This question already has answers here:
Resolve build errors due to circular dependency amongst classes
(12 answers)
Cross referencing included headers in c++ program
(5 answers)
Closed 9 years ago.
For example a little Computergame with three Classes Player, Bot and Game
Player has a Method that checks if the Player collide with a bot
// Player.h
#include Game.h
#include Bot.h
class Player {
private:
bool collision(Game g) {
for (Bot bot: g.bots)
...
}
};
The Bot.h (kept simple, of cause it has some other attributes like actual position and so far)
// Bot.h
class Bot {
public:
Bot()
};
The Gameclass handles the Gameloop and the List of Bots
//Game.h
#include Bot.h
#include Player.h
class Game {
public:
Player player:
std::vector<Bot> bots
void loop() { player.collision() }
};
So here we have the problem that Game.h includes Player.h and the other way around.
How can I resolve this?
While the other answers here are certainly technically correct, whenever I come across a situation like this I see it as pointing out a potential flaw in my design. What you have is cyclic dependencies, like so:
This isn't just a problem for your implementation, it means that there is too much coupling between classes, or, conversely, too little information hiding. This means that you can't design the Player independently of the Game, for example.
So if possible, I'd prefer a situation like this, where the Game, as the controller delegates work out to other classes.
One way to do this is for the Game to pass references to its own properties as and when the Player needs them.
For example have collision take a bots parameter rather than the `Game
bool collision(const std::vector<Bot&> bots) {
// note pass objects by const ref is usu preferred in C++ to pass by value
for (Bot bot: g.bots)
...
}
(Note in a more sophisticated approach, it could pass interfaces, i.e. abstract base classes, onto itself).
If all else fails, you can go back to using a forward declaration.
In this case the easiest thing would be a forward declaration, and moving some code from the header file to the source file.
Like this
Player.h
#include "Bot.h"
class Game; // forward declaration
class Player {
private:
bool collision(Game g);
};
Player.cpp
#include "Player.h"
#include "Game.h"
bool Player::collision(Game g) {
for (Bot bot: g.bots)
...
}
The forward declaration tells the compiler that Game is the name of a class but nothing else. So the Player::collision method must be moved to the Player.cpp file where the full definition of Game is available.
I dont think you can do this since If A contains B, and B contains A, it would be infinite size. Infact you can create two classes that store pointers to one another, by using the forward declaration, so that the two classes know of each other's existence
Forward declaration is required - http://en.wikipedia.org/wiki/Forward_declaration
Related
I am experimenting with the Builder/Fluent style of creating objects trying to extend some ideas presented in a course. One element I immediately didn't like with my test implementation was the large number of additional header files the client needs to include for the process to work, particularly when I wish to make use of public/private headers via the pImpl idiom for purposes of providing a library interface. I'm not entirely certain whether the problem lies with my implementation or I'm just missing an obvious 'last step' to achieve what I want.
The general gist is as follows (using the toy example of Pilots):
Firstly the client code itself:
(Note: for brevity, various boilerplate and irrelevant code has been omitted)
Pilot p = Pilot::create()
.works().atAirline("Sun Air").withRank("Captain")
.lives().atAddress("123 Street").inCity("London")
What's happening here is:
In Pilot.h, the Pilot class is defined with a static member method called create() that returns an instance of a PilotBuilder class defined in PilotBuilder.h and forward declared in Pilot.h
Essentially the PilotBuilder class is a convenience builder only used to present builders of the two different facets of a Pilot (.works() and .lives()), letting you switch from one builder to another.
