Member acces into incomplete type C-CodePudding

I am trying to have two classes in C - class1, class2. I want class2 to contain an instance of class1 and class1 to have a shared_ptr to the class2 – a pointer to the object, in which it is contained. So I have following 4 files:

class2.hpp

#include "class1.hpp"
#include <vector>

class class2 {
    friend class1;
private:
    class1 data;
    std::vector<int> numbers;
public:
    class2();
};

class2.cpp

#include "class2.hpp"

class2::class2() : data(std::shared_ptr<class2>(this)){}

class1.hpp

#include <memory>

class class2;
class class1 {
    std::shared_ptr<class2> ptr;
    class1(std::shared_ptr<class2> ptr);
    void add_item(int i);
};

and class1.cpp

#include "class1.hpp"

class1::class1(std::shared_ptr<class2> ptr) {
    ptr = ptr;
}

void class1::add_item(int i) {
    ptr->add_item(6); //Member access into incomplete type 'std::__1::shared_ptr<class2>::element_type' (aka 'class2')
}

I am getting the error in function add_item. I think that it has something to do with the forward declaration of class2. I found this related topic, but there they dont split the files into header file. error: member access into incomplete type : forward declaration of

If I understand well, I should put the function add_item after definition of class2, but I dont how to solve it, if I want to have the files splitted like this. Is it possible? Please note, that I am trying to solve this in bigger project, this is very simplified.

Thanks is advance for any answer.

CodePudding user response：

Forward declare class1 in class2.hpp and include class2.h in class1.cpp.

class2.hpp:

//#include "class1.hpp"       // REMOVE!
#include <vector>

class class1;                 // ADD
class class2 {
...

class2.cpp:

#include "class2.hpp"
#include "class1.hpp"         // ADD

class1.hpp:

#include <memory>

class class2;                // OK

class1.cpp:

#include "class1.hpp"
#include "class2.hpp"         // ADD

class1::class1(std::shared_ptr<class2> ptr) {
    ptr = ptr;
}

CodePudding user response：

I want class2 to contain an instance of class1 and class1 to have a shared_ptr to the class2 – a pointer to the object, in which it is contained.

No, you should not want that. There is no reason for a sub-object to own (even in shared capacity) their super object.

Furthermore, you cannot access members of an incomplete class. You must define the class first.