I'm having a little circular-dependency problem. It works fine, but it makes for ugly-seeming code. It's in the context of a Snake game.
I have a class, Snake, which contains a vector of SnakeSegments, and manages their interaction (for instance moving and growing as a unit, rather than as separate entities).
When a SnakeSegment collides with a Food object, it flips its hasEaten member to true. The Snake routinely queries the SnakeSegments, essentially for this member. If any of the queries return positive (i.e. one has hit food), then the Snake will grow as a unit (i.e. expand the head and shrink the tail). This is all fine and good, but I'd much prefer a more signal-based approach, where, when a SnakeSegment hits food, it sends an alert (signal, interrupt, etc.) to the Snake class, which tells it to grow. This means I wouldn't have ugly code in my Snake's Update function, checking all the segments; and I would instead have an OnEat() function in my Snake class.
However, this leads to circular dependencies; the Snake contains a vector of SnakeSegments, and the SnakeSegments have a Snake& or Snake* member, which tells them whom to alert when they eat. In the code, I essentially just have to pre-declare the Snake class:
class Snake;
class SnakeSegment
{
...
Snake* alertOnEat;
...
};
and my Snake class just works normally
#include "SnakeSegment.hpp"
class Snake
{
...
std::vector segments;
...
void OnEat();
...
};
Are there any nicer designs to this? Note that it isn't just a problem occuring here; a similar problem occurs in a number of areas (e.g. the GameWorld contains a Snake member, and the Snake alerts the GameWorld when it dies), so a solution specific to Snake and SnakeSegment isn't what I'm looking for.
Your current design is perfectly fine in most situations. Yes, it creates a circular dependency, but it also is the simplest and the clearest way to do it.
However, you should limit those dependencies between interfaces or base classes rather than to the derived classes directly. The world-snake dependency is a good example. You probablly don't necessarily want the World class to know about all the possible game object types. However, what you can do is derive all your game objects from a common GameObject class and make World and GameObject interdependant.
There are also more complicated ways to avoid dependencies all with their pros and cons, they mostly differ on the level of separation and clarity. Among those, function pointers, delegates, observers, listerners, functors, etc.
In the end, it always come down to how much complexity you are ready to introduce to your architecture in exchange of flexibility and clear separation of concerns.
Never forget that design is compromise.
What you may want to look at for this problem is the Observer/Observable design pattern. It allows you to create objects that observe (Snake) observable objects (SnakeSegment) and get notified right away when their state has changed.
Wikipedia has a good example of it written in many languages including C++.
It's a common pattern used in a lot of GUI development so the View layer can change when any of the underlying Model data changes.
Related
In code I have been writing recently I have been forced to directly access a member of an object to call its functions, however, it feels wrong to do this because it would seem to violate encapsulation and the Law of Demeter. Yet the only good alternative I can come up with is to write my own function in the class for every single function of that member I may want to call, which would be very tedious and redundant. Example:
class Object
{
public:
void setNum(int x)
{
num = x;
}
private:
int num;
};
class Object2
{
public:
Object obj;
};
int main()
{
Object2 obj2;
obj2.obj.setNum(5);
}
vs
class Object
{
public:
void setNum(int x)
{
num = x;
}
private:
int num;
};
class Object2
{
public:
void setNum(int x)
{
obj.setNum(x);
}
private:
Object obj;
};
int main()
{
Object2 obj2;
obj2.setNum(5);
}
The call to setNum in Object2 is forwarded to the same function in Object. Is such a design considered bad practice? Is accessing obj directly be any better?
I could also have Object2 inherit from Object, but in this case the class I would be inheriting from is not designed to be a base class, would expose protected members to Object2, and seems unfitting to begin with because it is not an is-a relationship and composition would be preferred.
My specific situation: I am making a game using SFML, there is a class Ship that of course needs a sprite to represent it in the world. Anytime I want to set or get the ship's position, rotation, etc. I have to either directly access its sprite or write a redundant forwarding function in Ship. The issue is that doing either one of those things seems like a code smell: either violate encapsulation and the Law of Demeter, or write redundant code.
What would be considered best practice here? Am I being overly picky about writing simple forwarding functions? Or is there really nothing wrong with directly accessing the sprite of Ship in this case?
This question: C++ Forward method calls to embed object without inheritance in fact is exactly what I'm asking, however, neither answer gives a good solution. One not being condoned and apparently having very poor encapsulation, the other simply using a getter which seems to be a placebo if anything, and no one addresses the main question I have, which is what is the most elegant and acceptable solution?