Pilot.h:
class PilotBuilder;
class Pilot {
private:
// Professional
string airline_name_, rank_;
// Personal
string street_address_, city_;
Pilot(){}
public:
Pilot(Pilot&& other) noexcept;
static PilotBuilder create();
friend class PilotBuilder;
friend class PilotProfessionalBuilder;
friend class PilotPersonalBuilder;
};
Pilot.cpp:
#include "PilotBuilder.h"
PilotBuilder Pilot::create() {
return PilotBuilder();
}
// Other definitions etc
PilotBuilder.h
#include "public/includes/path/Pilot.h"
class PilotProfessionalBuilder;
class PilotPersonalBuilder;
class PilotBuilder {
private:
Pilot p;
protected:
Pilot& pilot_;
explicit PilotBuilder(Pilot& pilot) : pilot_{pilot} {};
public:
PilotBuilder() : pilot_{p} {}
operator Pilot() {
return std::move(pilot_);
}
PilotProfessionalBuilder works();
PilotPersonalBuilder lives();
};
PilotBuilder.cpp
#include "PilotBuilder.h"
#include "PilotProfessionalBuilder.h"
#include "PilotPersonalBuilder.h"
PilotPersonalBuilder PilotBuilder::lives() {
return PilotPersonalBuilder{pilot_};
}
PilotProfessionalBuilder PilotBuilder::works() {
return PilotProfessionalBuilder{pilot_};
}
As you can imagine the PilotProfessionalBuilder class and the PilotPersonalBuilder class simply implement the methods relevant to that particular facet eg(.atAirline()) in the fluent style using the reference provided by the PilotBuilder class, and their implementation isn't relevant to my query.
Avoiding the slightly contentious issue of providing references to private members, my dilemma is that to make use of my pattern as it stands, the client has to look like this:
#include "public/includes/path/Pilot.h"
#include "private/includes/path/PilotBuilder.h"
#include "private/includes/path/PilotProfessionalBuilder.h"
#include "private/includes/path/PilotPersonalBuilder.h"
int main() {
Pilot p = Pilot::create()
.works().atAirline("Sun Air").withRank("Captain")
.lives().atAddress("123 Street").inCity("London");
}
What I cannot figure out is:
How do I reorder or reimplement the code so that I can simply use #include "public/includes/path/Pilot.h" in the client, imagining say, that I'm linking against a Pilots library where the rest of the implementation resides and still keep the same behaviour?
Provided someone can enlighten me on point 1., is there any way it would be then possible to move the private members of Pilot into a unique_ptr<Impl> pImpl and still keep hold of the static create() method? - because the following is obviously not allowed:
:
PilotBuilder Pilot::create() {
pImpl = make_unique(Impl); /* struct containing private members */
return PilotBuilder();
}
Finally, I am by no means an expert at any of this so if any of my terminology is incorrect or coding practices really need fixing I will gladly receive any advice people have to give. Thank you!
I'm pretty new to C++ and I am working in a little roguelike game. I have a generic class named Actor, which has 2 child classes, NPC and Player. The idea is for each child to contain specific data, such as experience provided by killing an NPC or the Player's stats, and special methods. On the other hand, Actor contains general methods such as move, because both player and NPC should move.
Now I have a vector of NPC pointers, and my move method should check if the target tile is occupied by an NPC (and some other NPC info), but I don't have access to that class from Actor. I added a forward declaration to NPC inside of Actor, but then I get this error:
pointer to incomplete class type is not allowed
because forward declaration is not enough to access NPC methods.
Actor.h:
class NPC; // Forward declaration.
class Actor
{
public:
void move(std::vector<std::unique_ptr<NPC>> & NPCs);
}
Actor.cpp:
void Actor::move(std::vector<std::unique_ptr<NPC>> & NPCs)
{
// Go through the NPCs.
for (const auto &NPC : NPCs)
{
if (NPC->getOutlook() > 0) ... // Error.
}
}
I could put the move method inside both NPC and Player, but I would be duplicating code, and that's a pretty bad idea.
What would be the best solution here? I guess there is a better way to organize this, but it seems pretty logical as it is. Maybe some kind of inheritance or virtual functions magic?
Thanks! :)
You will need to include the header where NPC is defined in Actor.cpp, otherwise the definition of NPC will be missing.
// Actor.cpp
#include "NPC.h"
void Actor::move(std::vector<std::unique_ptr<NPC>> & NPCs)
{
// Go through the NPCs.
for (const auto &NPC : NPCs)
{
if (NPC->getOutlook() > 0) ... // Now you'll be able to access this.
