I am building a racing game in OpenGL using Glut, and I'm a bit lost in the details. First of all, any suggestions or a road map would be more than great.
So far what I am thinking is this:
Tree implementation for transformations.
Simulated dynamics.(*)
Octree implementation for collision detection.
Actual collision detection.(*)
Modelling in Maya and export them as .OBJs.
Polishing the game with GLSL or something like that for graphics quality.
(*): I am not sure the order of these two.
So I started with the simulated dynamics without a tree implementation, and it turned out to be a huge chaos for me. Is there any way you can think of something that could help me to build such a tree to use in racing game?
I thought of something like this but I have no idea how to implement it.
Reds are static, yellows are dynamic nodes
I would suggest the exact opposite of #bezad.
Start with a single car and an infinite road.
Split the problem of rendering and dynamics into two completely different things. The common Car updates and/or is the link between the CarRenderModel and the CarPhysicsModel.
What shapes the Car puts into the GL scene is up to the Car.
Among other things, this means you can have a really simple Car show up on screen, and attach a really simple physics model to it, and either make the Car prettier or make it behave physically better without having to tie the two together. And, ideally, at each stage you have something playable.
So, a car that is a rectangle, 5 long 3 wide and 1 unit high. A road that is 13 units wide, and goes on forever. A stationary camera. Maybe a horizon. The first physics model is a rocket ship, where every second you push down on an arrow key the car gains x units/second of velocity in that direction. Note that this car doesn't rotate -- it is axis aligned. If the car leaves the road, it explodes, and the "game" ends.
You now have something on the screen that responds to user input. You could spend time making a fancier car model (wheels, etc), or you could improve the car physics and control model (direction! Angle! Breaking != speeding up!), or you could make the environment more interesting (add black-and-white stripes so you can see velocity at the edge of the road. An off-road portion near the road, and maybe trees that blow up the car), or you could make the camera more interesting (say, it stays behind the car, and looks over its shoulder).
Now, for dynamics, I'd treat universe-car interaction using distinct code from car-car interaction, just to keep my sanity intact. The car doesn't get to modify the environment.
This means you can write a bunch of the car-universe interaction easier than you can the car-car interaction.
...
Building an arbitrary tree in C++ is easy.
#include <vector>
#include <memory>
#include <string>
struct MyTree;
typedef std::unique_ptr<MyTree> upTree; // punt on memory management!
struct MyBaseNode;
typedef std::unique_ptr<MyBaseNode> upNode;
struct MyTree {
std::vector<upTree> children;
upNode node;
MyTree( upNode node_ ):node(std::move(node_)) {}
private:
// if C++11 compiler, use these:
MyTree( MyTree const& ) = delete;
MyTree& operator=( MyTree const& ) = delete;
// if C++03, use these:
// MyTree( MyTree const& ); // no implementation
// MyTree& operator=( MyTree const& ); // no implementation
};
upTree make_tree(upNode node) { return upTree( new MyTree(std::move(node)) ); }
enum EOrder{ eFirst, eMiddle, eLast };
template<typename Functor>
void walk_tree( upTree const& tree, Functor f, bool bFirst = true, bool bLast = true) {
if (!tree) return;
f( tree, bFirst, bLast );
for (auto it = tree->children.begin(); it != tree->children.end(); ++it) {
bool bChildFirst = (it == tree->children.begin());
bool bChildLast = ((it+1) == tree->children.end());
walk_tree( *it, f, bChildFirst, bChildLast );
}
}
struct MyBaseNode {
virtual ~MyBaseNode() {};
// put things that your tree nodes have to be able to do here
// as pure virtual functions...
virtual std::string MyName() const = 0;
};
struct CarsNode : MyBaseNode {
// cars node implementation!
virtual std::string MyName() const /*override*/ { return "I am a bunch of CARS!"; }
};
upNode make_cars() { return upNode( new CarsNode() ); }
struct CarNode : MyBaseNode {
// car node implementation!
virtual std::string MyName() const /*override*/ { return "I am a CAR!"; }
};
upNode make_car() { return upNode( new CarNode() ); }
struct RoadNode : MyBaseNode {
// car node implementation!
virtual std::string MyName() const /*override*/ { return "I am a ROAD!"; }
};
upNode make_road() { return upNode( new RoadNode() ); }
#include <iostream>
void tree_printer_func( upTree const& tree, bool bFirst, bool bLast ) {
if (bFirst) std::cout << "[ ";
if (tree->node) {
std::cout << tree->node->MyName().c_str();
} else {
std::cout << "nullNode";
}
if (bLast) {
std::cout << " ]\n";
} else {
std::cout << ", ";
}
}
int main() {
upTree root = make_tree(upNode());
upTree carsTree = make_tree(make_cars());
carsTree->children.push_back( make_tree( make_car() ) );
carsTree->children.push_back( make_tree( make_car() ) );
root->children.push_back( std::move(carsTree) );
upTree roadTree = make_tree(make_road());
root->children.push_back( std::move(roadTree) );
walk_tree( root, tree_printer_func );
}
the above is pretty rough (I didn't get end-node handling quite right in the printer, for example), but it demonstrates a non-homogenious, non-leaking, n-ary tree structure in C++.
If I understand you right, you are asking for ideas on how to go about developing the project (your game). I would start with the smaller objects like the tire, body, other car parts and the car, then the pieces of the environment, the terrain and road track etc. Finally the world and putting all other objects together into the world. Hope it helps.
You might consider posting this on gamdev.stackexchange; you'd probably get better answers there.
That being said, I agree with Michael IV's suggestion in the comment. Your best bet would be to start with a free open source game engine. Even if you want to make your own engine, starting out by building it with an existing engine would give you valuable ideas about how it can be accomplished.
Here is a relevant answered question on gamedev regarding which engine to use.
Related
In many places you can read that dynamic_cast means "bad design". But I cannot find any article with appropriate usage (showing good design, not just "how to use").
I'm writing a board game with a board and many different types of cards described with many attributes (some cards can be put on the board). So I decided to break it down to the following classes/interfaces:
class Card {};
class BoardCard : public Card {};
class ActionCard : public Card {};
// Other types of cards - but two are enough
class Deck {
Card* draw_card();
};
class Player {
void add_card(Card* card);
Card const* get_card();
};
class Board {
void put_card(BoardCard const*);
};
Some guys suggested that I should use only one class describing a card. But I would mean many mutually excluding attributes. And in the case of the Board class' put_card(BoardCard const&) - it is a part of the interface that I cannot put any card on the board. If I had only one type of card I would have to check it inside the method.
I see the flow like the following:
a generic card is in the deck (it's not important what its type is)
a generic card is drawn from the deck and given to a player (the same as above)
if a player chosen a BoardCard then it can be put on the board
So I use dynamic_cast before putting a card on the board. I think that using some virtual method is out of the question in this case (additionally I wouldn't make any sense to add some action about board to every card).
So my question is: What have I designed badly? How could I avoid dynamic_cast? Using some type attribute and ifs would be a better solution...?
P.S.
Any source treating about dynamic_cast usage in the context of design is more than appreciated.
Yes, dynamic_cast is a code smell, but so is adding functions that try to make it look like you have a good polymorphic interface but are actually equal to a dynamic_cast i.e. stuff like can_put_on_board. I'd go as far as to say that can_put_on_board is worse - you're duplicating code otherwise implemented by dynamic_cast and cluttering the interface.
As with all code smells, they should make you wary and they don't necessarily mean that your code is bad. This all depends on what you're trying to achieve.
