ok, after last night I've decided to rephrase my question for easier reading.
I have 2 classes, Army and Battle groups. heres the classes below, albeit with some parts missing:
class Battlegroups
{
private: battlegroup battlegroupobject[100];
public:
void AddBattleGroup(); //add a battlegroup object to the array
void removebattlegroup(); //remove a battle group objects from the array
};
class Army
{
private: battlegroups battlegroupsobject;
public:
void formbattlegroup()
{
battlegroupsobject.AddBattleGroup();
}
void disbandbattlegroup()
{
battlegroupsobject.removebattlegroup();
}
};
See, the problem is the formbattlegroup() and the disbandbattlegroup(), which seems like its adding a pointless interface.
if there is a way for me to access the Addbattlegroup and removebattlegroup() methods without doing the above, please tell me.
or, tell me if this is the way you do it, but to me, it seems like adding code, for the sake of code.
If Battlegroups is used only for containing objects, you may use battlegroup battlegroupobject[100] right in the Army instead. Or use std::vector<battlegroup> instead of battlegroups. If not (and anyway), your decision seems quite correct, because there are different responsibilities of these objects.
not very sure about your design, but here is my guess: you want to have Army, Navy etc. Then you may want to inherit Army, Navy from Battlegroups, such that you don't need to write AddBattleGroup and RemoveBattleGroup for every classes for Army, Navy etc . Like the following:
class Battlegroups
{
// side-note: you may want to replaced below by std::vector<battlegroup>
private: battlegroup battlegroupobject[100];
public:
void AddBattleGroup();
void RemoveBattleGroup();
};
class Army
: public Battlegroups
{
// other specific implementations
};
class Navy
: public Battlegroups
{
// other specific implementations
};
// usage
Army army1;
army1.AddBattleGroup();
Navy navy1;
navy1.AddBattleGroup();
If this is not what you want, you may want to consider Dmitry's answer.
Related
i'm creating particle system and i want to have possibility to choose what kind of object will be showing on the screen (like simply pixels, or circle shapes). I have one class in which all parameters are stored (ParticleSettings), but without those entities that stores points, or circle shapes, etc. I thought that i may create pure virtual class (ParticlesInterface) as a base class, and its derived classes like ParticlesVertex, or ParticlesCircles for storing those drawable objects. It is something like that:
class ParticlesInterface
{
protected:
std::vector<ParticleSettings> m_particleAttributes;
public:
ParticlesInterface(long int amount = 100, sf::Vector2f position = { 0.0,0.0 });
const std::vector<ParticleSettings>& getParticleAttributes() { return m_particleAttributes; }
...
}
and :
class ParticlesVertex : public ParticlesInterface
{
private:
std::vector<sf::Vertex> m_particleVertex;
public:
ParticlesVertex(long int amount = 100, sf::Vector2f position = { 0.0,0.0 });
std::vector<sf::Vertex>& getParticleVertex() { return m_particleVertex; }
...
}
So... I know that i do not have access to getParticleVertex() method by using polimorphism. And I really want to have that access. I want to ask if there is any better solution for that. I have really bad times with decide how to connect all that together. I mean i was thinking also about using template classes but i need it to be dynamic binding not static. I thought that this idea of polimorphism will be okay, but i'm really need to have access to that method in that option. Can you please help me how it should be done? I want to know what is the best approach here, and also if there is any good answer to that problem i have if i decide to make that this way that i show you above.
From the sounds of it, the ParticlesInterface abstract class doesn't just have a virtual getParticleVertex because that doesn't make sense in general, only for the specific type ParticlesVertex, or maybe a group of related types.
The recommended approach here is: Any time you need code that does different things depending on the actual concrete type, make those "different things" a virtual function in the interface.
So starting from:
void GraphicsDriver::drawUpdate(ParticlesInterface &particles) {
if (auto* vparticles = dynamic_cast<ParticlesVertex*>(&particles)) {
for (sf::Vertex v : vparticles->getParticleVertex()) {
draw_one_vertex(v, getCanvas());
}
} else if (auto* cparticles = dynamic_cast<ParticlesCircle*>(&particles)) {
for (CircleWidget& c : cparticles->getParticleCircles()) {
draw_one_circle(c, getCanvas());
}
}
// else ... ?
