Hide private members from a friend class

friend.cpp

#include <iostream>

// forward declare Foo so we can make it a friend of FooPrivate
class Foo;

// allows us to hide instance members from friends of Foo,
// but still allows Foo itself to access them.
class FooPrivate
{
private:

    friend class Foo;
    
    // only allow Foo to derive from this class
    FooPrivate() {};
    
    // hidden from friends of Foo
    int x = 0;

};

// this class hides some of its private members from friend classes
class Foo : public FooPrivate
{
public:

    int getX()
    {
        return x;
    }

private:

    // give Bar access to private method
    friend class Bar;
    
    void setX(int value)
    {
        x = value;
    }

};

class Bar
{
public:

    void modifyFoo(Foo& foo)
    {
        foo.setX(5);
        
        // error: 'x' is a private member of 'FooPrivate'
        //foo.x = 4;
    }

};

int main()
{
    Foo foo;
    
    // prints "0"
    std::cout << foo.getX() << std::endl;
    
    Bar bar;
    bar.modifyFoo(foo);
    
    // prints "5"
    std::cout << foo.getX() << std::endl;
}