If you're implementing a board game that will have 5k lines of code, two categories of cards, then anything that works is fine. If you're designing something larger, extensible and possibly allowing for cards being created by non-programmers (whether it's an actual need or you're doing it for research) then this probably won't do.
Assuming the latter, let's look at some alternatives.
You could put the onus of applying the card properly to the card, instead of some external code. E.g. add a play(Context& c) function to the card (the Context being a means to access the board and whatever may be necessary). A board card would know that it may only be applied to a board and a cast would not be necessary.
I would entirely give up using inheritance however. One of its many issues is how it introduces a categorisation of all cards. Let me give you an example:
you introduce BoardCard and ActionCard putting all cards in these two buckets;
you then decide that you want to have a card that can be used in two ways, either as an Action or a Board card;
let's say you solved the issue (through multiple-inheritance, a BoardActionCard type, or any different way);
you then decide you want to have card colours (as in MtG) - how do you do this? Do you create RedBoardCard, BlueBoardCard, RedActionCard etc?
Other examples of why inheritance should be avoided and how to achieve runtime polymorphism otherwise you may want to watch Sean Parent's excellent "Inheritance is the Base Class of Evil" talk. A promising looking library that implements this sort of polymorphism is dyno, I have not tried it out yet though.
A possible solution might be:
class Card final {
public:
template <class T>
Card(T model) :
model_(std::make_shared<Model<T>>(std::move(model)))
{}
void play(Context& c) const {
model_->play(c);
}
// ... any other functions that can be performed on a card
private:
class Context {
public:
virtual ~Context() = default;
virtual void play(Context& c) const = 0;
};
template <class T>
class Model : public Context {
public:
void play(Context& c) const override {
play(model_, c);
// or
model_.play(c);
// depending on what contract you want to have with implementers
}
private:
T model_;
};
std::shared_ptr<const Context> model_;
};
Then you can either create classes per card type:
class Goblin final {
void play(Context& c) const {
// apply effects of card, e.g. take c.board() and put the card there
}
};
Or implement behaviours for different categories, e.g. have a
template <class T>
void play(const T& card, Context& c);
template and then use enable_if to handle it for different categories:
template <class T, class = std::enable_if<IsBoardCard_v<T>>
void play(const T& card, Context& c) {
c.board().add(Card(card));
}
where:
template <class T>
struct IsBoardCard {
static constexpr auto value = T::IS_BOARD_CARD;
};
template <class T>
using IsBoardCard_v = IsBoardCard<T>::value;
then defining your Goblin as:
class Goblin final {
public:
static constexpr auto IS_BOARD_CARD = true;
static constexpr auto COLOR = Color::RED;
static constexpr auto SUPERMAGIC = true;
};
which would allow you to categorise your cards in many dimensions also leaving the possibility to entirely specialise the behaviour by implementing a different play function.
The example code uses std::shared_ptr to store the model, but you can definitely do something smarter here. I like to use a static-sized storage and only allow Ts of a certain maximum size and alignment to be used. Alternatively you could use a std::unique_ptr (which would disable copying though) or a variant leveraging small-size optimisation.
Why not use dynamic_cast
dynamic_cast is generally disliked because it can be easily abused to completely break the abstractions used. And it is not wise to depend on specific implementations. Of course it may needed, but really rarely, so nearly everyone takes a rule of thumb - probably you should not use it. It's a code smell that may imply that you should rethink Your abstractions because they may be not the ones needed in Your domain. Maybe in Your game the Board should not have put_card method - maybe instead card should have method play(const PlaySpace *) where Board implements PlaySpace or something like that. Even CppCoreGuidelines discourage using dynamic_cast in most cases.
When use
Generally few people ever have problems like this but I came across it multiple times already. The problem is called Double (or Multiple) Dispatch. Here is pretty old, but quite relevant article about double dispatch (mind the prehistoric auto_ptr):
http://www.drdobbs.com/double-dispatch-revisited/184405527
Also Scott Meyers in one of his books wrote something about building double dispatch matrix with dynamic_cast. But, all in all, these dynamic_casts are 'hidden` inside this matrix - users don't know what kind of magic happens inside.
Noteworthy - multiple dispatch is also considered code smell :-).
Reasonable alternative
Check out the visitor pattern. It can be used as replace for dynamic_cast but it is also some kind of code smell.
I generally recommend using dynamic_cast and visitor as a last resort tools for design problems as they break abstraction which increases complexity.
You could apply the principles behind Microsoft's COM and provide a series of interfaces, with each interface describing a set of related behaviors. In COM you determine if a specific interface is available by calling QueryInterface, but in modern C++ dynamic_cast works similarly and is more efficient.
class Card {
virtual void ~Card() {} // must have at least one virtual method for dynamic_cast
};
struct IBoardCard {
virtual void put_card(Board* board);
};
class BoardCard : public Card, public IBoardCard {};
class ActionCard : public Card {};
// Other types of cards - but two are enough
class Deck {
Card* draw_card();
};
class Player {
void add_card(Card* card);
Card const* get_card();
};
class Board {
void put_card(Card const* card) {
const IBoardCard *p = dynamic_cast<const IBoardCard*>(card);
if (p != null) p->put_card(this);
};
That may be a bad example, but I hope you get the idea.
It seems to me that the two types of cards are quite different. The things a board card and an action card can do are mutually exclusive, and the common thing is just that they can be drawn from the deck. Moreover, that's not a thing a card does, it's a player / deck action.
If this is true, a question one should ask is whether they should really descend from a common type, Card. An alternative design would be that of a tagged union: let Card instead be a std::variant<BoardCard, ActionCard...>, and contain an instance of the appropriate type. When deciding what to do with the card, you use a switch on the index() and then std::get<> only the appropriate type. This way you don't need any *_cast operator, and get a complete freedom of what methods (neither of which would make sense for the other types) each type of card supports.
If it's only almost true but not for all types, you can variate slightly: only group together those types of cards that can sensibly be superclassed, and put the set of those common types into the variant.
I always found the usage of a cast a code smell, and in my experience, the 90% of the time the cast was due to bad design.
I saw usage of dynamic_cast in some time-critical application where it was providing more performance improvement than inherit from multiple interfaces or retrieving an enumeration of some kind from the object (like a type). So the code smelt, but the usage of the dynamic cast was worth it in that case.
That said, I will avoid dynamic cast in your case as well as multiple inheritances from different interfaces.
Before reaching my solution, your description sounds like there are a lot of details omitted about the behavior of the cards or the consequence they have on the board
and the game itself. I used that as a further constraint, trying to keep thing boxed and maintainable.