}
(CircleWidget is made up. I'm not familiar with sf, but that's not the point here.)
Since getParticleVertex doesn't make sense for every kind of ParticleInterface, any code that would use it from the interface will necessarily have some sort of if-like check, and a dynamic_cast to get the actual data. The drawUpdate above also isn't extensible if more types are ever needed. Even if there's a generic else which "should" handle everything else, the fact one type needed something custom hints that some other future type or a change to an existing type might want its own custom behavior at that point too. Instead, change from a thing code does with the interface to a thing the interface can be asked to do:
class ParticlesInterface {
// ...
public:
virtual void drawUpdate(CanvasWidget& canvas) = 0;
// ...
};
class ParticlesVertex {
// ...
void drawUpdate(CanvasWidget& canvas) override;
// ...
};
class ParticlesCircle {
// ...
void drawUpdate(CanvasWidget& canvas) override;
// ...
};
Now the particles classes are more "alive" - they actively do things, rather than just being acted on.
For another example, say you find ParticlesCircle, but not ParticlesVertex, needs to make some member data updates whenever the coordinates are changed. You could add a virtual void coordChangeCB() {} to ParticlesInterface and call it after each motion model tick or whenever. With the {} empty definition in the interface class, any class like ParticlesVertex that doesn't care about that callback doesn't need to override it.
Do try to keep the interface's virtual functions simple in intent, following the Single Responsibility Principle. If you can't write in a sentence or two what the purpose or expected behavior of the function is in general, it might be too complicated, and maybe it could more easily be thought of in smaller steps. Or if you find the virtual overrides in multiple classes have similar patterns, maybe some smaller pieces within those implementations could be meaningful virtual functions; and the larger function might or might not stay virtual, depending on whether what remains can be considered really universal for the interface.
(Programming best practices are advice, backed by good reasons, but not absolute laws: I'm not going to say "NEVER use dynamic_cast". Sometimes for various reasons it can make sense to break the rules.)
Suppose I have a class structure like (simplifying the actual classes I have):
class Graph
{
};
class DerivedGraph : public Graph
{
};
class DerivedGraph2 : public Graph
{
};
I want to expand this structure to account for different variations of the same graph. Ideally I would like to be able to do something like:
class Graph
{
};
// Removed
//class DerivedGraph : public Graph
//{
//};
// Removed
//class DerivedGraph2 : public Graph
//{
//};
class DerivedGraph3 : public Graph // really just a mode of DerivedGraph
{
};
class DerivedGraph4 : public Graph // really just a second mode of DerivedGraph
{
};
class DerivedGraph5 : public Graph // really just a mode of DerivedGraph2
{
};
class DerivedGraph6 : public Graph // really just a second mode of DerivedGraph2
{
};
But you can quickly see the problem here -- I am having to create too many classes here. Also, the base class is extremely complex and large (the bottom line is that it just plain sucks) ... so I don't want to make too many structural changes. I want the flexibility of defining things at the level of just the graph itself but at the same time have the flexibility of defining things for a particular mode of one graph type. I would like to be able to use virtual functions such as DoesGraphSupportNormalizedData() or something like that (this is just a simple example). Each class would then override this method.
Another idea I had was to create a separate class structure for the modes themselves (the Graph class would create an instance of it), like:
class BaseMode
{
};
class Mode1 : public BaseMode
{
};
class Mode2 : public BaseMode
{
};
Now the problem is that these mode classes need access to several pieces of data from the Graph class ... and I really don't want to pass all of that information. The mode class would then become just as useless and wouldn't be flexible at all. I just can't think of a clean way to deal with this. The best I could come up with is to have the mode classes do what it can without having to pass all kinds of crap to it but now the interface is just goofy and awkward. Any ideas?
You can either user and interface or use inherited classes from what I can gather from your description.
If you use a base-class and inherit off of it just have the things you don't want derived classes to have just give them the private access modifier and then protected or public for the others (depending on the situation of course). That way your derived classes only take what information they need. You could also have a instance variable that needs to be set in each of lower classes to define things about each derived class. Access modifiers are your friends.
If you use an interface just include everything each graph will need and then when building the individual classes just customize them from there to include the specialties.
If it were up to me, personally, I would go with inheritance over an interface but that's just me.
