classes with a lot of members - c++

I am attempting to create a D&D encounter simulator.
I have therefore created a class called "Actor" to emulate the behaviour of the players and the monsters. The problem is that while the class for the moment only have 3 member variable, as the simulation gets more accurate it will become necessary to add more member variables to best simulate the stats of the monsters and player e.g. strength, dexterity etc. (possibly more than 10 member variables)
This leads to a constructor with a lot of parameters so the question then becomes; Is there a better way to organize this data, since it can all vary with every instance of the actor class?
Right now the user is required to type in all the states by hand, though I have plans to make file reading for monsters accessible later since monster stats only vary with monster type (dragon, skeleton etc.)
Note: These stats are very important and used to calculate the outcome of every action the "Actor" class can take in the encounter.
EDIT:
A lot of people suggest using inheritance, but the fact is that (Monsters and Players) never have different stats, the monsters like the players are controlled by a player (The Game Master) This is a tabletop game and the simulator is supposed to help the game master balance encounter's ahead of a real game.
Actor.h
#ifndef actor_h_
#define actor_h_
#include "dice.h"
class Actor
{
private:
signed int hp;
signed int ac; // Armor Class
signed int dmg;
public:
Actor( signed int hp, int ac, int dmg);
~Actor();
signed int getHP( void );
signed int getAC( void );
signed int getDmg( void );
void setHP(signed int newHP);
void setAC(signed int newAC);
void setDmg(signed int newDmg);
void attack(Actor* target);
};
#endif
Actor Constructor
Actor::Actor(signed int hp, signed int ac, signed int dmg)
{
this->hp = hp;
this->ac = ac;
this->dmg = dmg;
}
Actor::~Actor(){}

This leads to a constructor with a lot of parameters so the question then becomes; Is there a better way to organize this data, since it can all vary with every instance of the actor class?
Right now the user is required to type in all the states by hand, though I have plans to make file reading for monsters accessible later since monster stats only vary with monster type (dragon, skeleton etc.)
It sounds like this question is less about how to engineer the OOP, but rather how to define all the stats for the program without it being hugely cumbersome.
Reading stats from files is a good idea. You could represent them in one or more JSON files and use Nlohmann's JSON library to read it into your program. You'll still have to write code that says how your program should use the read JSON data, and of course you need to write the JSON in the first place---no free lunch. But it could nevertheless be helpful for organizing the program.
Another thought: for actors like an army of orcs, or wherever there are multiple instances of the same kind, a useful approach could be to define an orc factory function that creates an orc actor and its stats. The factory could pseudorandomly vary speed, strength, etc. so that each orc is personalized a little different without having to manually write stats for each instance.

As you bring up the example of stats yourself, those could go in a
struct StatsBlock {
int str, dex, con, /*others*/;
};
You can then either source them from static class variables or static methods:
class Skeleton : public Actor {
...
public:
static inline const StatsBlock = { 13, 8, ... };
static StatsBlock rollStats() { ... );
};
As a bonus, this also gives you a central place to apply or remove effects that change the stats temporarily, like equipping a "Ring of +1 strength" or being hit with a spell like Polymorph.

It's a basic concept of Polymorphism in C++, and I would suggest to start from reading about it
You can have for example your base class Actor will contain the basic information that every entity in your emulator has (monster, player etc.)
Then you can create a Monster and a Player class, which derive from your Actor class. Those will have their own unique information (tooth size, breaths under water, fly etc..)
If you want to dynamically create new stats for your player as the emulation goes on, I would suggest using smart pointers to hold these members for memory optimization.