}
}
(it is be possible that this question has been asked very often already and i am sorry about this repost, but anything i found just didnt help me, since i am relatively a beginner at c++)
so here is an example to show my problem
i have the class monster
class Monster{
public:
Monster();
void attack();
private:
int _health;
int _damage;
};
and i have the class Level
class Level{
Level();
};
i have created the object "snake" from the class Monster in my "main.cpp"
#include "Monster.h"
int main(){
Monster snake;
}
now what do i do if i want to use "snake" in my "Level" class? if i want to do "snake.attack();" inside of "Level.cpp" for example?
If i declare it again in "Level.cpp" it will be a seperate object with its own attributes wont it?
i have always been making the member functions of my classes static until now, so i could do "Monster::attack();" anywhere in my program but with this tachnique i cant have multiple objects doing different things depending on their attributes (snake1, snake2, bat1, etc...)
thanks for the help in advance!
(and sorry for the possibly reoccuring question)
Presuming those snips are your .h files.
Your level.cpp should something like this:
#include "level.h" // its own header
#include "monster.h" // header with Monster::attack() declaration
Level::DoAttack(Monster& monster) { // using snake as parameter.
health = health - monster.attack(); // monster hits us, subtract health.
}
monster.h would be
class Monster{
public:
Monster();
void attack();
private:
int _health;
int _damage;
};
and monster.cpp
Monster::attack() {
// code to calculate the attack
}
I could not completely understand your questions.But from what I understood.I think you want to access a Monster object instantiated in main() to be used inside level.So,here is what you can do.Add a constructor inside the level class which takes a monster object as an argument.Then instantiate a level object and pass the monster object in it.Like this,
Level l=new Level(snake);
By declaring a class you're not creating any objects. You normally declare a class by including the corresponding header file.
So, in Level.h you'd #include <Monster.h>, then you can reference it inside Level.
But seriously, you can't write much C++ code without understanding the basic things such as declaration vs. definition, header files (.h), classes vs. objects, pointers and references, etc. It would be best to invest in a book or at least to read some tutorials online.
I'm trying to make class functions I can tack on to other classes, like with nested classes. I'm still fairly new to C++, so I may not actually be trying to use nested classes, but to the best of my knowledge that's where I'm at.
Now, I've just written this in Chrome, so it has no real use, but I wanted to keep the code short.
I'm compiling on Windows 7, using Visual Studio 2015.
I have two classes in file_1.h:
#pragma once
#include "file_2.h"
class magic_beans{
public:
magic_beans();
~magic_beans();
int getTotal();
private:
double total[2]; //they have magic fractions
}
class magic_box{
public:
magic_box(); //initiate
~magic_box(); //make sure all objects have been executed
void update();
magic_beans beans; //works fine
magic_apples apples; //does not work
private:
int true_rand; //because it's magic
};
... And I have one class in file_2.h:
#pragma once
#include "file_1.h"
class magic_apples{
public:
magic_apples();
~magic_apples();
int getTotal();
private:
double total[2];
}
Now, I've found that I can simply change:
magic_apples apples;
To:
class magic_apples *apples;
And in my constructor I add:
apples = new magic_apples;
And in my destructor, before you ask:
delete apples;
Why must I refer to a class defined in an external file using pointers, whereas one locally defined is fine?
Ideally I would like to be able to define magic_apples the same way I can define magic_beans. I'm not against using pointers but to keep my code fairly uniform I'm interested in finding an alternative definition method.
I have tried a few alternative defines of magic_apples within my magic_box class in file_1.h but I have been unable to get anything else to work.
You have a circular dependency, file_1.h depends on file_2.h which depends on file_1.h etc. No amount of header include guards or pragmas can solve that problem.
There are two ways of solving the problem, and one way is by using forward declarations and pointers. Pointers solve it because using a pointer you don't need a complete type.