I would go for a composition instead of an inheritance. It will provide you evenly the chance of using the card as a 'factory':
it can spawn more game modifiers - something to be applied to the board, and one to a specific enemy
the card can be reused - the card could stays in the hands of the player and the effect on the game is detached from it (there is no 1-1 binding between cards and effects)
the card itself can sit back on the deck, while the effects of what it did are still alive on the board.
a card can have a representation (drawing methods) and react to the touch in a way, where instead the BoardElement can be evenly a 3d miniature with animation
See [https://en.wikipedia.org/wiki/Composition_over_inheritance for further details]. I'd like to quote:
Composition also provides a more stable business domain in the long term as it is less prone to the quirks of the family members.In other words, it is better to compose what an object can do (HAS - A) than extend what it is(IS - A).[1]
A BoardCard/Element can be something like this:
//the card placed on the board.
class BoardElement {
public:
BoardElement() {}
virtual ~BoardElement() {};
//up to you if you want to add a read() methods to read data from the card description (XML / JSON / binary data)
// but that should not be part of the interface. Talking about a potential "Wizard", it's probably more related to the WizardCard - WizardElement relation/implementation
//some helpful methods:
// to be called by the board when placed
virtual void OnBoard() {}
virtual void Frame(const float time) { /*do something time based*/ }
virtual void Draw() {}
// to be called by the board when removed
virtual void RemovedFromBoard() {}
};
the Card could represent something to be used in a deck or in the user's hands, I'll add an interface of that kind
class Card {
public:
Card() {}
virtual ~Card() {}
//that will be invoked by the user in order to provide something to the Board, or NULL if nothing should be added.
virtual std::shared_ptr<BoardElement*> getBoardElement() { return nullptr; }
virtual void Frame(const float time) { /*do something time based*/ }
virtual void Draw() {}
//usefull to handle resources or internal states
virtual void OnUserHands() {}
virtual void Dropped() {}
};
I'd like to add that this pattern allows many tricks inside the getBoardElement() method, from acting as a factory (so something should be spawned with its own lifetime),
returning an Card data member such as a std:shared_ptr<BoardElement> wizard3D; (as example), create a binding between the Card and the BoardElement as for:
class WizardBoardElement : public BoardElement {
public:
WizardBoardElement(const Card* owner);
// other members omitted ...
};
The binding can be useful in order to read some configuration data or whatever...
So inheritance from Card and from BoardElement will be used to implement the features exposed by the base classes and not for providing other methods that can be reached only through a dynamic_cast.
For completeness:
class Player {
void add(Card* card) {
//..
card->OnUserHands();
//..
}
void useCard(Card* card) {
//..
//someway he's got to retrieve the board...
getBoard()->add(card->getBoardElement());
//..
}
Card const* get_card();
};
class Board {
void add(BoardElement* el) {
//..
el->OnBoard();
//..
}
};
In that way, we have no dynamic_cast, Player and board do simple things without knowing about the inner details of the card they are handled, providing good separations between the different objects and increasing maintainability.
Talking about the ActionCard, and about "effects" that may be applied to other players or your avatar, we can think about having a method like:
enum EffectTarget {
MySelf, //a player on itself, an enemy on itself
MainPlayer,
Opponents,
StrongOpponents
//....
};
class Effect {
public:
//...
virtual void Do(Target* target) = 0;
//...
};
class Card {
public:
//...
struct Modifiers {
EffectTarget eTarget;
std::shared_ptr<Effect> effect;
};
virtual std::vector<Modifiers> getModifiers() { /*...*/ }
//...
};
class Player : public Target {
public:
void useCard(Card* card) {
//..
//someway he's got to retrieve the board...
getBoard()->add(card->getBoardElement());
auto modifiers = card->getModifiers();
for each (auto modifier in modifiers)
{
//this method is supposed to look at the board, at the player and retrieve the instance of the target
Target* target = getTarget(modifier.eTarget);
modifier.effect->Do(target);
}
//..
}
};
That's another example of the same pattern to apply the effects from the card, avoiding the cards to know details about the board and it's status, who is playing the card, and keep the code in Player pretty simple.
Hope this may help,
Have a nice day,
Stefano.
What have I designed badly?
The problem is that you always need to extend that code whenever a new type of Card is introduced.
How could I avoid dynamic_cast?
The usual way to avoid that is to use interfaces (i.e. pure abstract classes):
struct ICard {
virtual bool can_put_on_board() = 0;
virtual ~ICard() {}
};
class BoardCard : public ICard {
public:
bool can_put_on_board() { return true; };
};
class ActionCard : public ICard {
public:
bool can_put_on_board() { return false; };
};
This way you can simply use a reference or pointer to ICard and check, if the actual type it holds can be put on the Board.
But I cannot find any article with appropriate usage (showing good design, not just "how to use").
In general I'd say there aren't any good, real life use cases for dynamic cast.
Sometimes I have used it in debug code for CRTP realizations like
template<typename Derived>
class Base {
public:
void foo() {
#ifndef _DEBUG
static_cast<Derived&>(*this).doBar();
#else
// may throw in debug mode if something is wrong with Derived
// not properly implementing the CRTP
dynamic_cast<Derived&>(*this).doBar();
#endif
}
};
I think that I would end up with something like this (compiled with clang 5.0 with -std=c++17). I'm couroius about your comments. So whenever I want to handle different types of Cards I need to instantiate a dispatcher and supply methods with proper signatures.
#include <iostream>
#include <typeinfo>
#include <type_traits>
#include <vector>
template <class T, class... Args>
struct any_abstract {
static bool constexpr value = std::is_abstract<T>::value || any_abstract<Args...>::value;
};
template <class T>
struct any_abstract<T> {
static bool constexpr value = std::is_abstract<T>::value;
};
template <class T, class... Args>
struct StaticDispatcherImpl {
template <class P, class U>
static void dispatch(P* ptr, U* object) {
if (typeid(*object) == typeid(T)) {
ptr->do_dispatch(*static_cast<T*>(object));
return;
}
if constexpr (sizeof...(Args)) {
StaticDispatcherImpl<Args...>::dispatch(ptr, object);
}
}
};
template <class Derived, class... Args>
struct StaticDispatcher {
static_assert(not any_abstract<Args...>::value);
template <class U>
void dispatch(U* object) {
if (object) {
StaticDispatcherImpl<Args...>::dispatch(static_cast<Derived *>(this), object);
}
}
};
struct Card {
virtual ~Card() {}
};
struct BoardCard : Card {};
struct ActionCard : Card {};
struct Board {
void put_card(BoardCard const& card, int const row, int const column) {
std::cout << "Putting card on " << row << " " << column << std::endl;
}
};
struct UI : StaticDispatcher<UI, BoardCard, ActionCard> {
void do_dispatch(BoardCard const& card) {
std::cout << "Get row to put: ";
int row;
std::cin >> row;
std::cout << "Get row to put:";
int column;
std::cin >> column;
board.put_card(card, row, column);
}
void do_dispatch(ActionCard& card) {
std::cout << "Handling action card" << std::endl;
}
private:
Board board;
};
struct Game {};
int main(int, char**) {
Card* card;
ActionCard ac;
BoardCard bc;
UI ui;
card = ∾
ui.dispatch(card);
card = &bc;
ui.dispatch(card);
return 0;
}
As I can't see why you wouldn't use virtual methods, I'm just gonna present, how I would do it. First I have the ICard interface for all cards. Then I would distinguish, between the card types (i.e. BoardCard and ActionCard and whatever cards you have). And All the cards inherit from either one of the card types.
class ICard {
virtual void put_card(Board* board) = 0;
virtual void accept(CardVisitor& visitor) = 0; // See later, visitor pattern
}
class ActionCard : public ICard {
void put_card(Board* board) final {
// std::cout << "You can't put Action Cards on the board << std::endl;
// Or just do nothing, if the decision of putting the card on the board
// is not up to the user
}
}
class BoardCard : public ICard {
void put_card(Board* board) final {
// Whatever implementation puts the card on the board, mb something like:
board->place_card_on_board(this);
}
}
class SomeBoardCard : public BoardCard {
void accept(CardVisitor& visitor) final { // visitor pattern
visitor.visit(this);
}
void print_information(); // see BaseCardVisitor in the next code section
}
class SomeActionCard : public ActionCard {
void accept(CardVisitor& visitor) final { // visitor pattern
visitor.visit(this);
}
void print_information(); // see BaseCardVisitor
}
class Board {
void put_card(ICard* const card) {
card->put_card(this);
}
void place_card_on_board(BoardCard* card) {
// place it on the board
}
}
I guess the user has to know somehow what card he has drawn, so for that I would implement the visitor pattern. You could also place the accept-method, which I placed in the most derived classes/cards, in the card types (BoardCard, ActionCard), depeneding on where you want to draw the line on what information shall be given to the user.
template <class T>
class BaseCardVisitor {
void visit(T* card) {
card->print_information();
}
}
class CardVisitor : public BaseCardVisitor<SomeBoardCard>,
public BaseCardVisitor<SomeActionCard> {
}
class Player {
void add_card(ICard* card);
ICard const* get_card();
void what_is_this_card(ICard* card) {
card->accept(visitor);
}
private:
CardVisitor visitor;
};
Hardly a complete answer but just wanted to pitch in with an answer similar to Mark Ransom's but just very generally speaking, I've found downcasting to be useful in cases where duck typing is really useful. There can be certain architectures where it is very useful to do things like this:
for each object in scene:
{
if object can fly:
make object fly
}
Or:
for each object in scene that can fly:
make object fly
COM allows this type of thing somewhat like so:
for each object in scene:
{
// Request to retrieve a flyable interface from
// the object.