If you don't want to have a parameter for each property of the stats of the actor in the constructor of the actor you can consider having a separate stats object and pass an instance of that stats to the constructor:
struct ActorStats {
signed int hp;
signed int ac; // Armor Class
signed int dmg;
}
struct Actor {
Actor(ActorStats stats) : stats_(std::move(stats)) {}
signed int getHP( void );
signed int getAC( void );
signed int getDmg( void );
void setHP(signed int newHP);
void setAC(signed int newAC);
void setDmg(signed int newDmg);
void attack(Actor* target);
protected:
ActorStats stats_;
};
You could then have different strategies of creating an actor, like reading from a save file, or from a preset:
ActorStats read_stats_from_file() {
ActorStats stats;
// some reading logic
stats.hp = // value read from file
return stats;
}
Actor create_actor() {
ActorStats stats = read_stats_from_file();
Actor actor(stats);
return Actor;
}
That way you don't need to implement a getter/setter for each stats, and don't need to use friend for the functions that should initialize the stats. But the raw values are still protected, as soon as they are passed to the Actor.

Related

What is the best way to initialize a more complex class construct with many unchangeable members in c++

I'm currently designing classes that should represent a chaotic storage system.
Lets say we have slots in rows and columns with certain properties.
So the slots have different restrictions in min/max height, width, length, weight and some more that come from a parameter file.
Also the Slots have a max total weight that must be checked before a new parcel gets added to that slot. And also the max weight that a row of slots can hold is lower than the sum of the max weights of the single slots. So for example each individual slot might be able to hold 50kg but the row of 10 slots must not exceed 200kg, so it is not allowed to fill every slot by 100%. The same is true for the Columns where the maximum weight is lower than the sum of the individual weights of the single rows. The row_id and column_id are atrificial numbers for adressing the slot in the physical system with barcodes or whatever that get read for positioning.
As all this parameters do not change over the lifetime of the program, my intention was to design the classes in a way that this properties are readable by getter functions but there should not be any setter functions (maybe not even private ones) in the object o the values cannot be changed by accident.
There is one class/function that reads the config-file and generates the data structure for the rows and columns with the slots. This function should be able to read the config and create objects for every column holding a row of slots and pass all the values from the config down to the slot.
Later when the program is running I also need some way to search for the best matching slot to add the next parcel or for searching parcels and unload them in a certain sequence.
So the (simplfied) basic structure of the classes would be like this:
Class Parcel {
int width;
int height;
int length;
int weight;
}
Class Slot {
vector<Parcel> parcel;
int min_width;
int max_width;
int min_height;
int max_height;
int min_length;
int max_length;
int max_total_weight;
int act_total_weight;
int total_length;
int free_length;
}
Class Row {
vector<Slot> slot;
int row_id;
int max_total_weight;
int act_total_weight;
}
Class Column {
vector<Row> row;
int column_id;
int max_total_weight;
int act_total_weight;
}
Class Storage {
vector<Column> column;
}
So here are my thoughts about how to initialize the data structure:
First possibility would be to pass all the properties in the constructor(s) of the classes, but then the constructors has some huge parameter lists specially for the Slot class that has a lot of properties.
Second thing that came to my mind (and currently my fafourite way to go) is to use config-data-structures that hold all the parameters. This parameter-objects get filled by the config-function and passed to the constructor when initializing the class. Then it also may be useful to use the parameter class as such and not having all the parameters defined in the storage class once more.
Third way is to use private setter and public getter and make the config class friends with the data structure classes to be able to access the setter functions (but i would prefer to have no setters at all in the final storage structure classes.