What solution is the best highly depends on underlying semantics of encapsulation. You need to decouple your code as much as possible. I'll describe that on examples:
You have a Ship class and it has a Sprite. You may want to separate game logic and rendering. So all that Ships knows about the rendering is that it has a Sprite object which handles it. So in this case you are separating responsibilities and that's good. So simple getter is a good solution.
But if absolute coordinates and rotation must be stored in a sprite, then things change: game logic usually needs them two, so they must be set consistently both inside a Ship and a Sprite. The best way to achieve that is to make Ship's setPosition and setRotation methods to set Sprites position and rotation too. That way you simplify the code which works with the Ship at expense of Ship complexity. Given that Ship is manipulated from several places, that's worth it. NOTE: You still may want to expose Sprite through a getter for rendering purposes. And you may want to prevent anybody except a Ship to set Sprite position and rotation, if that does not bloat your code too much.
Let's imagine that Ship class is mostly devoted to rendering. In such situation you may want to hide from outer classes that you use sprites for graphics (because if you change rendering engine it will be good if you won't need to rewrite anything except rendering code). And in such situation you will want to proxy all setPosition and setRotation calls to a Sprite through Ship just to hide existence of the Sprite.
In none of this cases inheritance is used because inheritance means that child is a variation of it's ancestor. You can say that BattleShip is a variant of a Ship. But Ship is not a variant of a Sprite, they are too different and mean different things.
So: if encapsulated class is too specific and should not be visible outside or if it must be operated consistently with a master object, then writing a bunch of proxy methods is a good solution. Otherwise these methods will just bloat your code and it's better to provide a way to get nested object. But in that case I vote for a getter method instead of a public property.
Despite of how the classic javanese oop school can think, never forgot that also DRY (Don't Repeat Yourself) is ALSO considered a good practice.
And while OOP is just one of many programming paradigm C++ can support, DRY is the very essence of all programming since the first very assembler got macros, and this is true long before oop inventors was even far away from their own parent's thoughts and wishes.
For all what my unfair experience is... if respecting a good oop practice force you in writing useless boilerplates of repeating code it means either
The language you are using is fundamentally broken for the purpose you want to achieve, not supporting the paradigm correctly or...
OOP is broken for the purpose you are try to reach. And in C++ chances are that OOP is really the broken paradigm.
To come to your specific problem, delegation (that's how that pattern is commonly named) makes sense if:
the way it is delegating can be changed or
the delegation is to hide part of the member interface.
In you case, you have a function that calls a fixed method reachable from fixed member.
Is that only specific to this particular sample or in your implementation will be always that way by design?
If so, delegation adds no semantic value, if not just reducing a.b.m() to a.m(). If you are writing more than ... let's say three "do nothing just forward" functions you are wasting your time.
If b has 50 methods and you are making it private to delegate only 5 of them, than it makes perfectly sense.
(Refer Update #1 for a concise version of the question.)
We have an (abstract) class named Games that has subclasses, say BasketBall and Hockey (and probably many more to come later).
Another class GameSchedule, must contain a collection GamesCollection of various Games objects. The issue is that we would, at times, like to iterate only through the BasketBall objects of GamesCollection and call functions that are specific to it (and not mentioned in the Games class).
That is, GameSchedule deals with a number of objects that broadly belong to Games class, in the sense that they do have common functions that are being accessed; at the same time, there is more granularity at which they are to be handled.
We would like to come up with a design that avoids unsafe downcasting, and is extensible in the sense that creating many subclasses under Games or any of its existing subclasses must not necessitate the addition of too much code to handle this requirement.
Examples:
A clumsy solution that I came up with, that doesn't do any downcasting at all, is to have dummy functions in the Game class for every subclass specific function that has to be called from GameSchedule. These dummy functions will have an overriding implementation in the appropriate subclasses which actually require its implementation.
We could explicitly maintain different containers for various subclasses of Games instead of a single container. But this would require a lot of extra code in GameSchedule, when the number of subclasses grow. Especially if we need to iterate through all the Games objects.
Is there a neat way of doing this?
Note: the code is written in C++
Update# 1: I realized that the question can be put in a much simpler way. Is it possible to have a container class for any object belonging to a hierarchy of classes? Moreover, this container class must have the ability to pick elements belonging to (or derive from) a particular class from the hierarchy and return an appropriate list.