IFlyable* flyable = object.query_interface<IFlyable>();
// If the object provides such an interface, make
// it fly.
if (flyable)
flyable->fly();
}
Or:
for each flyable in scene.query<IFlyable>:
flyable->fly();
This implies a cast of some form somewhere in the centralized code to query and obtain interfaces (ex: from IUnknown to IFlyable). In such cases, a dynamic cast checking run-time type information is the safest type of cast available. First there might be a general check to see if an object provides the interface that doesn't involve casting. If it doesn't, this query_interface function might return a null pointer or some type of null handle/reference. If it does, then using a dynamic_cast against RTTI is the safest thing to do to fetch the actual pointer to the generic interface (ex: IInterface*) and return IFlyable* to the client.
Another example is entity-component systems. In that case instead of querying abstract interfaces, we retrieve concrete components (data):
Flight System:
for each object in scene:
{
if object.has<Wings>():
make object fly using object.get<Wings>()
}
Or:
for each wings in scene.query<Wings>()
make wings fly
... something to this effect, and that also implies casting somewhere.
For my domain (VFX, which is somewhat similar to gaming in terms of application and scene state), I've found this type of ECS architecture to be the easiest to maintain. I can only speak from personal experience, but I've been around for a long time and have faced many different architectures. COM is now the most popular style of architecture in VFX and I used to work on a commercial VFX application used widely in films and games and archviz and so forth which used a COM architecture, but I've found ECS as popular in game engines even easier to maintain than COM for my particular case*.
One of the reasons I find ECS so much easier is because the bulk of the systems in this domain like PhysicsSystem, RenderingSystem, AnimationSystem, etc. boil down to just data transformers and the ECS model just fits beautifully for that purpose without abstractions getting in the way. With COM in this domain, the number of subtypes implementing an interface like a motion interface like IMotion might be in the hundreds (ex: a PointLight which implements IMotion along with 5 other interfaces), requiring hundreds of classes implementing different combinations of COM interfaces to maintain individually. With the ECS, it uses a composition model over inheritance, and reduces those hundreds of classes down to just a couple dozen simple component structs which can be combined in endless ways by the entities that compose them, and only a handful of systems have to provide behavior: everything else is just data which the systems loop through as input to then provide some output.
Between legacy codebases that used a bunch of global variables and brute force coding (ex: sprinkling conditionals all over the place instead of using polymorphism), deep inheritance hierarchies, COM, and ECS, in terms of maintainability for my particular domain, I'd say ECS > COM, while deep inheritance hierarchies and brute force coding with global variables all over the place were both incredibly hard to maintain (OOP using deep inheritance with protected data fields is almost as hard to reason about in terms of maintaining invariants as a boatload of global variables IMO, but further can invite the most nightmarish cascading changes spilling across entire hierarchies if designs need to change -- at least the brute force legacy codebase didn't have the cascading problem since it was barely reusing any code to begin with).
COM and ECS are somewhat similar except with COM, the dependencies flow towards central abstractions (COM interfaces provided by COM objects, like IFlyable). With an ECS, the dependencies flow towards central data (components provided by ECS entities, like Wings). At the heart of both is often the idea that we have a bunch of non-homogeneous objects (or "entities") of interest whose provided interfaces or components are not known in advance, since we're accessing them through a non-homogeneous collection (ex: a "Scene"). As a result we need to discover their capabilities at runtime when iterating through this non-homogeneous collection by either querying the collection or the objects individually to see what they provide.
Either way, both involve some type of centralized casting to retrieve either an interface or a component from an entity, and if we have to downcast, then a dynamic_cast is at least the safest way to do that which involves runtime type checking to make sure the cast is valid. And with both ECS and COM, you generally only need one line of code in the entire system which performs this cast.
That said, the runtime checking does have a small cost. Typically if dynamic_cast is used in COM and ECS architectures, it's done in a way so that a std::bad_cast should never be thrown and/or that dynamic_cast itself never returns nullptr (the dynamic_cast is just a sanity check to make sure there are no internal programmer errors, not as a way to determine if an object inherits a type). Another type of runtime check is made to avoid that (ex: just once for an entire query in an ECS when fetching all PosAndVelocity components to determine which component list to use which is actually homogeneous and only stores PosAndVelocity components). If that small runtime cost is non-negligible because you're looping over a boatload of components every frame and doing trivial work to each, then I found this snippet useful from Herb Sutter in C++ Coding Standards:
template<class To, class From> To checked_cast(From* from) {
assert( dynamic_cast<To>(from) == static_cast<To>(from) && "checked_cast failed" );
return static_cast<To>(from);
}
template<class To, class From> To checked_cast(From& from) {
assert( dynamic_cast<To>(from) == static_cast<To>(from) && "checked_cast failed" );
return static_cast<To>(from);
}
It basically uses dynamic_cast as a sanity check for debug builds with an assert, and static_cast for release builds.
I want to make an app where a user can edit a diagram (for example), which would provide the standard mechanisms of: Save, Load, Undo, and Redo.
A simple way to do it is to have classes for the diagram and for the various shapes in it, which implement serialization via save and load methods, and where all methods to edit them return UndoableActions that can be added to an UndoManager which calls their perform method and adds them to an undo stack.
The problem with the simple way described above is that it requires a lot of error-prone boilerplate work.
I know that the serialization (save/load) part of the work can be solved by using something like Google's Protocol Buffers or Apache Thrift, which generates the boiler-plate serialization code for you, but it doesn't solve the undo+redo problem. I know that for Objective C and Swift, Apple provides Core Data which does solve serialization + undo, but I'm not familiar with anything similar for C++.
Is there a good way non-error-prone to solve save+load+undo+redo with little boilerplate?
The problem with the simple way described above is that it requires a lot of error-prone boilerplate work.
I am not convinced that this is the case. Your approach sounds reasonable and using Modern C++ features and abstractions you can implement a safe and elegant interface for it.
For starters, you could use std::variant as a sum type for "undoable actions" - this will give you a type-safe tagged union for every action. (Consider using boost::variant or other implementations that can be easily found on Google if you do not have access to C++17). Example:
namespace action
{
// User dragged the shape to a separate position.
struct move_shape
{
shape_id _id;
offset _offset;
};
// User changed the color of a shape.
struct change_shape_color
{
shape_id _id;
color _previous;
color _new;
};
// ...more actions...
}
using undoable_action = std::variant<
action::move_shape,
action::change_shape_color,
// ...
