class Foo;

class Bar{
    private:
        int x_;
        Foo foo_; // error: incomplete type

    public:
        void setx(int x) {x_ = x;};
        void increment(int);

};


class Foo{

public:
    void run(int y,Bar& bar) {bar.setx(y);};
};

void Bar::increment(int i){foo_.run(i,*this);}

class Bar;

class Foo{
public:
    void run(int y,Bar& bar);
};

class Bar { ... };

void Foo::run(int y, Bar& bar) {
    bar.setx(y);
}

class Foo;

class Bar{
    Foo foo_; // error: incomplete type
};

class Foo{
    // put details here
};

class Bar{
    Foo foo_; // OK
};

class Foo;

class Bar{
    std::unique_ptr<Foo> foo_; // OK
};