I ran in this kind of a problem before (and still now and then...)
In this case, you may be taking it the wrong way, what you're looking into is device a specialized function depending on the type of graph and mode. Inheritance is nice, but it has its limits as you mentioned. Especially because the user may want to switch the type of graph, but keep is existing graph object. Inheritance is not helpful in that case.
One way to do something like this is to create functions that get called depending on the current type and mode. Say you have to draw lines and the mode can be set to LINE or DOTS. You could have two functions that draw a line and are specific to a mode or another:
void Graph::draw_line_line(line l)
{
// draw a line
}
void Graph::draw_line_dots(line l)
{
// draw a dots along the line
}
Now you can define a type which represents that type of render functions and a variable member for it:
typedef void (Graph::*draw_line_func)(line l);
draw_line_func m_draw_line;
With that in hands, you can program your set_mode() function, something like this:
void Graph::set_mode(mode_t mode)
{
m_mode = mode; // save for get_mode() to work
switch(mode)
{
case LINE:
m_draw_line = &Draw::draw_line_line;
break;
case DOTS:
m_draw_line = &Draw::draw_line_dots;
break;
...
}
}
Now when you want to render the line, you do call this specialized function and you do not need to know whether it is a LINE or a DOTS...
void Graph::draw_line(line l)
{
this->*m_draw_line(l);
}
This way you create an indirection and make it a lot cleaner in the existing large functions that have large switch or many if() statements without breaking up the existing "powerful" class in many pieces that may become hard to use (because if it's that big it's probably already in use...)
One important and essential rule I have learnt as a C++ programmer is the preference of Composition over Inheritance (http://en.wikipedia.org/wiki/Composition_over_inheritance).
I totally agree with this rule which mostly makes things much more simple than It would be if we used Inheritance.
I have a problem which should be solved using Composition but I'm really struggling to do so.
Suppose you have a Vendor Machine, and you have two types of products:
Discrete product - like a snack.
Fluid product - like a drink.
These two types of products will need to be represented in a class called VendorCell which contains the cell content.
These two products share some identical attributes(dm) as Price, Quantity and so on...BUT also contain some different attributes.
Therefore using Composition here might lead for the following result:
class VendorCell {
private : // default access modifier
int price;
int quantity;
// int firstProductAttributeOnly
// char secondProductAttributeOnly
};
As you can see the commented lines show that for a single VendorCell depending on the product it containing, only one of these two commented lines will be significant and useable (the other one is only relevant for the other type - fluid for example).
Therefore I might have a VendorCell with a snack inside and its secondProductAttributeOnly is not needed.
Is composition (for the VendorCell) is the right solution? is It seems proper to you guys that someone will determine the VendorCell type via a constructor and one DM (the DM dedicated for the other type) will not be used at all (mark it as -1 for example) ?>
Thanks you all!
Your general rule of favoring composition over inheritance is right. The problem here is that you want a container of polymorphic objects, not a giant aggregate class that can hold all possible products. However, because of the slicing problem, you can't hold polymorphic objects directly, but you need to hold them by (preferably smart) pointer. You can hold them directly by (smart) pointer such as
class AbstractProduct { /* price, quauntity interface */ };
class AbstractSnack: public AbstractProduct { /* extended interface */ };
class AbstractDrink: public AbstractProduct { /* extended interface */ };
typedef std::unique_ptr<AbstractProduct> VendorCell;
typedef std::vector< VendorCell > VendorMachine;
You simply define your snacks/drinks by deriving from AbstractSnack/AbstractDrink
class SnickersBar: public AbstractSnack { /* your implementation */ };
class CocaColaBottle: public AbstractDrink { /* your implementation */ };
and then you can insert or extract products like this:
// fill the machine
VendorMachine my_machine;
my_machine.emplace_back(new SnickersBar());
my_machine.emplace_back(new CocaColaBottle());
my_snack = my_machine[0]; // get a Snickers bar
my_drink = my_machine[1]; // get a Coca Cola bottle;
There are also other solutions such as Boost.Any that uses a wrapper class that internally holds a pointer to a polymorphic object. You could also refactor this code by replacing the typedef with a separate class VendorMachine that holds a std::vector< VendorCell >, so that you can get a nicer interface (with money exchange functionality e.g.)