>;
Now that you have a sum type for all your possible "undoable actions", you can define undo behavior by using pattern matching. I wrote two articles on variant "pattern matching" by overloading lambdas that you could find interesting:
"visiting variants using lambdas - part 1"
"visiting variants using lambdas - part 2"
Here's an example of how your undo function could look like:
void undo()
{
auto action = undo_stack.pop_and_get();
match(action, [&shapes](const move_shape& y)
{
// Revert shape movement.
shapes[y._id].move(-y._offset);
},
[&shapes](const change_shape_color& y)
{
// Revert shape color change.
shapes[y._id].set_color(y._previous);
},
[](auto)
{
// Produce a compile-time error.
struct undo_not_implemented;
undo_not_implemented{};
});
}
If every branch of match gets large, it could be moved to its own function for readability. Trying to instantiate undo_not_implemented or using a dependent static_assert is also a good idea: a compile-time error will be produced if you forget to implement behavior for a specific "undoable action".
That's pretty much it! If you want to save the undo_stack so that the history of actions is preserved in saved documents, you can implement a auto serialize(const undoable_action&) that, again, uses pattern matching to serialize the various actions. You could then implement a deserialize function that repopulates the undo_stack on file load.
If you find implementing serialization/deserialization for every action too tedious, consider using BOOST_HANA_DEFINE_STRUCT or similar solutions to automatically generate serialization/deserialization code.
Since you're concerned about battery and performance, I would also like to mention that using std::variant or similar tagged union constructs is on average faster and more lightweight compared to polymorphic hierarchies, as heap allocation is not required and as there is no run-time virtual dispatch.
About redo functionality: you could have a redo_stack and implement an auto invert(const undoable_action&) function that inverts the behavior of an action. Example:
void undo()
{
auto action = undo_stack.pop_and_get();
match(action, [&](const move_shape& y)
{
// Revert shape movement.
shapes[y._id].move(-y._offset);
redo_stack.push(invert(y));
},
// ...
auto invert(const undoable_action& x)
{
return match(x, [&](move_shape y)
{
y._offset *= -1;
return y;
},
// ...
If you follow this pattern, you can implement redo in terms of undo! Simply call undo by popping from the redo_stack instead of the undo_stack: since you "inverted" the actions it will perform the desired operation.
EDIT: here's a minimal wandbox example that implements a match function that takes in a variant and returns a variant.
The example uses boost::hana::overload to generate the visitor.
The visitor is wrapped in a lambda f that unifies the return type to the type of the variant: this is necessary as std::visit requires that the visitor always returns the same type.
If returning a type which is different from the variant is desirable, std::common_type_t could be used, otherwise the user could explicitly specify it as the first template parameter of match.
Two reasonable approaches to this problem are implemented in the frameworks Flip and ODB.
Code-generation / ODB
With ODB you need to add #pragma declarations to your code, and have it's tool generate methods that you use for save/load and for editing the model, like so:
#pragma db object
class person
{
public:
void setName (string);
string getName();
...
private:
friend class odb::access;
person () {}
#pragma db id
string email_;
string name_;
};
Where the accessors declared in the class are auto-generated by ODB so that all changes to the model can get captured and undo-transactions may be made for them.
Reflection with minimal boilerplate / Flip
Unlike ODB, Flip doesn't generate C++ code for you, but rather it requires your program to call Model::declare to re-declare your structures like so:
class Song : public flip::Object
{
public:
static void declare ();
flip::Float tempo;
flip::Array <Track> tracks;
};
void Song::declare ()
{
Model::declare <Song> ()
.name ("acme.product.Song")
.member <flip::Float, &Song::tempo> ("tempo");
.member <flip::Array <Track>, &Song::tracks> ("tracks");
}
int main()
{
Song::declare();
...
}
With the structured declared like so, flip::Object's constructor can initialize all the fields so that they can point to the undo stack, and all the edits on them are recorded. It also has a list of all the members so that flip::Object can implement the serialization for you.
The problem with the simple way described above is that it requires a lot of error-prone boilerplate work.
I would say that the actual problem is that your undo/redo logic is part of a component, that should ship only a bunch of data as a position, a content and so on.
A common OOP way to decouple the undo/redo logic from the data is the command design pattern.
The basic idea is that all the user interactions are converted to commands and those commands are executed on the diagram itself. They contain all the information required to perform an operation and to rollback it, as long as you maintain a sorted list of commands and undo/redo them in order (that is usually the user expectation).
Another common OOP pattern that can help you either to design a custom serialization utility or to use the most common ones is the visitor design pattern.
Here the basic idea is that your diagram should not care about the kind of components it contains. Whenever you want to serialize it, you provide a serializer and the components promote themselves to the right type when queried (see double dispatching for further details on this technique).
That being said, a minimal example is worth more than a thousand words:
#include <memory>
#include <stack>
#include <vector>
#include <utility>
#include <iostream>
#include <algorithm>
#include <string>
struct Serializer;
struct Part {
virtual void accept(Serializer &) = 0;
virtual void draw() = 0;
};
struct Node: Part {
void accept(Serializer &serializer) override;
void draw() override;
std::string label;
unsigned int x;
unsigned int y;
};
struct Link: Part {
void accept(Serializer &serializer) override;
void draw() override;
std::weak_ptr<Node> from;
std::weak_ptr<Node> to;
};
struct Serializer {
void visit(Node &node) {
std::cout << "serializing node " << node.label << " - x: " << node.x << ", y: " << node.y << std::endl;
}
void visit(Link &link) {
auto pfrom = link.from.lock();
auto pto = link.to.lock();
std::cout << "serializing link between " << (pfrom ? pfrom->label : "-none-") << " and " << (pto ? pto->label : "-none-") << std::endl;
}
};
void Node::accept(Serializer &serializer) {
serializer.visit(*this);
}
void Node::draw() {
std::cout << "drawing node " << label << " - x: " << x << ", y: " << y << std::endl;
}
void Link::accept(Serializer &serializer) {
serializer.visit(*this);
}
void Link::draw() {
auto pfrom = from.lock();
auto pto = to.lock();
std::cout << "drawing link between " << (pfrom ? pfrom->label : "-none-") << " and " << (pto ? pto->label : "-none-") << std::endl;
}
struct TreeDiagram;
struct Command {
virtual void execute(TreeDiagram &) = 0;
virtual void undo(TreeDiagram &) = 0;
};
struct TreeDiagram {
std::vector<std::shared_ptr<Part>> parts;
std::stack<std::unique_ptr<Command>> commands;
void execute(std::unique_ptr<Command> command) {
command->execute(*this);
commands.push(std::move(command));
}
void undo() {
if(!commands.empty()) {
commands.top()->undo(*this);
commands.pop();
}
}
void draw() {
std::cout << "draw..." << std::endl;
for(auto &part: parts) {
part->draw();
}
}
void serialize(Serializer &serializer) {
std::cout << "serialize..." << std::endl;
for(auto &part: parts) {
part->accept(serializer);
}
}
};
struct AddNode: Command {
AddNode(std::string label, unsigned int x, unsigned int y):
label{label}, x{x}, y{y}, node{std::make_shared<Node>()}
{
node->label = label;
node->x = x;
node->y = y;
}
void execute(TreeDiagram &diagram) override {
diagram.parts.push_back(node);
}
void undo(TreeDiagram &diagram) override {
auto &parts = diagram.parts;
parts.erase(std::remove(parts.begin(), parts.end(), node), parts.end());
}
std::string label;
unsigned int x;
unsigned int y;
std::shared_ptr<Node> node;
};
struct AddLink: Command {
AddLink(std::shared_ptr<Node> from, std::shared_ptr<Node> to):
link{std::make_shared<Link>()}
{
link->from = from;
link->to = to;
}
void execute(TreeDiagram &diagram) override {
diagram.parts.push_back(link);
}
void undo(TreeDiagram &diagram) override {
auto &parts = diagram.parts;
parts.erase(std::remove(parts.begin(), parts.end(), link), parts.end());
}
std::shared_ptr<Link> link;
};
struct MoveNode: Command {
MoveNode(unsigned int x, unsigned int y, std::shared_ptr<Node> node):
px{node->x}, py{node->y}, x{x}, y{y}, node{node}
{}
void execute(TreeDiagram &) override {
node->x = x;
node->y = y;
}
void undo(TreeDiagram &) override {
node->x = px;
node->y = py;
}
unsigned int px;
unsigned int py;
unsigned int x;
unsigned int y;
std::shared_ptr<Node> node;
};
int main() {
TreeDiagram diagram;
Serializer serializer;
auto addNode1 = std::make_unique<AddNode>("foo", 0, 0);
auto addNode2 = std::make_unique<AddNode>("bar", 100, 50);
auto moveNode2 = std::make_unique<MoveNode>(10, 10, addNode2->node);
auto addLink = std::make_unique<AddLink>(addNode1->node, addNode2->node);
diagram.serialize(serializer);
diagram.execute(std::move(addNode1));
diagram.execute(std::move(addNode2));
diagram.execute(std::move(addLink));
diagram.serialize(serializer);
diagram.execute(std::move(moveNode2));
diagram.draw();
diagram.undo();
diagram.undo();
diagram.serialize(serializer);
}
I've not implemented the redo action and the code is far from being a production-ready piece of software, but it acts quite well as a starting point from which to create something more complex.
