class Software
{
    friend class SoftwareProducer;

    SoftwareProducer* m_producer;
    int m_key;
    // Only producers can produce software
    Software(SoftwareProducer* producer) : m_producer(producer) { }

public:
    void buy()
    {
        m_key = m_producer->next_key();
    }
};

class SoftwareProducer
{
    friend class Software;

public:
    Software* produce()
    {
        return new Software(this);
    }

private:
    // Only software from this producer can get a valid key for registration
    int next_key()
    {
        return ...;
    }
};

class Software {
private:
    Software() : m_key(0) { /* whatever */ }
public:
    void buy() { m_key = new_key(); }
public:
    static Software *create() { return new Software; }
private:
    static int new_key() { static int example_id = 1; return example_id++; }
private:
    int m_key;
};

Software *soft = Software::create();
soft->buy();