Problem with static variable definitions

Discussion in 'C++' started by Jaco Naude, Jul 3, 2009.

  1. Jaco Naude

    Jaco Naude Guest

    Hi

    I'm trying to implement a template based factory, following the
    guidelines of the following article:
    http://www.codeproject.com/KB/architecture/all_kinds_of_factories.aspx
    (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
    `ZN9Qtilities24AbstractFormattingEngineD1Ev':
    D:/ScinericSoftware/Products/Qtilities/trunk/include/Qtilities/
    AbstractFormattingEngine.h:(.bss+0x8): multiple definition of
    `Qtilities::FileLoggerEngine::factory'
    ../tmp\FileLoggerEngine.o:c:/Tools/Qt/2009.02/qt/include/QtCore/../../
    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
    {
    public:
    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>
    {
    public:
    FactoryItem() { tag = QString("");}
    virtual ~FactoryItem() {}
    virtual BaseClass *createInstance() {return new
    ActualClass;}
    QString getTag() { return tag; }
    bool setTag(const QString& iface_tag) {
    if (tag == QString("")) {
    tag = iface_tag;
    return true;
    } else
    return false;
    }
    private:
    QString tag;
    };

    //! A factory class which can produce class instances through
    registered factory interfaces.
    /*!
    */
    template <class BaseClass>
    class Factory
    {
    public:
    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();
    else
    return 0;
    }

    private:
    QMap<QString,FactoryInterface<BaseClass>* > reg_ifaces;
    };

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

    class FormattingEngine_Default : virtual public
    AbstractFormattingEngine
    {
    public:
    FormattingEngine_Default() : AbstractFormattingEngine() {
    factory_item.setTag("Default");
    }
    ~FormattingEngine_Default() {}

    static FactoryItem<AbstractFormattingEngine,
    FormattingEngine_Default> factory_item;

    };
    FactoryItem<AbstractFormattingEngine, FormattingEngine_Default>
    FormattingEngine_Default::factory_item;

    And another class like this:

    class QTILITIES_SHARED_EXPORT FileLoggerEngine : virtual public
    AbstractLoggerEngine
    {

    public:
    FileLoggerEngine();
    ~FileLoggerEngine();

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

    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,
    Jaco
     
    Jaco Naude, Jul 3, 2009
    #1
    1. Advertising

  2. * Jaco Naude:
    > Hi
    >
    > I'm trying to implement a template based factory, following the
    > guidelines of the following article:
    > http://www.codeproject.com/KB/architecture/all_kinds_of_factories.aspx
    > (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
    > `ZN9Qtilities24AbstractFormattingEngineD1Ev':
    > D:/ScinericSoftware/Products/Qtilities/trunk/include/Qtilities/
    > AbstractFormattingEngine.h:(.bss+0x8): multiple definition of
    > `Qtilities::FileLoggerEngine::factory'


    That's probably because you've defined the variable in a header file.

    Define it in a separately compiled file.

    Or return a reference to it from a function.


    > ./tmp\FileLoggerEngine.o:c:/Tools/Qt/2009.02/qt/include/QtCore/../../
    > 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
    > {
    > public:
    > FactoryInterface() {}
    > virtual ~FactoryInterface() {}
    > virtual BaseClass *createInstance() = 0;


    This one should probably be 'const'.


    > virtual QString getTag() = 0;


    This one should probably be 'const'.


    > };
    >
    > //! 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>
    > {
    > public:
    > FactoryItem() { tag = QString("");}


    Just do

    FactoryItem(): tag( "" ) {}

    or assuming any reasonable default constructor in QString,

    FactoryItem() {}


    > virtual ~FactoryItem() {}
    > virtual BaseClass *createInstance() {return new
    > ActualClass;}


    With modern compilers you can make that function covariant and thus more useful,

    virtual ActualClass* createInstance() const { return new ActualClass; }

    Note the 'const'.