In the context of the above problem, the container class must have functions like GetCricketGames, GetTestCricketGames, GetBaseballGame etc.,
This is exactly one of the problems that The "Tell, Don't Ask" principle was created for.
You're describing an object that holds onto references to other objects, and wants to ask them what type of object they are before telling them what they need to do. From the article linked above:
The problem is that, as the caller, you should not be making decisions based on the state of the called object that result in you then changing the state of the object. The logic you are implementing is probably the called object’s responsibility, not yours. For you to make decisions outside the object violates its encapsulation.
If you break the rules of encapsulation, you not only introduce the runtime risks incurred by rampant downcasts, but also make your system significantly less maintainable by making it easier for components to become tightly coupled.
Now that that's out there, let's look at how the "Tell, Don't Ask" could be applied to your design problem.
Let's go through your stated constraints (in no particular order):
GameSchedule needs to iterate over all games, performing general operations
GameSchedule needs to iterate over a subset of all games (e.g., Basketball), to perform type-specific operations
No downcasts
Must easily accommodate new Game subclasses
The first step to following the "Tell, Don't Ask" principle is identifying the actions that will take place in the system. This lets us take a step back and evaluate what the system should be doing, without getting bogged down into the details of how it should be doing it.
You made the following comment in #MarkB's answer:
If there's a TestCricket class inheriting from Cricket, and it has many specific attributes concerning the timings of the various innings of the match, and we would like to initialize the values of all TestCricket objects' timing attributes to some preset value, I need a loop that picks all TestCricket objects and calls some function like setInningTimings(int inning_index, Time_Object t)
In this case, the action is: "Initialize the inning timings of all TestCricket games to a preset value."
This is problematic, because the code that wants to perform this initialization is unable to differentiate between TestCricket games, and other games (e.g., Basketball). But maybe it doesn't need to...
Most games have some element of time: Basketball games have time-limited periods, while Baseball games have (basically) innings with basically unlimited time. Each type of game could have its own completely unique configuration. This is not something we want to offload onto a single class.
Instead of asking each game what type of Game it is, and then telling it how to initialize, consider how things would work if the GameSchedule simply told each Game object to initialize. This delegates the responsibility of the initialization to the subclass of Game - the class with literally the most knowledge of what type of game it is.
This can feel really weird at first, because the GameSchedule object is relinquishing control to another object. This is an example of the Hollywood Principle. It's a completely different way of solving problems than the approach most developers initially learn.
This approach deals with the constraints in the following ways:
GameSchedule can iterate over a list of Games without any problem
GameSchedule no longer needs to know the subtypes of its Games
No downcasting is necessary, because the subclasses themselves are handling the subclass-specific logic
When a new subclass is added, no logic needs to be changed anywhere - the subclass itself implements the necessary details (e.g., an InitializeTiming() method).
Edit: Here's an example, as a proof-of-concept.
struct Game
{
std::string m_name;
Game(std::string name)
: m_name(name)
{
}
virtual void Start() = 0;
virtual void InitializeTiming() = 0;
};
// A class to demonstrate a collaborating object
struct PeriodLengthProvider
{
int GetPeriodLength();
}
struct Basketball : Game
{
int m_period_length;
PeriodLengthProvider* m_period_length_provider;
Basketball(PeriodLengthProvider* period_length_provider)
: Game("Basketball")
, m_period_length_provider(period_length_provider)
{
}
void Start() override;
void InitializeTiming() override
{
m_period_length = m_time_provider->GetPeriodLength();
}
};
struct Baseball : Game
{
int m_number_of_innings;
Baseball() : Game("Baseball") { }
void Start() override;
void InitializeTiming() override
{
m_number_of_innings = 9;
}
}
struct GameSchedule
{
std::vector<Game*> m_games;
GameSchedule(std::vector<Game*> games)
: m_games(games)
{
}
void StartGames()
{
for(auto& game : m_games)
{
game->InitializeTiming();
game->Start();
}
}
};
You've already identified the first two options that came to my mind: Make the base class have the methods in question, or maintain separate containers for each game type.
The fact that you don't feel these are appropriate leads me to believe that the "abstract" interface you provide in the Game base class may be far too concrete. I suspect that what you need to do is step back and look at the base interface.
You haven't given any concrete example to help, so I'm going to make one up. Let's say your basketball class has a NextQuarter method and hockey has NextPeriod. Instead, add to the base class a NextGameSegment method, or something that abstracts away the game-specific details. All the game-specific implementation details should be hidden in the child class with only a game-general interface needed by the schedule class.
