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VarnishedOtter

Circular Dependency Problem

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Hi,

I am having a problem with circular dependency and forward declaration doesn't seem to solve it.

Here is the situation:


class A contains an object of class B
class B contains a std::vector of class A objects

class B is declared first with a forward declaration
class A is defined
class B is defined

I am getting an error that class B is undefined when I use it in class A.

Thanks for any help.

Regards,
-Matt

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You will need to use pointers and forward declarations for this to work.

//In B.h
struct A;

struct B {
std::vector<A*> theAs;
};

//In A.h
struct B;

struct A {
B* parent;
};

#include <vector>
#include "A.h"
#include "B.h"

int main() {
B b;
A* a = new A();
a.parent = &b;
b.theAs.push_back(a);

for(size_t i = 0; i < b.theAs.size(); ++i)
delete b.theAs; //what you new, you delete
}


Additionally, you'll need to new the A's and B's (depending on various factors), you should use smart pointers, and if you don't you have to delete the pointers to everything you new...

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[color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif]

Hi Washu,

[/font]

[color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif]

Thanks for the solution, it works. :)

[/font]


[color=#282828][font=helvetica, arial, verdana, tahoma, sans-serif]

Regards,

[/font]

-Matt

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You will need to use pointers and forward declarations for this to work.

//In B.h
struct A;

struct B {
std::vector<A*> theAs;
};

//In A.h
struct B;

struct A {
B* parent;
};

#include <vector>
#include "A.h"
#include "B.h"

int main() {
B b;
A* a = new A();
a.parent = &b;
b.theAs.push_back(a);

for(size_t i = 0; i < b.theAs.size(); ++i)
delete b.theAs; //what you new, you delete
}


Additionally, you'll need to new the A's and B's (depending on various factors), you should use smart pointers, and if you don't you have to delete the pointers to everything you new...


It's not just pointers, you can do this with references as well:

class SomeClass;

class MyClass
{
public:
MyClass(SomeClass &someOtherClass);

private:
SomeClass &mSomeOtherClass;
};

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You generally do not want to store references as members (because you cannot reseat references). I believe it is illegal to try to store references in a standard container.

So while yes, sometimes you can use references in circular situations, I don't think it would be appropriate here.

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