    > QString getTag() { return tag; }


    No matter what purpose the "tag" plays, this should probably be 'const'.


    > bool setTag(const QString& iface_tag) {
    > if (tag == QString("")) {
    > tag = iface_tag;
    > return true;
    > } else
    > return false;
    > }


    This function implies that the "tag" should only be set once.

    Why not set it on construction?

    And since presumably it would be an error to try to set it more than once, if
    you do not set it on construction, throw an exception on attempt to set twice.


    > private:
    > QString tag;
    > };
    >
    > //! A factory class which can produce class instances through
    > registered factory interfaces.
    > /*!
    > */
    > template <class BaseClass>
    > class Factory
    > {
    > public:
    > Factory() {}
    > ~Factory() {}


    Are you sure that this destructor should be empty?


    > void registerFactoryInterface(FactoryInterface<BaseClass>*
    > interface) {
    > if (interface->getTag() != QString(""))
    > reg_ifaces[interface->getTag()] = interface;
    > }


    Should failure to register really just be ignored?


    > void unregisterFactoryInterface(const QString& tag)
    > { reg_ifaces.remove(tag); }
    > QStringList registeredTags() { return reg_ifaces.keys(); }


    This one should probably be 'const'.


    > bool isTagValid(const QString& tag) { return
    > reg_ifaces.contains(tag); }


    This one should probably be 'const'.


    > BaseClass* createInstance(const QString& tag) {
    > if (isTagValid(tag))
    > return reg_ifaces.value(tag)->createInstance();
    > else
    > return 0;
    > }


    First, this implies that in order to create an instance you need a non-empty tag.

    Therefore, that tag *should* be set on construction.

    Second, failure to create an instance is serious. It should produce an exception.


    >
    > private:
    > QMap<QString,FactoryInterface<BaseClass>* > reg_ifaces;
    > };


    Does the QMap really deallocate the dynamically allocated FactoryInterface
    instances?


    > I then create classes with static FactoryItem factories in them as
    > shown below:
    >
    > class FormattingEngine_Default : virtual public
    > AbstractFormattingEngine
    > {
    > public:
    > FormattingEngine_Default() : AbstractFormattingEngine() {
    > factory_item.setTag("Default");
    > }
    > ~FormattingEngine_Default() {}
    >
    > static FactoryItem<AbstractFormattingEngine,
    > FormattingEngine_Default> factory_item;
    >
    > };




    > FactoryItem<AbstractFormattingEngine, FormattingEngine_Default>
    > FormattingEngine_Default::factory_item;


    This one should not be in a header file.


    > And another class like this:
    >
    > class QTILITIES_SHARED_EXPORT FileLoggerEngine : virtual public
    > AbstractLoggerEngine
    > {
    >
    > public:
    > FileLoggerEngine();
    > ~FileLoggerEngine();
    >
    > // Make this class a factory item
    > static FactoryItem<AbstractLoggerEngine, FileLoggerEngine>
    > factory;
    > };




    > FactoryItem<AbstractLoggerEngine, FileLoggerEngine>
    > FileLoggerEngine::factory;


    This one should not be in a header file.


    Cheers & hth.,

    - Alf
     
    Alf P. Steinbach, Jul 3, 2009
    #2
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Patrice
    Replies:
    1
    Views:
    1,681
    =?Utf-8?B?REZyYW5rbHlu?=
    May 13, 2004
  2. Rakesh Sinha
    Replies:
    4
    Views:
    1,869
    Rakesh Sinha
    Jan 13, 2005
  3. LuB

    static definitions

    LuB, May 28, 2005, in forum: C++
    Replies:
    2
    Views:
    283
  4. Roman Mashak

    question about 'static' definitions

    Roman Mashak, May 21, 2005, in forum: C Programming
    Replies:
    3
    Views:
    336
    Malcolm
    May 21, 2005
  5. Replies:
    1
    Views:
    176
    Florian GroƟ
    Oct 26, 2005
Loading...

Share This Page