Deriving from singleton : one object each of base and derived?

Question

I have a singleton class A, with virtual protected destructor and private constructor. I derived publicly from it and create class B.

Can i create two objects one each of base and derived class ?

If i cannot is there any other way to share same code ?

When i tried it i am getting compile time errors :

warning C4356: 'A::variable' : static data member cannot be initialized 
via derived class

B.cpp(4): error C2371: 'variable' : redefinition; different basic types

Theoretically you can, if the base class has a protected constructor. — BartoszKP, Oct 08 '13 at 10:11
Don’t use singletons and you won’t have problems related to singletons. — , Oct 08 '13 at 10:12
Don't write software and you won't have problems related to software engineering. Sound logic. :P — Tony The Lion, Oct 08 '13 at 10:13
@TonyTheLion Except that singletons are a rather problematic pattern (it's just a thinly veiled global variable, after all), so avoiding singletons generally leads to fewer problems and better design. I understand singleton utility when dealing with legacy code, but I try hard to avoid the (anti-)pattern in any new code I write. — Angew is no longer proud of SO, Oct 08 '13 at 10:16
http://stackoverflow.com/questions/137975 (although that just describes language-independent conceptual problems; C++ adds a whole pile of lifetime-management issues to the mix). — Mike Seymour, Oct 08 '13 at 10:20
@TonyTheLion: Sometimes we need to write software, so we have to deal with some problems. There's no need to create more problems than necessary when we do that. — Mike Seymour, Oct 08 '13 at 10:30

score 0 · Answer 1 · answered Oct 08 '13 at 10:20

It all depends on how your singleton is implemented. If it has a public constructor of some sort than you're probably fine with extending it's singleton-ness to a derived instance of B. If it's some kind of factory method then probably not. The fact that it has a protected dtor is a sign that it was built for use as a base class.

score 0 · Answer 2 · answered Oct 08 '13 at 10:22

0

Don't be confused with two independent idioms:

Inheritance is a part of C++ language mechanics
Singleton is an instantiation policy, defined in your code

So the answer to your question is "it depends on how you've implemented the instantiation policy".

answered Oct 08 '13 at 10:22

Yury Schkatula

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score 0 · Answer 3 · edited May 23 '17 at 10:31

If I understand right, what you want to achieve is a reusable singleton pattern base class. You can do this using a template base class, using a pattern named CRTP. This question also provides a simple explanation of this technique.

Concretely, it would mean doing something like this:

template<typename TDerived>
class Singleton {
public:
    static TDerived& instance() {
         static TDerived instance_;
         return instance_;
    }
protected:
    Singleton() {}
    virtual ~Singleton() {}
};

class ConcreteSingleton : public Singleton<ConcreteSingleton> {
public:
    ...
protected:
    ConcreteSingleton() {
         ...
    }
    virtual ~ConcreteSingleton() {
         ...
    }

};

This particular version requires that your base classes have a default constructor. Other, more complex interfaces can alleviate that issue.

Overall, as pointed out in some comments, singleton as a pattern has scalability issues, though const singletons are generally considered safer.

Deriving from singleton : one object each of base and derived?

3 Answers3