As you can see, the goal is to create a tree diagram that contains both nodes an links. A component contains a bunch of data and knows how to draw itself. Moreover, as anticipated, a component accepts a serializer in case you want to write it down on a file or whatever.
All the logic is contained in the so called commands. In the example there are three commands: add node, add link and move node. Neither the diagram nor the components know anything about what's going on under the hood. All what the diagram knows is that it's executing a set of commands and those commands can be executed back a step at the time.
A more complex undo/redo system can contain a circular buffer of commands and a few indexes that indicate the one to substitute with the next one, the one valid when going forth and the one valid when going back.
It's quite easy to implement indeed.
This approach will help you decoupling the logic from the data and it's quite common when dealing with user interfaces.
To be honest, it's not something that came up suddenly to my mind. I found something similar while looking at how open-source software solved the issue and I've used it a few years ago in a software of mine. The resulting code is really easy to maintain.
Another approach you might want to consider is working with inmutable data structures and objects. Then, the undo/redo stack can be implemented as a stack of versions of the scene/diagram/document. Undo() replaces the current version with an older version from the stack, and so on. Because all data is inmutable, you can keep references instead of copies, so it is fast and (relatively) cheap.
Pros:
simple undo/redo
multithread-friendly
clean separation of "structure" and transient state (e.g. current selection)
may simplify serialization
caching/memoization/precomputation-friendly (e.g. bounding-box, gpu buffers)
Cons:
consumes a bit more memory
forces separation of "structure" and transient state
probably more difficult: for example, for a typical tree-like scenegraph, to change a node you would also need to change all the nodes along the path to the root; the old and new versions can share the rest of the nodes
Assuming that you're calling save() on a temporary file for each edit of the diagram (even if user doesn't explicitly call the save action) and that you undo only the latest action, you can do as follows:
LastDiagram load(const std::string &path)
{
/* Check for valid path (e.g. boost::filesystem here) */
if(!found)
{
throw std::runtime_exception{"No diagram found"};
}
//read LastDiagram
return LastDiagram;
}
LastDiagram undoLastAction()
{
return loadLastDiagram("/tmp/tmp_diagram_file");
}
and in your main app you handle the exception if thrown. In case you want to allow more undos, then you should think to a solution like sqlite or a tmp file with more entries.
If performance in time and space are issues due large diagrams, think to implement some strategy like keeping an incremental difference for each element of a diagram in a std::vector (limit it to 3/5 if objects are big) and call the renderer with the current statuses. I'm not an OpenGL expert, but I think it's the way it's done there. Actually you could 'steal' this strategy from game development best practices, or generally graphics related ones.
One of those strategies could be something like this:
A structure for efficient update, incremental redisplay and undo in graphical editors
I'm working on a code which needs to be extremely flexible in nature, i.e. especially very easy to extend later also by other people. But I'm facing a problem now about which I do not even know in principal how to properly deal with:
I'm having a rather complex Algorithm, which at some point is supposed to converge. But due to its complexity there are several different criteria to check for convergence, and depending on the circumstances (or input) I would want to have different convergence criteria activated. Also it should easily be possible to create new convergence criteria without having to touch the algorithm itself. So ideally I would like to have an abstract ConvergenceChecker class from which I can inherit and let the algorithm have a vector of those, e.g. like this:
//Algorithm.h (with include guards of course)
class Algorithm {
//...
vector<ConvergenceChecker*> _convChecker;
}
//Algorithm.cpp
void runAlgorithm() {
bool converged=false;
while(true){
//Algorithm performs a cycle
for (unsigned i=0; i<_convChecker.size(); i++) {
// Check for convergence with each criterion
converged=_convChecker[i]->isConverged();
// If this criterion is not satisfied, forget about the following ones
if (!converged) { break; }
}
// If all are converged, break out of the while loop
if (converged) { break; }
}
}
The problem with this is that each ConvergenceChecker needs to know something about the currently running Algorithm, but each one might need to know totally different things from the algorithm. Say the Algorithm changes _foo _bar and _fooBar during each cycle, but one possible ConvergenceChecker only needs to know _foo, another one _foo and _bar, and it might be that some day a ConvergenceChecker needing _fooBar will be implemented. Here are some ways I already tried to solve this:
Give the function isConverged() a large argument list (containing _foo, _bar, and _fooBar). Disadvantages: Most of the variables used as arguments will not be used in most cases, and if the Algorithm would be extended by another variable (or a similar algorithm inherits from it and adds some variables) quite some code would have to be modified. -> possible, but ugly
Give the function isConverged() the Algorithm itself (or a pointer to it) as an argument. Problem: Circular dependency.
Declare isConverged() as a friend function. Problem (among others): Cannot be defined as a member function of different ConvergenceCheckers.
Use an array of function pointers. Does not solve the problem at all, and also: where to define them?
(Just came up with this while writing this question) Use a different class which holds the data, say AlgorithmData having Algorithm as a friend class, then provide the AlgorithmData as a function argument. So, like 2. but maybe getting around circular dependency problems. (Did not test this yet.)
I'd be happy to hear your solutions about this (and problems you see with 5.).
Further notes:
Question title: I'm aware that 'strongly dependent classes' already says that most probably one is doing something very wrong with designing the code, still I guess a lot of people might end up with having that problem and would like to hear possibilities to avoid it, so I'd rather keep that ugly expression.
Too easy?: Actually the problem I presented here was not complete. There will be a lot of different Algorithms in the code inheriting from each other, and the ConvergenceCheckers should of course ideally work in appropriate cases without any further modification even if new Algorithms come up. Feel free to comment on this as well.
Question style: I hope the question is neither too abstract nor too special, and I hope it did not get too long and is understandable. So please also don't hesitate to comment on the way I ask this question so that I can improve on that.
Actually, your solution 5 sounds good.
When in danger of introducing circular dependencies, the best remedy usually is to extract the part that both need, and moving it to a separate entity; exactly as extracting the data used by the algorithm into a separate class/struct would do in your case!
