base class any more and it does not exhibit the maintenance problem pointed out by n.m.

object->registerWindow (this, EventTypes);
Of course you need to implement registerWindow for all EventListener heirs. Let them check for event types which are relevant to them.
UPDATE
If this means you need to redesign your code, then you need to redesign your code. Why is it so? Because dynamic_cast is not a proper way to do switch-on-types. It is not a proper way because every time you add a class in your hierarchy, you need to go over and possibly update all switches-by-dynamic-cast in your old code. This becomes very messy and unmaintainable very quickly, and this is exactly the reason why virtual functions were invented.
If you do your switch-on-types with virtual functions, every time you change your hierarchy you have to do... nothing. The virtual call mechanism will take care of your changes.

This is what I ended up doing:
template <int EventType> void register_(EventListener<EventType> Listener)
{
// do stuff with Listener, using more templates
};
It turned out that static polymorphism was better suited for my needs - I just wanted to avoid writing
register_mouse_motion_event(...)
register_keyboard_event(...)
and so on. This approach also nicely eliminates the need for an empty base class.

Using an interface, and telling it all implementors _will also_ inherit from a common base class

Sometimes, you invent an interface to say that a certain type of object supports a certain type of behavior. For example, both Dog and ScaryDog have the MeleeAttacker interface.
But now say I pass a MeleeAttacker* into some function,
void Attack( MeleeAttacker* attacker, Living *victim ) ;
Because MeleeAttacker is separated from the hierarchy, I can't access the vitals of attacker -- I cannot access members of Living for attacker even though I know attacker is a Living. Because of how I constructed the hierarchy, I know that MeleeAttackers will always be inheritors of the common class WarmBlooded. But the code doesn't know that. My question is, how can I tell the code that?
I think I have just confused things by introducing the MeleeAttacker interface, but I didn't want to crowd the base WarmBlooded class with functions that won't be implemented in some branches of the hierarchy (for example, the "implementation" for a Bird's melee attack might be {} (nothing)).

Why not simply cast your argument to a Living * using dyanmic_cast? You know it will, since you guarantee that by virtue of your concrete classes implementing the latter whenever the former is implemented.
Basically the idea is kind-of-like queryInterface in COM, only you don't have to actually implement queryInterface yourself in each class, since you know the cast will work, and you don't have to implement graceful handling in your calling code, since, again, you know the cast will succeed.

Virtual event handlers from several classes: multiple inheritance or composition?

My team has written several C++ classes which implement event handling via pure virtual callbacks - for example, when a message is received from another process, the base class which handles IPC messaging calls its own pure virtual function, and a derived class handles the event in an override of that function. The base class knows the event has occurred; the derived class knows what to do with it.
I now want to combine the features provided by these base classes in a higher-level class, so for example when a message arrives from another process, my new class can then forward it on over its network connection using a similar event-driven networking class. It looks like I have two options:
(1) composition: derive classes from each of the event-handling base classes and add objects of those derived classes to my new class as members, or:
(2) multiple inheritance: make my new class a derived class of all of the event-handling base classes.
I've tried both (1) and (2), and I'm not satisfied with my implementation of either.
There's an extra complication: some of the base classes have been written using initialisation and shutdown methods instead of using constructors and destructors, and of course these methods have the same names in each class. So multiple inheritance causes function name ambiguity. Solvable with using declarations and/or explicit scoping, but not the most maintainable-looking thing I've ever seen.
Even without that problem, using multiple inheritance and overriding every pure virtual function from each of several base classes is going to make my new class very big, bordering on "God Object"-ness. As requirements change (read: "as requirements are added") this isn't going to scale well.
On the other hand, using separate derived classes and adding them as members of my new class means I have to write lots of methods on each derived class to exchange information between them. This feels very much like "getters and setters" - not quite as bad, but there's a lot of "get this information from that class and hand it to this one", which has an inefficient feel to it - lots of extra methods, lots of extra reads and writes, and the classes have to know a lot about each other's logic, which feels wrong. I think a full-blown publish-and-subscribe model would be overkill, but I haven't yet found a simple alternative.
There's also a lot of duplication of data if I use composition. For example, if my class's state depends on whether its network connection is up and running, I have to either have a state flag in every class affected by this, or have every class query the networking class for its state every time a decision needs to be made. If I had just one multiply-inherited class, I could just use a flag which any code in my class could access.
So, multiple inheritance, composition, or perhaps something else entirely? Is there a general rule-of-thumb on how best to approach this kind of thing?