The other way to solve it is to break the circular dependency. By looking at your structures that you show, it seems magic_apples doesn't need the magic_beans type, so you can break the circle by simply not includeing file_1.h. So file_2.h should look like
#pragma once
// Note no include file here!
class magic_apples{
public:
magic_apples();
~magic_apples();
int getTotal();
private:
double total[2];
}
I am working on a little game engine but I got stuck at something. Explanation : I have two classes, cEntity And ObjectFactory :
cEntity
class cEntity:public cEntityProperty
{
Vector2 position;
Vector2 scale;
public:
cEntity(void);
cEntity(const cEntity&);
~cEntity(void);
public:
void init();
void render();
void update();
void release();
};
ObjectFactory
#include "cEntity.h"
#include <vector>
class ObjectFactory
{
static std::vector<cEntity> *entityList;
static int i, j;
public:
static void addEntity(cEntity entity) {
entityList->push_back(entity);
}
private:
ObjectFactory(void);
~ObjectFactory(void);
};
std::vector<cEntity> *ObjectFactory::entityList = new std::vector<cEntity>();
Now I am adding new cEnity to ObjectFactory in cEntity constructor but facing an error related to circular references: for using ObjectFactor::addEntity() I need to define the ObjectFactory.h in cEntity class but it creates a circular reference.
I think your code might have an underlying architectural issue given how you have described the problem.
Your ObjectFactory should be handling the cEntities, which in turn should be unaware of the "level above". From the description of the problem you are having, it implies that you're not sure what class is in charge of what job.
Your cEntitys should expose an interface (i.e. all the stuff marked "public" in a class) that other bits of code interact with. Your ObjectFactory (which is a bit badly named if doing this job, but whatever) should in turn use that interface. The cEntitys shouldn't care who is using the interface: they have one job to do, and they do it. The ObjectFactory should have one job to do that requires it to keep a list of cEntitys around. You don't edit std::string when you use it elsewhere: why is your class any different?
That being said, there's two parts to resolving circular dependencies (beyond "Don't create code that has circular dependencies in the first place" - see the first part to this answer. That's the best way to avoid this sort of problem in my opinion)
1) Include guards. Do something like this to each header (.h) file:
#ifndef CENTITY_H
#define CENTITY_H
class cEntity:public cEntityProperty
{
Vector2 position;
Vector2 scale;
public:
cEntity(void);
cEntity(const cEntity&);
~cEntity(void);
public:
void init();
void render();
void update();
void release();
};
#endif
What this does:
The first time your file is included, CENTITY_H is not defined. The ifndef macro is thus true, and moves to the next line (defining CENTITY_H), before it moves onto the rest of your header.
The second time (and all future times), CENTITY_H is defined, so the ifndef macro skips straight to the endif, skipping your header. Subsequently, your header code only ever ends up in your compiled program once. If you want more details, try looking up how the Linker process.
2) Forward-declaration of your classes.
If ClassA needs a member of type ClassB, and ClassB needs a member of type ClassA you have a problem: neither class knows how much memory it needs to be allocated because it's dependant on another class containing itself.
The solution is that you have a pointer to the other class. Pointers are a fixed and known size by the compiler, so we don't have a problem. We do, however, need to tell the compiler to not worry too much if it runs into a symbol (class name) that we haven't previously defined yet, so we just add class Whatever; before we start using it.
In your case, change cEntity instances to pointers, and forward-declare the class at the start. You are now able to freely use ObjectFactory in cEntity.
#include "cEntity.h"
#include <vector>
class cEntity; // Compiler knows that we'll totally define this later, if we haven't already
class ObjectFactory
{
static std::vector<cEntity*> *entityList; // vector of pointers
static int i, j;
public:
static void addEntity(cEntity* entity) {
entityList->push_back(entity);
}
// Equally valid would be:
// static void addEntity(cEntity entity) {
// entityList->push_back(&entity);}
// (in both cases, you're pushing an address onto the vector.)
// Function arguments don't matter when the class is trying to work out how big it is in memory
private:
ObjectFactory(void);
~ObjectFactory(void);
};
std::vector<cEntity*> *ObjectFactory::entityList = new std::vector<cEntity*>();