2

Given two classes:

class A {
    B* b_;
    A(B* b){
        b_ = b;
    }
}

class B {}

B is a class that should be used only inside A so I would like not to expose its creation to whatever class uses A. Moreover, B should not be created every time an instance of A is needed (it has a slow initialization).

Given that I would like to reuse the same instance of B, but I do not really need to make it Singleton, could be a good idea creating a Factory which holds an internal reference to an instance of B?

Something like:

class Factory {
    B* b_;
    A* createA() {
        if(b_ == NULL)
            b_ = new B();
        return new A(b_);
    }
}

My doubt is that I do not find anything useful googling for "stateful factory" and maybe I overlook some drawback in this approach.

3
  • Do you intend the instance of B to last forever, e.g. until the program shuts down? – rwong Mar 3 '17 at 8:54
  • Probably yes. As I said B is slow in creation so it is better to reuse the same instance, but it is not mandatory to have only one instance: multiple instances don't break application logic, they only waste resources. – Marco Stramezzi Mar 3 '17 at 9:05
  • Try stackoverflow.com/questions/26013650/… – rwong Mar 3 '17 at 9:14
1

In a multi-threaded environment, you may have multiple instances of B created simultaneously. Other than that, your design solves all of your requirements with the least amount of code. In general, singletons complicate multi-threading in the future.

I would not call it a factory. It is a lazily initialized reusable instance.

2
  • So, I should mark as static the field b_ of Factory (so I use the same instance of B at every creation of A) and protect its initialization with sinchronization logic, right? – Marco Stramezzi Mar 6 '17 at 8:03
  • @MarcoStramezzi that would protect the initialization but not the usage (i.e. any mutable data in the B instance). It depends on your needs. – Frank Hileman Mar 6 '17 at 16:53

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