From your description I think you've gone for a "template method" style approach where the base does work and then calls a pure virtual that the derived class implements rather than a "callback interface" approach which is pretty much the same except that the pure virtual method is on a completely separate interface that's passed in to the "base" as a parameter to the constructor. I personally prefer the later as I find it considerably more flexible when the time comes to plug objects together and build higher level objects.
I tend to go for composition with the composing class implementing the callback interfaces that the composed objects require and then potentially composing again in a similar style at a higher level.
You can then decide if it's appropriate to compose by having the composing object implement the callback interfaces and pass them in to the "composed" objects in their constructors OR you can implement the callback interface in its own object possibly with a simpler and more precise callback interface that your composing object implements, and compose both the "base object" and the "callback implementation object"...
Personally I wouldn't go with an "abstract event handling" interface as I prefer my code to be explicit and clear even if that leads to it being slightly less generic.

I'm not totally clear on what your new class is trying to achieve, but it sounds like you're effectively having to provide a new implementation somewhere for all of these abstract event classes.
Personally I would plump for composition. Multiple inheritance quickly becomes a nightmare, especially when things have to change, and composition keeps the existing separation of concerns.
You state that each derived object will have to communicate with the network class, but can you try and reduce this to the minimum. For instance, each derived event object is purely responsible for packaging up the event info into some kind of generic packet, and then that packet is passed to the network class to do the guts of sending?
Without knowing exactly what your new class is doing it's hard to comment, or suggest better patterns, but the more I code, the more I am learning to agree with the old adage "favour composition over inheritance"

Large abstract base classes

I'm writing a large abstract base class with 30 something purely virtual methods*.
Finding all the functions to implement in a base class in the implementation classes is a little tedious, mostly because MSVC++ does not tell you which function you failed to implement with compiler error "Cannot construct abstract class"
So, I'm wondering is my large abstract base class a bad idea, or should I split it up into several interfaces, or is there a compiler warning I can activate that'll tell me WHICH method I failed to provide an implementation for.. or is this just a part of coding with abstract classes and I should get used to it.
*What it does is provide a layer of common functionality between a few different rendering subsystems.

There's no obvious correct answer to this question. Deciding whether to factor apart the base class into multiple abstract base classes should probably be a decision you make based on whether or not the base class logically represents several different concepts, rather than on poor compiler error messages. If the only reason you'd do this is for the compiler error messages, you might want to check and see if you can upgrade the compiler or if there's some other reason to do this. Most modern compilers should provide very nice, detailed errors about this.
Splitting the interface into pieces may be a good idea if your design suggests that you might actually want to have multiple different classes that implement just small pieces of the base class. If you expect to do this, it may be advantageous to factor the interface apart. You will see some added complexity from this, however. For example, if you have a pointer of one interface type to an object implementing multiple interfaces, you may have to do some sort of cross-cast to obtain the correct type, or you may have to introduce a new abstract class that represents something inheriting from all the different interface types. Multiple inheritance with interface classes might also lead to some name collisions, though this typically isn't a problem if the interfaces are designed correctly.
In short, I'd strongly suggest not doing this for the compiler error reason, but if you think it's a good design decision then by all means go for it. Compilers are good enough these days that you rarely (but not never) need to build your design around them.

Interface classes in my opinion are inherently bad, however the question as posed makes this particular application sound suspect.
If you have classes that are deriving from this interface, and it's not clear exactly what functions you need to override, that seems to indicate that all those functions may not be necessary.
When you craft an abstract base class, the number of pure virtual methods isn't important (to me), but it should be clear why every class that derives from this interface has to implement each pure virtual function. If you find yourself thinking "Why do I have to implement this function?", it may be appropriate to break the abstract class up into several distinct interfaces.

Anyway such a big class is a mess, God class antipatern. Use aggregation/composition to split a class and take a look at SOLID development principles, looks like 30 methods for a single class do not follows single responsibility principle, at least... so I wold like recommend to reconsider a design of class. Good luck!

Usually, right after the error "Cannot instantiate abstract class" (which is thrown at the line it was called), if you copied and pasted the interface into the class before writing the implementations, you'll get a linker error "Unresolved external error"pointing to the method you forgot to implement.