How to solve this forward class declaration problem?

Question

I am having a following situation:

#include <iostream>
#include <vector>
#include <string>
using namespace std;

// Forward declarations
class Owner;
class Item;

class Owner {
public:
    string name;
    vector<Item> item;
    Owner(string _name, initializer_list<Item> _item) : name(_name), item(_item) {
        for (size_t i = 0; i < item.size(); i++)
            item[i].owner = this;
    }
    // change the owner when copying...
    Owner(const Owner& o) {
        name = o.name;
        item = o.item;
        for (size_t i = 0; i < item.size(); i++)
            item[i].owner = this; // is this OK? 
    }
    Owner& operator = (const Owner& o) {
        name = o.name;
        item = o.item;
        for (size_t i = 0; i < item.size(); i++)
            item[i].owner = this; 
        return *this;
    }
};
class Item {
public:
    string name;
    Owner* owner;
    Item(string _name) : name(_name) {}
    string getInfo() {
        return name + " owned by " + owner->name;
    }
};
int main(){
    Item Cup("Cup"), Plate("Plate");
    Owner John("John", { Cup, Plate });
    cout << Cup.getInfo() << endl; // expecting "Cup owned by John"
    return 0;
}

One owner can have many items, but each item also must contain info about its owner. When I run this I get

Error C2027 use of undefined type 'Item'

I thought this would be solved by using forward class declarations, but no such luck. Also, I'm guessing my copy constructor and assignment operators are OK?

Answer 1

Forward declaring the type is only part of the solution. You also need move defintions of functions of at least one of the types when dependencies are circular - usually put in .cpp file (but not neccesary as seen in example).

Working example:

#include <iostream>
#include <vector>
#include <string>
using namespace std;

// Forward declarations
class Owner;

class Item {
public:
    string name;
    Owner* owner;
    Item(string _name) : name(_name) {}
    string getInfo();//<-- only declaration now
};

class Owner {
public:
    string name;
    vector<Item> item;
    Owner(string _name, initializer_list<Item> _item) : name(_name), item(_item) {
        for (size_t i = 0; i < item.size(); i++)
            item[i].owner = this;
    }
    // change the owner when copying...
    Owner(const Owner& o) {
        name = o.name;
        item = o.item;
        for (size_t i = 0; i < item.size(); i++)
            item[i].owner = this; // is this OK? 
    }
    Owner& operator = (const Owner& o) {
        name = o.name;
        item = o.item;
        for (size_t i = 0; i < item.size(); i++)
            item[i].owner = this; 
        return *this;
    }
};

//or put in .cpp file
    string Item::getInfo() {
        return name + " owned by " + owner->name;
    }

int main(){
    Item Cup("Cup"), Plate("Plate");
    Owner John("John", { Cup, Plate });
    cout << Cup.getInfo() << endl; // expecting "Cup owned by John"
    return 0;
}

Try it yourself

Furthermore, you copy the Items into the owner, so when you print the original, it only prints "Cup owned by " and "John", the owner, is missing. It's not clear which way this should be fixed - but could eg. be fixed by storing std::shared_ptr instead if the items should be shared.