How to use friend function in multiple classes

Question

I am trying to create a non-member operator<<. However, I want the operator to be accessible to two of my classes. The operator is

void operator<< (ClassA & a, ClassB & b)

In the two classes public part, I say:

friend void operator<< (ClassA & a, ClassB & b);

However, it turned out that the operator can access the private member variable in CLass B but cannot access private member variable in Class A.

Why?

The real code: In the cpp file:

void operator<< (Hand& h, Deck& d){

    h.hand.push_back(d.card[0]);
    sort(h.hand.begin(),h.hand.end());
    d.card.erase(d.card.begin());
}

In the header file:

class Deck {

private:
    vector<Card> card;

public:
    friend void operator<< (Hand& , Deck& );
};

class Hand {

private:
    vector<Card> hand;

public:
    friend void operator<< (Hand& , Deck& );
};

And the card file didn't work.

Answer 1

Update to the edited question: the following code compiles no problem for me:

#include <vector>
#include <algorithm>
typedef int Card;

class Hand; // NOTE forward declaration

class Deck {
private:
    std::vector<Card> card;

public:
    friend void operator<< (Hand& , Deck&);
};

class Hand {
private:
    std::vector<Card> hand;

public:
    friend void operator<< (Hand& , Deck&);
};

void operator<< (Hand& h, Deck& d) {
    h.hand.push_back(d.card[0]);
    std::sort(h.hand.begin(),h.hand.end());
    d.card.erase(d.card.begin());
}

int main()
{
}

Did you forget to forward declare Hand in the header file?

---

You might be confused because you can define the body of the static friend function inside one of the class's declarations.

Still, a friend declaration is always just a declaration. So, in fact

struct A;
struct B;

struct A
{
    friend bool operator<<(A const&, B const&);
};

struct B
{
    friend bool operator<<(A const&, B const&);
};

bool operator<<(A const&, B const&)
{
    // access to both A and B
    return false;
}

Is equivalent, to

struct A;
struct B;

struct A
{
    friend bool operator<<(A const&, B const&)
    {
        // access to both A and B
        return false;
    }
};

struct B
{
    friend bool operator<<(A const&, B const&);
};