You inherit in order to be re-used.
You compose in order to re-use.
If you have different attributes then you probably want to inherit, otherwise compose.
Some variation:
class ProductVariety {
public:
virtual void display(Screen& screen) = 0;
};
An implementation:
class Liquid : public ProductVariety {
public:
virtual void display(Screen& screen) {
//...
}
}
Composing variation:
class Product
{
int price;
int quantity;
unique_ptr<ProductVariety> variety;
}
I have a C background and am a newb on C++. I have a basic design question. I have a class (I'll call it "chef" b/c the problem I have seems very analogous to this, both in terms of complexity and issues) that basically works like this
class chef
{
public:
void prep();
void cook();
void plate();
private:
char name;
char dish_responsible_for;
int shift_working;
etc...
}
in pseudo code, this gets implemented along the lines of:
int main{
chef my_chef;
kitchen_class kitchen;
for (day=0; day < 365; day++)
{
kitchen.opens();
....
my_chef.prep();
my_chef.cook();
my_chef.plate();
....
kitchen.closes();
}
}
The chef class here seems to be a monster class, and has the potential of becoming one. chef also seems to violate the single responsibility principle, so instead we should have something like:
class employee
{
protected:
char name;
int shift_working;
}
class kitchen_worker : employee
{
protected:
dish_responsible_for;
}
class cook_food : kitchen_worker
{
public:
void cook();
etc...
}
class prep_food : kitchen_worker
{
public:
void prep();
etc...
}
and
class plater : kitchen_worker
{
public:
void plate();
}
etc...
I'm admittedly still struggling with how to implement it at run time so that, if for example plater (or "chef in his capacity as plater") decides to go home midway through dinner service, then the chef has to work a new shift.
This seems to be related to a broader question I have that if the same person invariably does the prepping, cooking and plating in this example, what is the real practical advantage of having this hierarchy of classes to model what a single chef does? I guess that runs into the "fear of adding classes" thing, but at the same time, right now or in the foreseeable future I don't think maintaining the chef class in its entirety is terribly cumbersome. I also think that it's in a very real sense easier for a naive reader of the code to see the three different methods in the chef object and move on.
I understand it might threaten to become unwieldy when/if we add methods like "cut_onions()", "cut_carrots()", etc..., perhaps each with their own data, but it seems those can be dealt with by having making the prep() function, say, more modular. Moreover, it seems that the SRP taken to its logical conclusion would create a class "onion_cutters" "carrot_cutters" etc... and I still have a hard time seeing the value of that, given that somehow the program has to make sure that the same employee cuts the onions and the carrots which helps with keeping the state variable the same across methods (e.g., if the employee cuts his finger cutting onions he is no longer eligible to cut carrots), whereas in the monster object chef class it seems that all that gets taken care of.
Of course, I understand that this then becomes less about having a meaningful "object oriented design", but it seems to me that if we have to have separate objects for each of the chef's tasks (which seems unnatural, given that the same person is doing all three function) then that seems to prioritize software design over the conceptual model. I feel an object oriented design is helpful here if we want to have, say, "meat_chef" "sous_chef" "three_star_chef" that are likely different people. Moreover, related to the runtime problem is that there is an overhead in complexity it seems, under the strict application of the single responsibility principle, that has to make sure the underlying data that make up the base class employee get changed and that this change is reflected in subsequent time steps.
I'm therefore rather tempted to leave it more or less as is. If somebody could clarify why this would be a bad idea (and if you have suggestions on how best to proceed) I'd be most obliged.
To avoid abusing class heirarchies now and in future, you should really only use it when an is relationship is present. As yourself, "is cook_food a kitchen_worker". It obviously doesn't make sense in real life, and doesn't in code either. "cook_food" is an action, so it might make sense to create an action class, and subclass that instead.
Having a new class just to add new methods like cook() and prep() isn't really an improvement on the original problem anyway - since all you've done is wrapped the method inside a class. What you really wanted was to make an abstraction to do any of these actions - so back to the action class.
class action {
public:
virtual void perform_action()=0;
}
class cook_food : public action {
public:
virtual void perform_action() {
//do cooking;
}
}
A chef can then be given a list of actions to perform in the order you specify. Say for example, a queue.
class chef {
...
perform_actions(queue<action>& actions) {
for (action &a : actions) {
a.perform_action();
}
}
...