Another solution would be passing your checker an object that provides the current algorithm state in response to parameter names expressed as string names. This makes it possible to separately compile your conversion strategies, because the interface of this "callback" interface stays the same even if you add more parameters to your algorithm:
struct AbstractAlgorithmState {
virtual double getDoubleByName(const string& name) = 0;
virtual int getIntByName(const string& name) = 0;
};
struct ConvergenceChecker {
virtual bool converged(const AbstractAlgorithmState& state) = 0;
};
That is all the implementers of the convergence checker need to see: they implement the checker, and get the state.
You can now build a class that is tightly coupled with your algorithm implementation to implement AbstractAlgorithmState and get the parameter based on its name. This tightly coupled class is private to your implementation, though: the callers see only its interface, which never changes:
class PrivateAlgorithmState : public AbstractAlgorithmState {
private:
const Algorithm &algorithm;
public:
PrivateAlgorithmState(const Algorithm &alg) : algorithm(alg) {}
...
// Implement getters here
}
void runAlgorithm() {
PrivateAlgorithmState state(*this);
...
converged=_convChecker[i]->converged(state);
}
Using a separate data/state structure seems easy enough - just pass it to the checker as a const reference for read-only access.
class Algorithm {
public:
struct State {
double foo_;
double bar_;
double foobar_;
};
struct ConvergenceChecker {
virtual ~ConvergenceChecker();
virtual bool isConverged(State const &) = 0;
}
void addChecker(std::unique_ptr<ConvergenceChecker>);
private:
std::vector<std::unique_ptr<ConvergenceChecker>> checkers_;
State state_;
bool isConverged() {
const State& csr = state_;
return std::all_of(checkers_.begin(),
checkers_.end(),
[csr](std::unique_ptr<ConvergenceChecker> &cc) {
return cc->isConverged(csr);
});
}
};
Maybe the decorator pattern can help in simplifying an (unknown) set of convergence checks. This way you can keep the algorithm itself agnostic to what convergence checks may occur and you don't require a container for all the checks.
You would get something along these lines:
class ConvergenceCheck {
private:
ConvergenceCheck *check;
protected:
ConvergenceCheck(ConvergenceCheck *check):check(check){}
public:
bool converged() const{
if(check && check->converged()) return true;
return thisCheck();
}
virtual bool thisCheck() const=0;
virtual ~ConvergenceCheck(){ delete check; }
};
struct Check1 : ConvergenceCheck {
public:
Check1(ConvergenceCheck* check):ConvergenceCheck(check) {}
bool thisCheck() const{ /* whatever logic you like */ }
};
You can then make arbitrary complex combinations of convergence checks while only keeping one ConvergenceCheck* member in Algorithm. For example, if you want to check two criteria (implemented in Check1 and Check2):
ConvergenceCheck *complex=new Check2(new Check1(nullptr));
The code is not complete, but you get the idea. Additionally, if you are a performance fanatic and are afraid of the virtual function call (thisCheck), you can apply the curiously returning template pattern to eliminate that.
Here is a complete example of decorators to check constraints on an int, to give an idea of how it works:
#include <iostream>
class Check {
private:
Check *check_;
protected:
Check(Check *check):check_(check){}
public:
bool check(int test) const{
if(check_ && !check_->check(test)) return false;
return thisCheck(test);
}
virtual bool thisCheck(int test) const=0;
virtual ~Check(){ delete check_; }
};
class LessThan5 : public Check {
public:
LessThan5():Check(NULL){};
LessThan5(Check* check):Check(check) {};
bool thisCheck(int test) const{ return test < 5; }
};
class MoreThan3 : public Check{
public:
MoreThan3():Check(NULL){}
MoreThan3(Check* check):Check(check) {}
bool thisCheck(int test) const{ return test > 3; }
};
int main(){
Check *morethan3 = new MoreThan3();
Check *lessthan5 = new LessThan5();
Check *both = new LessThan5(new MoreThan3());
std::cout << morethan3->check(3) << " " << morethan3->check(4) << " " << morethan3->check(5) << std::endl;
std::cout << lessthan5->check(3) << " " << lessthan5->check(4) << " " << lessthan5->check(5) << std::endl;
std::cout << both->check(3) << " " << both->check(4) << " " << both->check(5);
}
Output:
0 1 1
1 1 0
0 1 0
Let's say we already have a hierarchy of classes, e.g.
class Shape { virtual void get_area() = 0; };
class Square : Shape { ... };
class Circle : Shape { ... };
etc.
Now let's say that I want to (effectively) add a virtual draw() = 0 method to Shape with appropriate definitions in each sub-class. However, let's say I want to do this without modifying those classes (as they are part of a library that I don't want to change).
What would be the best way to go about this?
Whether or not I actually "add" a virtual method or not is not important, I just want polymorphic behaviour given an array of pointers.
My first thought would be to do this:
class IDrawable { virtual void draw() = 0; };
class DrawableSquare : Square, IDrawable { void draw() { ... } };
class DrawableCircle : Circle, IDrawable { void draw() { ... } };
and then just replace all creations of Squares and Circles with DrawableSquares and DrawableCircles, respectively.
Is that the best way to accomplish this, or is there something better (preferably something that leaves the creation of Squares and Circles intact).
Thanks in advance.
(I do propose a solution down further... bear with me...)
One way to (almost) solve your problem is to use a Visitor design pattern. Something like this:
class DrawVisitor
{
public:
void draw(const Shape &shape); // dispatches to correct private method
private:
void visitSquare(const Square &square);
void visitCircle(const Circle &circle);
};
Then instead of this:
Shape &shape = getShape(); // returns some Shape subclass
shape.draw(); // virtual method
You would do:
DrawVisitor dv;
Shape &shape = getShape();
dv.draw(shape);
Normally in a Visitor pattern you would implement the draw method like this:
DrawVisitor::draw(const Shape &shape)
{
shape.accept(*this);
}
But that only works if the Shape hierarchy was designed to be visited: each subclass implements the virtual method accept by calling the appropriate visitXxxx method on the Visitor. Most likely it was not designed for that.
Without being able to modify the class hierarchy to add a virtual accept method to Shape (and all subclasses), you need some other way to dispatch to the correct draw method. One naieve approach is this:
DrawVisitor::draw(const Shape &shape)
{
if (const Square *pSquare = dynamic_cast<const Square *>(&shape))
{
visitSquare(*pSquare);
}
else if (const Circle *pCircle = dynamic_cast<const Circle *>(&shape))
{
visitCircle(*pCircle);
}
// etc.
}
That will work, but there is a performance hit to using dynamic_cast that way. If you can afford that hit, it is a straightforward approach that is easy to understand, debug, maintain, etc.
Suppose there was an enumeration of all shape types:
enum ShapeId { SQUARE, CIRCLE, ... };
and there was a virtual method ShapeId Shape::getId() const = 0; that each subclass would override to return its ShapeId. Then you could do your dispatch using a massive switch statement instead of the if-elsif-elsif of dynamic_casts. Or perhaps instead of a switch use a hashtable. The best case scenario is to put this mapping function in one place, so that you can define multiple visitors without having to repeat the mapping logic each time.
So you probably don't have a getid() method either. Too bad. What's another way to get an ID that is unique for each type of object? RTTI. This is not necessarily elegant or foolproof, but you can create a hashtable of type_info pointers. You can build this hashtable in some initialization code or build it dynamically (or both).
DrawVisitor::init() // static method or ctor
{
typeMap_[&typeid(Square)] = &visitSquare;
typeMap_[&typeid(Circle)] = &visitCircle;
// etc.