Fourth way that i was thinking off, was to derive child classes from the structure classes that hold the setter functions (and also some other logic needed for creating the data structure) so the child has no own variables but only additional functions. So the child class is used to fill the properties but the base class gets added to the data structure vector.
I also want to use Factory pattern to initialize the data structure because the objects have often similar or only slightly different properties. So with the second aproach after creating one row of slots I would maybe want to change the max weight of the slots in that row. Therefore I would need to change the setting in the factory and the factory then fills the parameter data structure differently and passes it to the Slot class. Or is it better to pass the data structure to the factory directly and the factory assigns it but then i think this is not what the factory pattern is meant to be.
I don't know if this is a good aproach or which of the above is best practice.
Or am I missing something and there is a way more convenient solution or this?
Thank you (and sorry if the question is maybe not the way it should be)
When constructing your classes as you describe it you can have a look at the creational design patterns.
Your second proposed solution is almost a builder design pattern. This will help you to construct the Slot e.g. piecewise.
As an example:
#include <iostream>
class Slot {
public:
int GetMinWidth() const { return min_width_; };
int GetMaxWidth() const { return max_width_; };
// Builder class
class SlotBuilder {
public:
SlotBuilder& SetMinWidth(int min_width) {
min_width_ = min_width;
return *this;
}
SlotBuilder& SetMaxWidth(int max_width) {
max_width_ = max_width;
return *this;
}
Slot Build() {
return Slot(min_width_, max_width_);
}
private:
int min_width_{/* you can add default value here*/};
int max_width_{/* you can add default value here*/};
};
// This is optional, but creates a neat interface
static SlotBuilder Create() {
static SlotBuilder slot_builder;
return slot_builder;
}
private:
// Make constructor private to restrict access and force the use of the builder
Slot(int min_width, int max_width) : min_width_(min_width), max_width_(max_width) {}
const int min_width_;
const int max_width_;
// .
// .
// Continue with your const attributes
};
int main() {
// Create the slot with the builder
Slot slot = Slot::Create()
.SetMinWidth(10)
.SetMaxWidth(20)
.Build();
std::cout << slot.GetMinWidth() << ", " << slot.GetMaxWidth();
return 0;
}
You can see the example working here
For having different types that are almost the same a Prototype pattern could work if you want to "clone" a class or in your case a Factory pattern could do the job.
There is never an ideal solution or that one pattern that solves it all, so I can't give you a definitive answer, but here are some collected thoughts:
Default values
Primitive types like int don't have a default value, so make sure you give them one explicitly:
struct Parcel {
int width{};
int height = 0;
int length = {};
int weight{};
}
All those different versions above are equivalent, but you really should use one of them. Otherwise you will probably run into UB down the line.
Const correctness
One thing that I love about C++ and that I dearly miss in languages like C# is const correctness. If you want an object to be immutable, declare it as const. To prevent changes to your objects, either instantiate the object as a const:
const Parcel x;
x.width = 10; // compiler error
or make the members of your classes const:
struct Parcel {
const int width{};
const int height{};
const int length{};
const int weight{};
};
Parcel x;
x.width = 10; // compiler error
Aggregate initialization
If you keep your types simple enough you can initialize the class members with curly braces directly:
const Parcel x { 1, 2, 3, 4 };
In C++ 20, you can also name the members, so this code is equivalent to the line above:
const Parcel x { .width = 1, .height = 2, .length = 3, .weight = 4 };
Note that this can bite you later though if you have to deal with ABI stability and versioning. In that case you are better off using getter and setter functions, because that allows you to still change the data layout of your members.
I have to think about design patterns a bit more. I'll update this post if something useful comes out of it.