C# supports reflections and by using the "is" keyword or GetType() member function you could do these easily. If you are writing your code in unmanaged C++, I think the best way to do this is add a GetType() method in your base class (Games?). Which in its turn would return an enum, containing all the classes that derive from it (so you would have to create an enum too) for that. That way you can safely determine the type you are dealing with only through the base type. Below is an example:
enum class GameTypes { Game, Basketball, Football, Hockey };
class Game
{
public:
virtual GameTypes GetType() { return GameTypes::Game; }
}
class BasketBall : public Game
{
public:
GameTypes GetType() { return GameTypes::Basketball; }
}
and you do this for the remaining games (e.g. Football, Hockey). Then you keep a container of Game objects only. As you get the Game object, you call its GetType() method and effectively determine its type.
You're trying to have it all, and you can't do that. :) Either you need to do a downcast, or you'll need to utilize something like the visitor pattern that would then require you to do work every time you create a new implementation of Game. Or you can fundamentally redesign things to eliminate the need to pick the individual Basketballs out of a collection of Games.
And FWIW: downcasting may be ugly, but it's not unsafe as long as you use pointers and check for null:
for(Game* game : allGames)
{
Basketball* bball = dynamic_cast<Basketball*>(game);
if(bball != nullptr)
bball->SetupCourt();
}
I'd use the strategy pattern here.
Each game type has its own scheduling strategy which derives from the common strategy used by your game schedule class and decouples the dependency between the specific game and game schedule.
I'm studying for an exam and am trying to figure this question out. The specific question is "Inheritance and object composition both promote code reuse. (T/F)", but I believe I understand the inheritance portion of the question.
I believe inheritance promotes code reuse because similar methods can be placed in an abstract base class such that the similar methods do not have to be identically implemented within multiple children classes. For example, if you have three kinds of shapes, and each shape's method "getName" simply returns a data member '_name', then why re-implement this method in each of the child classes when it can be implemented once in the abstract base class "shape".
However, my best understanding of object composition is the "has-a" relationship between objects/classes. For example, a student has a school, and a school has a number of students. This can be seen as object composition since they can't really exist without each other (a school without any students isn't exactly a school, is it? etc). But I see no way that these two objects "having" each other as a data member will promote code reuse.
Any help? Thanks!
Object composition can promote code reuse because you can delegate implementation to a different class, and include that class as a member.
Instead of putting all your code in your outermost classes' methods, you can create smaller classes with smaller scopes, and smaller methods, and reuse those classes/methods throughout your code.
class Inner
{
public:
void DoSomething();
};
class Outer1
{
public:
void DoSomethingBig()
{
// We delegate part of the call to inner, allowing us to reuse its code
inner.DoSomething();
// Todo: Do something else interesting here
}
private:
Inner inner;
};
class Outer2
{
public:
void DoSomethingElseThatIsBig()
{
// We used the implementation twice in different classes,
// without copying and pasting the code.
// This is the most basic possible case of reuse
inner.DoSomething();
// Todo: Do something else interesting here
}
private:
Inner inner;
};
As you mentioned in your question, this is one of the two most basic Object Oriented Programming principals, called a "has-a relationship". Inheritance is the other relationship, and is called an "is-a replationship".
You can also combine inheritance and composition in quite useful ways that will often multiply your code (and design) reuse potential. Any real world and well-architected application will constantly combine both of these concepts to gain as much reuse as possible. You'll find out about this when you learn about Design Patterns.
Edit:
Per Mike's request in the comments, a less abstract example:
// Assume the person class exists
#include<list>
class Bus
{
public:
void Board(Person newOccupant);
std::list<Person>& GetOccupants();
private:
std::list<Person> occupants;
};
In this example, instead of re-implementing a linked list structure, you've delegated it to a list class. Every time you use that list class, you're re-using the code that implements the list.
In fact, since list semantics are so common, the C++ standard library gave you std::list, and you just had to reuse it.
1) The student knows about a school, but this is not really a HAS-A relationship; while you would want to keep track of what school the student attends, it would not be logical to describe the school as being part of the student.
2) More people occupy the school than just students. That's where the reuse comes in. You don't have to re-define the things that make up a school each time you describe a new type of school-attendee.