}
DrawVisitor::draw(const Shape &shape)
{
type_info *ti = typeid(shape);
typedef void (DrawVisitor::*VisitFun)(const Shape &shape);
VisitFun visit = 0; // or default draw method?
TypeMap::iterator iter = typeMap_.find(ti);
if (iter != typeMap_.end())
{
visit = iter->second;
}
else if (const Square *pSquare = dynamic_cast<const Square *>(&shape))
{
visit = typeMap_[ti] = &visitSquare;
}
else if (const Circle *pCircle = dynamic_cast<const Circle *>(&shape))
{
visit = typeMap_[ti] = &visitCircle;
}
// etc.
if (visit)
{
// will have to do static_cast<> inside the function
((*this).*(visit))(shape);
}
}
Might be some bugs/syntax errors in there, I haven't tried compiling this example. I have done something like this before -- the technique works. I'm not sure if you might run into problems with shared libraries though.
One last thing I'll add: regardless of how you decide to do the dispatch, it probably makes sense to make a visitor base class:
class ShapeVisitor
{
public:
void visit(const Shape &shape); // not virtual
private:
virtual void visitSquare(const Square &square) = 0;
virtual void visitCircle(const Circle &circle) = 0;
};
What you're describing is somewhat like the decorator pattern. Which is very suitable to change runtime behaviour of existing classes.
But I don't really see how to implement your practical example, if shapes have no way to be drawn, then there's no way to change drawing behaviour at runtime either...
But I suppose this is just a very simplified example for stackoverflow? If all the basic building blocks for the desired functionality are available, then implementing the exact runtime behaviour with such a pattern is certainly a decent option.
One 'off the wall' solution you might like to consider, depending on the circumstance, is to use templates to give you compile time polymorphic behaviour. Before you say anything, I know that this will not give you traditional runtime polymorphism so it may well not be useful but depending on the limitations of the environment in which you're working, it can prove useful:
#include <iostream>
using namespace std;
// This bit's a bit like your library.
struct Square{};
struct Circle{};
struct AShape{};
// and this is your extra stuff.
template < class T >
class Drawable { public: void draw() const { cout << "General Shape" << endl; } };
template <> void Drawable< Square >::draw() const { cout << "Square!" << endl; };
template <> void Drawable< Circle >::draw() const { cout << "Circle!" << endl; };
template < class T >
void drawIt( const T& obj )
{
obj.draw();
}
int main( int argc, char* argv[] )
{
Drawable<Square> a;
Drawable<Circle> b;
Drawable<AShape> c;
a.draw(); // prints "Square!"
b.draw(); // prints "Circle!"
c.draw(); // prints "General Shape" as there's no specific specialisation for an Drawable< AShape >
drawIt(a); // prints "Square!"
drawIt(b); // prints "Circle!"
drawIt(c); // prints "General Shape" as there's no specific specialisation for an Drawable< AShape >
}
The drawIt() method is probably the key thing here as it represents generic behaviour for any class meeting the requirement of having a draw() method. Do watch out for code bloat here though as the compiler will instantiate a separate method for each type passed.
This can be useful in situations where you need to write one function to work on many types which have no common base class. I'm aware that this is not the question you asked, but I thought I'd throw it just as an alternative.
I building a game with QT. Every objects on my GraphicsScene inherits from GraphicsPixmapItem (Player, Obstacles, bombs...). I would like to implment collision effects. For example when the player gets hover a bonus he can pick it.
With the QT framework I can get the collidings items but I don't know which type they are as there isn't instanceof function. Any tips ?
edit: I get the collision "event" the thing I want to do is handle the different collisions. I made another question with better wording.
Design considerations:
I can't recommend inheriting Game objects from their graphic representation. Why? You may want to have multiple graphic representations of one game object (like one in game view or another one in minimap, or whatever). The relation is "Player 'has-a' graphic representation" and not "Player 'is-a' graphic representation". Better solution is to use composition and not inheritance. Other nice effect is possible encapsulation of other collision detection if you are not happy with one provided by Qt, decoupling, ... Truth also is, that for simple game it can be sufficient though.
For simple enough game logic, inheritance where other objects react to active object. Probably too simplistic for any more complex game mechanics.
class Asteroid {
public:
virtual void CollideWithPlayer(Player&) { p.loseHealth(100); }
};
class ExplodingAsteroid: Asteroid {
public:
virtual void CollideWithPlayer(Player&) { explode(); p.loseHealth(1000); }
};
If interaction gets complex(many active objects behaving on their own) you may need to identify your objects:
There's is RTTI, but erm it's hard to recommend see: How expensive is RTTI?
In short: expensive, hard to maintain.
You can use double-dispatch. Identifies objects using two virtual calls.
Problems: Quite a bit of syntax, sometimes difficult to maintain (especially when you add new objects), ownership problems (see more).
Game example from Wikipedia:
class SpaceShip {};
class GiantSpaceShip : public SpaceShip {};
class Asteroid {
public:
virtual void CollideWith(SpaceShip&) {
cout << "Asteroid hit a SpaceShip" << endl;
}
virtual void CollideWith(GiantSpaceShip&) {
cout << "Asteroid hit a GiantSpaceShip" << endl;
}
};
class ExplodingAsteroid : public Asteroid {
public:
virtual void CollideWith(SpaceShip&) {
cout << "ExplodingAsteroid hit a SpaceShip" << endl;
}
virtual void CollideWith(GiantSpaceShip&) {
cout << "ExplodingAsteroid hit a GiantSpaceShip" << endl;
}
};
"enumeration"
virtual function id
class GameObject() {
virtual getId() { return GAME_OBJECT; }
};
class Asteroid() {
virtual getId() { return ASTEROID; }
};
or as a member
class GameObject() {
ID getId() { return id; }
protected:
GameObject(ID id):id(id) {}
private:
ID id;
};
or using template with auto initialization of id (a little mind-boggling syntax, let's omit it :o)
and others
Now for game loop like this:
for each object
update by (fixed) time step
detect collisions and resolve them
you will encounter:
Ownership problems:
player loses health when being hit by asteroid and asteroid is destroyed afterwards..
Asteorid::collideWithPlayer(Player& p) { p.loseHealth(100); this->explode(); }
now consider also
Player::collideWithAsteroid(Asteroid& a) { this->loseHealth(100); a.explode(); }
result: code duplicity or unclear game mechanics
poor man's solution: call someone else to help you :o)
Asteorid::collideWithPlayer(Player& p) { resolveCollision(p, *this); }
Player::collideWithAsteroid(Asteroid& a) { resolveCollision(*this, a); }
resolveCollision(Player, Asteroid) { p.loseHealth(100); a.explode(); }
one idea : Inheritance : Each object a player can collide with has a CollideWithPlayer method that does whatever the object needs
c++ like psuedo code for example
class Item : GameObject {
public:
CollideWithPlayer( Player p );
}
class PointBag : Item {
public:
CollideWithPlayer( Player p ) { p.points += 5000; }
}
You could use an external library like Nvidia PhysX or Bullet which would let you set up callbacks for when collisions occur. By using such libraries, who would not use Qt's collision system, instead, every frame you would simulate the physics and then update the properties of the GraphicsPixmapItem to reflect their state within the physics simulation.
You could have a look at the following Qt Example : Colliding Mice Example.
It's quite easy and it will give you a pretty much nice introduction to basic collisions handling with Qt.
Quickly summarized, each object will have a bounding box that will give you the space it uses... You'll then use these bounding boxes to know whether items are touching each others or not...
Hope this helps !