Problem with static variable definitions

Jaco Naude <>
Fri, 3 Jul 2009 00:28:30 -0700 (PDT)

I'm trying to implement a template based factory, following the
guidelines of the following article:
(I'm trying Factory 6: the template-factory).

I get it to work fine, but as soon as I define more than 1 static
factory, each one in a different class, I get the following error:

Creating library file: bin\libQtilities.a
../tmp\Logger.o: In function
AbstractFormattingEngine.h:(.bss+0x8): multiple definition of
src/corelib/tools/qstring.h:(.bss+0x0): first defined here
collect2: ld returned 1 exit status

Below is some code to show exactly what I'm doing:

    //! Factory interface which is used by factories to create
instances of registered class types.
    template <class BaseClass>
    class FactoryInterface
          FactoryInterface() {}
          virtual ~FactoryInterface() {}
          virtual BaseClass *createInstance() = 0;
          virtual QString getTag() = 0;

    //! Factory item class which is used inside classes which can
register themselves as items in factories.
    template <class BaseClass,class ActualClass>
    class FactoryItem : public FactoryInterface<BaseClass>
          FactoryItem() { tag = QString("");}
          virtual ~FactoryItem() {}
          virtual BaseClass *createInstance() {return new
          QString getTag() { return tag; }
          bool setTag(const QString& iface_tag) {
              if (tag == QString("")) {
                  tag = iface_tag;
                  return true;
              } else
                  return false;
          QString tag;

    //! A factory class which can produce class instances through
registered factory interfaces.
    template <class BaseClass>
    class Factory
          Factory() {}
          ~Factory() {}

          void registerFactoryInterface(FactoryInterface<BaseClass>*
interface) {
              if (interface->getTag() != QString(""))
                reg_ifaces[interface->getTag()] = interface;
          void unregisterFactoryInterface(const QString& tag)
{ reg_ifaces.remove(tag); }
          QStringList registeredTags() { return reg_ifaces.keys(); }
          bool isTagValid(const QString& tag) { return
reg_ifaces.contains(tag); }
          BaseClass* createInstance(const QString& tag) {
              if (isTagValid(tag))
                  return reg_ifaces.value(tag)->createInstance();
                  return 0;

          QMap<QString,FactoryInterface<BaseClass>* > reg_ifaces;

I then create classes with static FactoryItem factories in them as
shown below:

    class FormattingEngine_Default : virtual public
        FormattingEngine_Default() : AbstractFormattingEngine() {
        ~FormattingEngine_Default() {}

        static FactoryItem<AbstractFormattingEngine,
FormattingEngine_Default> factory_item;

    FactoryItem<AbstractFormattingEngine, FormattingEngine_Default>

And another class like this:

    class QTILITIES_SHARED_EXPORT FileLoggerEngine : virtual public


        // Make this class a factory item
        static FactoryItem<AbstractLoggerEngine, FileLoggerEngine>
    FactoryItem<AbstractLoggerEngine, FileLoggerEngine>

If needed I can post the base classes as well. I hope I posted
everything that is necessary. To be honest, I wondered why the
FileLoggerEngine::factory variable needs to be defined outside the
class definition while I was reading the article, and now this gives
the problem.

Any ideas on why I get this error will be greatly appreciated.

Thanks in advance,

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