Change base class from derived class permanently?

So, my problem is that i want to modify my parent class Board from a derived class in such a way that it applies to all other objects of the derived class. Ex. If I input a 3 on getTest() in players[1] players[2] will be able to print that same value. Is this posible?
class Board {
public:
int test;
virtual void getTest() = 0;
};
class Player : public Board{
public:
int playerNum;
Player(int _playerNum){
playerNum = _playerNum;
}
void printTest(){
cout << "The value of test is: " << Board::test;
}
void getTest(){
cin >> Board::test;
}
};
int main(){
Player players[] = {1,2};
players[1].getTest();
players[0].printTest();
return 0;
}
It is indeed possible to share state between all instances that derive from Board.
As mentioned in the comments, if Board::test were given the static storage-class duration, this would be a variable shared across all instances of Player and any other classes that derive from Board now and forever. Technically, the variable is actually a member of the type rather than instances of the type. This will work, however in terms of design, it has some strange implications.
Namely, Player::getTest() is a non-static member-function, which sets static state that will be shared across all derived classes of Board. This can work as a quick and dirty change to code, but can lead to maintenance burdens and cognitive overhead. For example, if you have code in different subsystems like a Player and Widget that both implement Board, you get into a case where:
// subsystem A:
player.getState();
// subsystem B:
widget.printTest(); // not obvious that this comes from the player in "subsystem A"
The bigger the code gets, the harder it is to understand.
Additionally, static variables have issues working across translation units that can lead to initialization-order problems -- and it's often a mess that's not worth fighting with.
Depending on what it is you actually intend to share, often times it can be better for design to explicitly state as such using an object that explicitly indicates its shared ownership -- such as a std::shared_ptr.
This won't be implicitly passed to everything as you originally requested -- but that's actually a good thing since you can explicitly state what data you want shared and where. This also lets you decide if ever there's a case where you don't want everything shared between them, you can design it as such.
A simple example with your current code would instead be done like:
class Board {
public:
virtual void getTest() = 0;
};
class Player : public Board{
public:
int playerNum;
std::shared_ptr<int> test;
Player(int _playerNum, std::shared_ptr<int> _test){
playerNum = _playerNum;
test = _test;
}
void printTest(){
cout << "The value of test is: " << *test;
}
void getTest(){
cin >> *test;
}
};
int main(){
std::shared_ptr<int> test = std::make_shared<int>(0);
// explicitly share 'test' between the two players
Player players[] = {Player{1,test}, Player{2,test}};
players[1].getTest();
players[0].printTest();
return 0;
}
This would produce the same results as you originally requested, but it explicitly shared test rather than implicitly doing this behind static variables.
Explicitly stating the shared state here also allows you to produce more Player objects later that might share a different set of test state than the first created ones (depending on what this state is, this can be quite useful).
Overall, it's often better in terms of design for readability and overall cognitive overhead to be explicit about the data you want shared.

Use case of dynamic_cast

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 = &ac;
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.