I have to agree with #Karl Knechtel -- this is a pretty poor question. As he said, it's hard to explain why, but I'll give it a shot.
The first problem is that it uses a term without defining it -- and "code reuse" means a lot of different things to different people. To some people, cutting and pasting qualifies as code reuse. As little as I like it, I have to agree with them, to at least some degree. Other people define cod reuse in ways that rule out cutting and pasting as being code reuse (classing another copy of the same code as separate code, not reusing the same code). I can see that viewpoint too, though I tend to think their definition is intended more to serve a specific end than be really meaningful (i.e., "code reuse"->good, "cut-n-paste"->bad, therefore "cut-n-paste"!="code reuse"). Unfortunately, what we're looking at here is right on the border, where you need a very specific definition of what code reuse means before you can answer the question.
The definition used by your professor is likely to depend heavily upon the degree of enthusiasm he has for OOP -- especially during the '90s (or so) when OOP was just becoming mainstream, many people chose to define it in ways that only included the cool new OOP "stuff". To achieve the nirvana of code reuse, you had to not only sign up for their OOP religion, but really believe in it! Something as mundane as composition couldn't possibly qualify -- no matter how strangely they had to twist the language for that to be true.
As a second major point, after decades of use of OOP, a few people have done some fairly careful studies of what code got reused and what didn't. Most that I've seen have reached a fairly simple conclusion: it's quite difficult (i.e., essentially impossible) correlate coding style with reuse. Nearly any rule you attempt to make about what will or won't result in code reuse can and will be violated on a regular basis.
Third, and what I suspect tends to be foremost in many people's minds is the fact that asking the question at all makes it sound as if this is something that can/will affect a typical coder -- that you might want to choose between composition and inheritance (for example) based on which "promotes code reuse" more, or something on that order. The reality is that (just for example) you should choose between composition and inheritance primarily based upon which more accurately models the problem you're trying to solve and which does more to help you solve that problem.
Though I don't have any serious studies to support the contention, I would posit that the chances of that code being reused will depend heavily upon a couple of factors that are rarely even considered in most studies: 1) how similar of a problem somebody else needs to solve, and 2) whether they believe it will be easier to adapt your code to their problem than to write new code.
I should add that in some of the studies I've seen, there were factors found that seemed to affect code reuse. To the best of my recollection, the one that stuck out as being the most important/telling was not the code itself at all, but the documentation available for that code. Being able to use the code without basically reverse engineer it contributes a great deal toward its being reused. The second point was simply the quality of the code -- a number of the studies were done in places/situations where they were trying to promote code reuse. In a fair number of cases, people tried to reuse quite a bit more code than they really did, but had to give up on it simply because the code wasn't good enough -- everything from bugs to clumsy interfaces to poor portability prevented reuse.
Summary: I'll go on record as saying that code reuse has probably been the most overhyped, under-delivered promise in software engineering over at least the last couple of decades. Even at best, code reuse remains a fairly elusive goal. Trying to simplify it to the point of treating it as a true/false question based on two factors is oversimplifying the question to the point that it's not only meaningless, but utterly ridiculous. It appears to trivialize and demean nearly the entire practice of software engineering.
I have an object Car and an object Engine:
class Engine {
int horsepower;
}
class Car {
string make;
Engine cars_engine;
}
A Car has an Engine; this is composition. However, I don't need to redefine Engine to put an engine in a car -- I simply say that a Car has an Engine. Thus, composition does indeed promote code reuse.
Object composition does promote code re-use. Without object composition, if I understand your definition of it properly, every class could have only primitive data members, which would be beyond awful.
Consider the classes
class Vector3
{
double x, y, z;
double vectorNorm;
}
class Object
{
Vector3 position;
Vector3 velocity;
Vector3 acceleration;
}
Without object composition, you would be forced to have something like
class Object
{
double positionX, positionY, positionZ, positionVectorNorm;
double velocityX, velocityY, velocityZ, velocityVectorNorm;
double accelerationX, accelerationY, accelerationZ, accelerationVectorNorm;
}
This is just a very simple example, but I hope you can see how even the most basic object composition promotes code reuse. Now think about what would happen if Vector3 contained 30 data members. Does this answer your question?
The more I get into writing unit tests the more often I find myself writing smaller and smaller classes. The classes are so small now that many of them have only one public method on them that is tied to an interface. The tests then go directly against that public method and are fairly small (sometimes that public method will call out to internal private methods within the class). I then use an IOC container to manage the instantiation of these lightweight classes because there are so many of them.
