c++ Design pattern for CoW, inherited classes, and variable shared data?
- by krunk
I've designed a copy-on-write base class. The class holds the default set of data needed by all children in a shared data model/CoW model.
The derived classes also have data that only pertains to them, but should be CoW between other instances of that derived class.
I'm looking for a clean way to implement this. If I had a base class FooInterface with shared data FooDataPrivate and a derived object FooDerived. I could create a FooDerivedDataPrivate.
The underlying data structure would not effect the exposed getters/setters API, so it's not about how a user interfaces with the objects. I'm just wondering if this is a typical MO for such cases or if there's a better/cleaner way?
What peeks my interest, is I see the potential of inheritance between the the private data classes. E.g. FooDerivedDataPrivate : public FooDataPrivate, but I'm not seeing a way to take advantage of that polymorphism in my derived classes.
class FooDataPrivate
{
public:
Ref ref; // atomic reference counting object
int a;
int b;
int c;
};
class FooInterface
{
public:
// constructors and such
// ....
// methods are implemented to be copy on write.
void setA(int val);
void setB(int val);
void setC(int val);
// copy constructors, destructors, etc. all CoW friendly
private:
FooDataPrivate *data;
};
class FooDerived : public FooInterface
{
public:
FooDerived() : FooInterface() {}
private:
// need more shared data for FooDerived
// this is the ???, how is this best done cleanly?
};