Handling derived class creation using mappers in C++

I'm reading through Martin Fowler's PoEAA right now on object-relational structural patterns. As a project to do while learning them, I thought I'd build a mini eCommerce system in C++. I'm having trouble figuring out how to return the objects from the mapper.
I have a Product base class, which has derived classes Hat and Shirt. Products have a type member to identify which derived class they are. I also have a ProductMapper class, with derived classes HatMapper and ShirtMapper, all of which implement a bunch of finder methods which let me try and retrieve certain hats and shirts.
class Product
{
unsigned long long int id;
std::string name;
unsigned int type;
};
// Derived classes don't necessarily have the same members.
class Hat : public Product
{
unsigned char fitted;
unsigned char color;
unsigned char style;
};
class Shirt : public Product
{
unsigned char size;
};
In the logic part of my application where I'd instantiate these mappers and retrieve products is where I'm having trouble. I can instantiate a HatMapper and pull back Hat objects without any problem, same with a ShirtMapper and Shirt objects. The patterns work great in these cases (in particular I'm using class table inheritance, i.e. one product table with product data, one table for hats with hat-specific data, and one table for shirts with shirt-specific data).
My problem is, what do I do if I want to retrieve all products, both hats and shirts? I can instantiate a ProductMapper and fetch all of the product data, but that seems like the wrong approach since I'd have to loop through all the Products I retrieve and build up Hats and Shirts based on their type in my logic portion of the program. Additionally, I'd have to modify any code that handles the situation this way when I add new product types. Seems bad.
The Fowler book has examples of the mappers with the base mapper using the derived mappers which seems completely wrong to me (have to modify the base mapper every time I add a new product, not great). Here's a quick example of how it's done there:
class ProductMapper
{
unsigned long long int productId;
unsigned long long int productType;
HatMapper * hm;
ShirtMapper * sm;
Product * FindById(unsigned long long int id)
{
// Query database for data.
if (this->productType == PRODUCT_TYPE_HAT)
{
return hm->FindById(id); // Hat object.
}
else if (this->productType == PRODUCT_TYPE_SHIRT)
{
return sm->FindById(id); // Shirt object.
}
return NULL;
}
};
Here's how I'd use this in the logic part of my program. Examples of this aren't provided in the book:
ProductMapper * pm = new ProductMapper();
Product * p = pm->FindById(1); // It's a Product, but a Hat or Shirt?
// Have to check type since a Product was returned.
switch (p->type)
{
case PRODUCT_TYPE_HAT:
{
Hat * h = (Hat) p;
break;
}
// Etc. Modify this every time a new product type is added or removed.
}
This will introduce circular dependencies. Additionally, assuming I somehow eliminate the circular dependencies, the result of the HatMapper and ShirtMapper classes are Hat objects and Shirt objects. Thus when I return from the ProductMapper, I'll be downcasting, so I'd have to again manipulate the result in my logic, which again introduces the issue of modifying code when I introduce new product types.
I'm at a loss for what to do. In a perfect world, I'd like to have a Product class and a ProductMapper class, both of which I can extend quickly, introducing new product types without having to modify existing code (at least too much).
I would like to be able to use these patterns from PoEAA, they do seem nice and useful, but I'm not sure if it's just something I can't do in C++ or I'm missing something that's preventing me from doing it. Alternative patterns and approaches are also really welcomed.
It feels like the Type Object pattern could help in this case, I know the link is about game programming but it is sufficient to apply the pattern to other domains.
The problem right now is that if you want to add products you have to add several classes, which can become hard to maintain as you noticed.
Edit: Maybe you could use something like that (code is C++11 to simplify the example):
class ProductProperty
{
typedef std::map<std::string, unsigned char> PropertyMap;
PropertyMap properties;
public:
ProductProperty(std::initializer_list<PropertyMap::value_type> il):
properties(il)
{}
// Use of at() is intended to only deal with the defined properties
const PropertyMap::value_type::second_type&
get(const PropertyMap::value_type::first_type& prop) const
{
return properties.at(prop);
}
PropertyMap::value_type::second_type&
get(const PropertyMap::value_type::first_type& prop)
{
return properties.at(prop);
}
};
// Some helpers to illustrate
std::shared_ptr<ProductProperty> makeHatProperty()
{
return std::make_shared<ProductProperty>(
ProductProperty{
{"fitted", ***whatever**},
{"color", ***whatever**},
{"style", ***whatever**}
});
}
std::shared_ptr<ProductProperty> makeShirtProperty()
{
return std::make_shared<ProductProperty>(
ProductProperty{{"size", ***whatever**}}
);
}
class Product
{
unsigned long long int id;
std::string name;
unsigned int type;
std::shared_ptr<ProductProperty> properties;
public:
Product(std::shared_ptr<ProductProperty> props):
properties(props)
{}
// Whatever function you need to get/set/check properties
};

Efficient way to generate id unique to class?

Is there any efficient way in C++ of generating an ID unique to the class, not to the instance? I'm looking for something of this level of simplicity (this generates an ID for every instance, not for every class type):
MyClass::MyClass()
{
static unsigned int i = 0;
id_ = i++;
}
Edit: Why I want unique IDs.
I'm writing a game. All entities in my game will have different states they can be in (walking left, jumping, standing, etc); these states are defined in classes. Each state needs to have its own ID so I can identify it.
You can try this, but it's not-deterministic.
int id_count = 0;
template <typename T>
int get_id()
{
static int id = id_count++;
return id;
}
Then just use:
get_id<int>(); // etc.
Of course, this isn't thread safe.
Again, it's not deterministic: the IDs are generated the first time you call the function for each type. So, if on one run you call get_id<int>() before get_id<float>() then on another run you call them the other way round then they'll have different IDs. However, they will always be unique for each type in a single run.
Basically you are asking for a custom rolled RTTI solution, that you can selectively apply to classes.
This can start from very crude preprocessor stuff like :
#define DECLARE_RTTI_CLASS(a) class a { \
inline const char * class_id() { return #a };
.. to a more sophisticated solutions that track inheritance etc, essentially partially duplicating compiler RTTI functionality. For an example, see Game Programming Gems #2, Dynamic Type Information
Previous discussions on gamedev on the same subject are also worth reading
Use your MyClass as a primitive, and incorporate a static instance of one into each class you want to ID.
class MyOtherClass1 {
static MyClass id;
};
class MyOtherClass2 {
static MyClass id;
};
[etc.]