GenericFactory as Singleton

Question

I read the article Abstract Factory, Template Style by Jim Hyslop and Herb Sutter. This factory is implemented as a Singleton. They provided an easy way to register classes automatically with the RegisterInFactory helper class.

Now i've read several times that Singletons should be avoided, some even consider them as Anti-Patterns and that there are just a few cases where they are usefull. Is this one of them? Or is there an alternative approach which provides such an easy way to autoregister classes?

Answer 1

As ever for this sort of subject, there is no answer that applies to every problems. Some said that singleton should be avoided when they are used as an access to a service. It is a use that is akin to the use of global variables. This way you mask the fact that you use service X in your implementation :

// in header
class MyUsefulClass
{
    public:
    void doSomethingUseful();
};

// in .cpp
MyUsefulClass::doSomethingUseful()
{
    // ...
    MyWonderfulService::instance().doService1();
    // ...
    MyWonderfulService::instance().doService2();
}

You create a coupling with MyWonderfulService that the users of your class can not guess. Moreover, You can not easily test your useful class with a mock service ...

That's why I usually prefer dependancy inversion :

// in header
class MyUsefulClass
{
    public:
    void setServiceToUse(MyService&);
    void doSomethingUseful();

    // [...]
};

// in .cpp
MyUsefulClass::doSomethingUseful()
{
    // ...
    _myService->doService1();
    // ...
    _myService->doService2();
}

This way is usually considered better as the coupling between class is lighter. Nevertheless, for some services, which are well known to be of widespread use in a framework, it is simplier to use a singleton. It makes sense for a single service that is the service which give you access to all other services in a framework for example ^^ It is often use for technical services like logging for instance.

my2c

Edit: I read the article, as the focus is on AbstractFactories, the use of a singleton is a casual one, not a design decision. It is understandable in an article in which you do not want to write things that will not get you to your point.