}
This is more commonly known as the Strategy Pattern. It promotes the open/closed principle, by allowing you to add new actions without modifying your existing classes.
An alternative approach you could use is a Template Method, where you specify a sequence of abstract steps, and use subclasses to implement the specific behaviour for each one.
class dish_maker {
protected:
virtual void prep() = 0;
virtual void cook() = 0;
virtual void plate() = 0;
public:
void make_dish() {
prep();
cook();
plate();
}
}
class onion_soup_dish_maker : public dish_maker {
protected:
virtual void prep() { ... }
virtual void cook() { ... }
virtual void plate() { ... }
}
Another closely related pattern which might be suitable for this is the Builder Pattern
These patterns can also reduce of the Sequential Coupling anti-pattern, as it's all too easy to forget to call some methods, or call them in the right order, particularly if you're doing it multiple times. You could also consider putting your kitchen.opens() and closes() into a similar template method, than you don't need to worry about closes() being called.
On creating individual classes for onion_cutter and carrot_cutter, this isn't really the logical conclusion of the SRP, but in fact a violation of it - because you're making classes which are responsible for cutting, and holding some information about what they're cutting. Both cutting onions and carrots can be abstracted into a single cutting action - and you can specify which object to cut, and add a redirection to each individual class if you need specific code for each object.
One step would be to create an abstraction to say something is cuttable. The is relationship for subclassing is candidate, since a carrot is cuttable.
class cuttable {
public:
virtual void cut()=0;
}
class carrot : public cuttable {
public:
virtual void cut() {
//specific code for cutting a carrot;
}
}
The cutting action can take a cuttable object and perform any common cutting action that's applicable to all cuttables, and can also apply the specific cut behaviour of each object.
class cutting_action : public action {
private:
cuttable* object;
public:
cutting_action(cuttable* obj) : object(obj) { }
virtual void perform_action() {
//common cutting code
object->cut(); //specific cutting code
}
}
So i need to design something like that :
I have a spreadsheet which can contain numerous sheets (maybe of different kinds).
each sheet has let's say 1A-9Z cells.
in each cell i can have one the above :
String,
Number,
Formula - which means the cell gets an operation like +,-,/,* etc... and cells numbers,
and in the cell i have the result of the operation.
I need to design the classes, so in the future i can add other type of cells (besides string/number/formula) and add also different kind of operation to the formula - all in an easy manner.
how would u design it ?
I though about something like this :
class SpreadSheet
{
private:
vector<Isheet> sheets;
public:
write(Isheet sheet,int CellNum,ICell value);
GetValue(Isheet sheet,int CellNum,ICell value);
AddSheet(ISheet sheet);
};
class Isheet
{
vector<ICell> cells; // can i do something like that ? cause ICell is a template
};
template<class T>
class ICell
{
Vector<Iobserver> observers;
public:
T GetValue() {return m_value;};
SetValue(T val) {m_value=val;};
AddObserver(Iobserver obs);
NotifyAll();
GetPos() {return m_pos;};
private:
T m_value;
int m_pos;
};
class CInt : public ICell<int>
{
};
class CString : public ICell<std:string>
{
};
class CFormula : public ICell<int>, Iobserver
{
};
class Iobserver
{
Update(int pos);
};
anyway, i'm really not sure where should i create the cell Concrete classes (CInt,Cstring,CFormula), should i use some kind of factory ? where to put the factory ? inside ISheet ?
and my main concern, where should i calculate the the right result for a CFormula cell ?
i used the observer pattern to keep the formula cell updated in case of other cell's changes.
any advices would be great
As an engineer to whom 'design pattern' means very little, I recently had the pleasure of working on a project (ongoing) with some computer scientists. I have implemented all of the things they call 'observer patterns', 'factories', 'reactors', etc before, but to be honest I found it a thoroughly unhelpful way to think about design.
What you need is a solution to your problem. Think about: data flow, think about execution paths, think about what needs to cause what.
Don't try to shoe-horn your problem into a textbook solution, likelihood is, it won't fit very well, and you'll end up with far more levels of abstraction than are useful, and which make the code very difficult to understand when you return to it months later.
My two cents, hope this is helpful.