Is this typical of trying to do things in a more of a TDD manner? I fear that I have now refactored a legacy 3,000 line class that had one method in it into something that is also difficult to maintain on the other side of the spectrum because there is now literally about 100 different class files.
Is what I am doing going too far? I am trying to follow the single responsibility principle with this approach but I may be treading into something that is an anemic class structure where I do not have very intelligent "business objects".
This multitude of small classes would drive me nuts. With this design style it becomes really hard to figure out where the real work gets done. I am not a fan of having a ton of interfaces each with a corresponding implementation class, either. Having lots of "IWidget" and "WidgetImpl" pairings is a code smell in my book.
Breaking up a 3,000 line class into smaller pieces is great and commendable. Remember the goal, though: it's to make the code easier to read and easier to work with. If you end up with 30 classes and interfaces you've likely just created a different type of monster. Now you have a really complicated class design. It takes a lot of mental effort to keep that many classes straight in your head. And with lots of small classes you lose the very useful ability to open up a couple of key files, pick out the most important methods, and get an idea of what the heck is going on.
For what it's worth, though, I'm not really sold on test-driven design. Writing tests early, that's sensible. But reorganizing and restructuring your class design so it can be more easily unit tested? No thanks. I'll make interfaces only if they make architectural sense, not because I need to be able to mock up some objects so I can test my classes. That's putting the cart before the horse.
You might have gone a bit too far if you are asking this question. Having only one public method in a class isn't bad as such, if that class has a clear responsibility/function and encapsulates all logic concerning that function, even if most of it is in private methods.
When refactoring such legacy code, I usually try to identify the components in play at a high level that can be assigned distinct roles/responsibilities and separate them into their own classes. I think about which functions should be which components's responsibility and move the methods into that class.
You write a class so that instances of the class maintain state. You put this state in a class because all the state in the class is related.You have function to managed this state so that invalid permutations of state can't be set (the infamous square that has members width and height, but if width doesn't equal height it's not really a square.)
If you don't have state, you don't need a class, you could just use free functions (or in Java, static functions).
So, the question isn't "should I have one function?" but rather "what state-ful entity does my class encapsulate?"
Maybe you have one function that sets all state -- and you should make it more granular, so that, e.g., instead of having void Rectangle::setWidthAndHeight( int x, int y) you should have a setWidth and a separate setHeight.
Perhaps you have a ctor that sets things up, and a single function that doesIt, whatever "it" is. Then you have a functor, and a single doIt might make sense. E.g., class Add implements Operation { Add( int howmuch); Operand doIt(Operand rhs);}
(But then you may find that you really want something like the Visitor Pattern -- a pure functor is more likely if you have purely value objects, Visitor if they're arranged in a tree and are related to each other.)
Even if having these many small objects, single-function is the correct level of granularity, you may want something like a facade Pattern, to compose out of primitive operations, often-used complex operations.
There's no one answer. If you really have a bunch of functors, it's cool. If you're really just making each free function a class, it's foolish.
The real answer lies in answering the question, "what state am I managing, and how well do my classes model my problem domain?"
I'd be speculating if I gave a definite answer without looking at the code.
However it sounds like you're concerned and that is a definite flag for reviewing the code. The short answer to your question comes back to the definition of Simple Design. Minimal number of classes and methods is one of them. If you feel like you can take away some elements without losing the other desirable attributes, go ahead and collapse/inline them.
Some pointers to help you decide:
Do you have a good check for "Single Responsibility" ? It's deceptively difficult to get it right but is a key skill (I still don't see it like the masters). It doesn't necessarily translate to one method-classes. A good yardstick is 5-7 public methods per class. Each class could have 0-5 collaborators. Also to validate against SRP, ask the question what can drive a change into this class ? If there are multiple unrelated answers (e.g. change in the packet structure (parsing) + change in the packet contents to action map (command dispatcher) ) , maybe the class needs to be split. On the other end, if you feel that a change in the packet structure, can affect 4 different classes - you've run off the other cliff; maybe you need to combine them into a cohesive class.
If you have trouble naming the concrete implementations, maybe you don't need the interface. e.g. XXXImpl classes implmenting XXX need to be looked at. I recently learned of a naming convention, where the interface describes a Role and the implementation is named by the technology used to implement the role (or falling back to what it does). e.g. XmppAuction implements Auction (or SniperNotifier implements AuctionEventListener)
Lastly are you finding it difficult to add / modify / test existing code (e.g. test setup is long or painful ) ? Those can be signs that you need to go refactoring.
