C++ friend classes

Question

I realize that there are a lot of questions regarding friend classes in C++. My question, though, is tied to a specific scenario. Given the below code, is it appropriate to use friend in such a manner?

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 ...;
    }
};

Thanks,

Best regards,

Answer 1

Sure, that's perfectly reasonable. Basically what you have done is very similar to a factory pattern. I see no issue with that since your code seems to imply that every Software object should have a pointer to its creator.

Though, usually you can avoid to have "Manager" classes like SoftwareProducer, and just have static methods in Software. Since it seems likely that there will be only one SoftwareProducer. Something like this perhaps:

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;
};

Then you can just do this:

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

Of course if you do plan on having more than one SoftwareProducer object, then what you have done seems appropriate.