What is the proper method to grant class access to modified value of private member of different class.

Using Friend Class is granting me access to value of private member if provided but won't let me access to modified value data of that member.

For example when I am building vector in one Class and I would like to work on build up data of that vector in another class.

How to grant access for

bar_start.Print()

have the same values as

foo_start.PrintData()

I believe its still in memory and it wasn't deleted

class Foo
{
public:
    Foo();
    ~Foo();
    void ChangeData();
    void PrintData();

private:
    int k = 0;
    std::vector<int> m_vector;
    friend class Bar;
};

class Bar
{
public:
    Bar();
    ~Bar();
    void Print();
};

void Foo::ChangeData()
{
    m_vector.push_back(1);
    int k = 5;
}

void Foo::PrintData()
{
    std::cout << k << std::endl;
    std::cout << m_vector[0] << std::endl;
}

void Bar::Print()
{
    Foo obj;
    std::cout << obj.k << std::endl;
    std::cout << obj.m_vector[0] << std::endl;
}

// printing in main() function

Foo foo_start;
Bar bar_start;

foo_start.ChangeData();
foo_start.PrintData(); // k = 5, m_vector[0] = 1;

bar_start.Print(); // k = 0, m_vector[0] empty and error due not existing element in vector

CodePudding user response：

You may want to store a reference / pointer to a particular Foo inside each Bar instance.

class Bar {
public:
    Bar(Foo& f) : foo(&f) {} // take a Foo by reference and store a pointer to it

    void Print();

private:
    Foo* foo;
};

void Bar::Print() { // use the pointer to the Foo in here:
    std::cout << foo->k << std::endl;
    std::cout << foo->m_vector[0] << std::endl;
}

You would then need to create each Bar supplying the Foo instance you'd like it to be connected to so to speak:

int main() {
    Foo foo_start;
    Bar bar_start(foo_start); // this Bar now stores a pointer to foo_start

    foo_start.ChangeData();
    bar_start.Print();
}

Also note that the k you instantiate inside ChangeData is not the member variable k. You initialize a new variable named k with 5 and then that k is forgotten.

void Foo::ChangeData()
{
    m_vector.push_back(1);
    int k = 5;
}

If you want to make a change to the member variable k, remove the declaration part and just assign 5 to it:

void Foo::ChangeData()
{
    m_vector.push_back(1);
    k = 5;
} 

CodePudding user response：

It seems you mean something like the following

void Bar::Print( Foo &obj )
{
    std::cout << obj.k << std::endl;
    std::cout << obj.m_vector[0] << std::endl;
}

and call the method like

bar_start.Print( foo_start );

Or just

void Bar::Print( Foo &obj )
{
    obj.PrintData()
}

and call the method like

bar_start.Print( foo_start );

That is you need to pass to the member function Print an object of the type Foo by reference.

To use the last shown function definition there is no need to declare the class Bar as a friend of the class Foo because the member function PrintData is a public member function.