I've got a game engine where I'm splitting off the physics simulation from the game object functionality. So I've got a pure virtual class for a physical body
class Body
from which I'll be deriving various implementations of a physics simulation. My game object class then looks like
class GameObject {
public:
// ...
private:
Body *m_pBody;
};
and I can plug in whatever implementation I need for that particular game. But I may need access to all of the Body functions when I've only got a GameObject. So I've found myself writing tons of things like
Vector GameObject::GetPosition() const { return m_pBody->GetPosition(); }
I'm tempted to scratch all of them and just do stuff like
pObject->GetBody()->GetPosition();
but this seems wrong (i.e. violates the Law of Demeter). Plus, it simply pushes the verbosity from the implementation to the usage. So I'm looking for a different way of doing this.
The idea of the law of Demeter is that your GameObject isn't supposed to have functions like GetPosition(). Instead it's supposed to have MoveForward(int) or TurnLeft() functions that may call GetPosition() (along with other functions) internally. Essentially they translate one interface into another.
If your logic requires a GetPosition() function, then it makes sense turn that into an interface a la Ates Goral. Otherwise you'll need to rethink why you're grabbing so deeply into an object to call methods on its subobjects.
One approach you could take is to split the Body interface into multiple interfaces, each with a different purpose and give GameObject ownership of only the interfaces that it would have to expose.
class Positionable;
class Movable;
class Collidable;
//etc.
The concrete Body implementations would probably implement all interfaces but a GameObject that only needs to expose its position would only reference (through dependency injection) a Positionable interface:
class BodyA : public Positionable, Movable, Collidable {
// ...
};
class GameObjectA {
private:
Positionable *m_p;
public:
GameObjectA(Positionable *p) { m_p = p; }
Positionable *getPosition() { return m_p; }
};
BodyA bodyA;
GameObjectA objA(&bodyA);
objA->getPosition()->getX();
Game hierarchies should not involve a lot of inheritance. I can't point you to any web pages, but that is the feeling I've gather from the several sources, most notably the game gem series.
You can have hierarchies like ship->tie_fighter, ship->x_wing. But not PlaysSound->tie_fighter. Your tie_fighter class should be composed of the objects it needs to represent itself. A physics part, a graphics part, etc. You should provide a minimal interface for interacting with your game objects. Implement as much physics logic in the engine or in the physic piece.
With this approach your game objects become collections of more basic game components.
All that said, you will want to be able to set a game objects physical state during game events. So you'll end up with problem you described for setting the various pieces of state. It's just icky but that is best solution I've found so far.
I've recently tried to make higher level state functions, using ideas from Box2D. Have a function SetXForm for setting positions etc. Another for SetDXForm for velocities and angular velocity. These functions take proxy objects as parameters that represent the various parts of the physical state. Using methods like these you could reduce the number of methods you'd need to set state but in the end you'd probably still end up implementing the finer grained ones, and the proxy objects would be more work than you would save by skipping out on a few methods.
So, I didn't help that much. This was more a rebuttal of the previous answer.
In summary, I would recommend you stick with the many method approach. There may not always be a simple one to 1 relationship between game objects and physic objects. We ran into that where it was much simpler to have one game object represent all of the particles from an explosion. If we had given in and just exposed a body pointer, we would not have been able to simplify the problem.
Do I understand correctly that you're separating the physics of something from it's game representation?
i.e, would you see something like this:
class CompanionCube
{
private:
Body* m_pPhysicsBody;
};
?
If so, that smells wrong to me. Technically your 'GameObject' is a physics object, so it should derive from Body.
It sounds like you're planning on swapping physics models around and that's why you're attempting to do it via aggregation, and if that's the case, I'd ask: "Do you plan on swapping physics types at runtime, or compile time?".
If compile time is your answer, I'd derive your game objects from Body, and make Body a typedef to whichever physics body you want to have be the default.
If it's runtime, you'd have to write a 'Body' class that does that switching internally, which might not be a bad idea if your goal is to play around with different physics.
Alternatively, you'll probably find you'll have different 'parent' classes for Body depending on the type of game object (water, rigid body, etc), so you could just make that explicit in your derivation.
Anyhow, I'll stop rambling since this answer is based on a lot of guesswork. ;) Let me know if I'm off base, and I'll delete my answer.