Unresolved external symbol when connecting with new syntax

Mwoua

Hello,

I have a class that inherits from QLineEdit to implement clicked signal :

#if defined(LD_LIBRARY)
#  define LD_DLL Q_DECL_EXPORT
#else
#  define LD_DLL Q_DECL_IMPORT
#endif

class LdLineEdit: public QLineEdit
{
    Q_OBJECT
public:
    LD_DLL explicit LdLineEdit( QWidget *aParent = nullptr );
signals:
    LD_DLL void LdClicked();
protected:
    void mouseReleaseEvent(QMouseEvent *){emit LdClicked();}
}

And when I try to connect to this signal with the new syntax :

connect(&mLESerial, &LdLineEdit::LdClicked,this,&PrReportViewer::LESerialTextEdited);

I have this error :

PrReportViewer.obj:-1: error: LNK2001: unresolved external symbol "public: static struct QMetaObject const LdLineEdit::staticMetaObject" (?staticMetaObject@LdLineEdit@@2UQMetaObject@@B)

However, if i use the old syntax :

connect( &mLESerial, SIGNAL( LdClicked() ), this, SLOT( LESerialTextEdited() ) ) ;

it works (it compiles, and its actually connected)

I need the new syntax to plug a Lambda instead of that slot, and it would be nice to understand anyway.

VRonin

What type is mLESerial? LdLineEdit or LdLineEdit*?

Mwoua

LdLineEdit

Taz742

@Mwoua
And What is LdLineEdit ?

VRonin

Strange error. replace private slots: with protected: btw, the event is not a slot. Also add a constructor or you'll potentially break parent/child dependency.

Last question, what is LD_DLL?

class LdLineEdit: public QLineEdit
{
    Q_OBJECT
Q_DISABLE_COPY(LdLineEdit)
public:
LdLineEdit(QWidget* parent = Q_NULLPTR)
:QLineEdit(parent)
{}
signals:
    LD_DLL void LdClicked();
protected:
    void mouseReleaseEvent(QMouseEvent *){emit LdClicked();}
}

Mwoua

@Taz742
See first post

@VRonin
The constructor is there in my class, I forgot it because I wanted to simplify (I'll edit my first post)

The macro is :

#if defined(LD_LIBRARY)
#  define LD_DLL Q_DECL_EXPORT
#else
#  define LD_DLL Q_DECL_IMPORT
#endif

LdLineEdit is in a dll that I use in my other function

Same issue with the function in protected.

EDIT : it might be linked to the dll, if I do this inside a function of LdLineEdit :

connect( this, &LdLineEdit::LdClicked, this, &LdLineEdit::ThisValueChanged);

It compiles (ThisValueChanged is private slot of LdLineEdit)

VRonin

@Mwoua said in Unresolved external symbol when connecting with new syntax:

LdLineEdit is in a dll

BINGO!
you should export the whole class

class LD_DLL LdLineEdit: public QLineEdit
{
    Q_OBJECT
Q_DISABLE_COPY(LdLineEdit)
public:
LdLineEdit(QWidget* parent = Q_NULLPTR)
:QLineEdit(parent)
{}
signals:
    void LdClicked();
protected:
    void mouseReleaseEvent(QMouseEvent *){emit LdClicked();}
}

kshegunov

Export the whole class, not only a handful of methods.

Edit: Crossposted with our Ronin, but it happens. ;)

Mwoua

Ok, is there a rule to export a whole class or just a part of it?

kshegunov

The rule is you need to export Qt's methods and variables from your class - the ones Q_OBJECT introduces (in this case the staticMetaObject, see your linker error in the OP), so when working with QObjects always export the whole class.

Mwoua

Thanks for the help !

kshegunov

No problem. As to why it works with the old syntax - it works because there's no compile time checking and all is done in runtime.