[SIZE="6"]a simple question:error lnk2019 unresolved external symbol[/SIZE]

[weixj2003ld]

I define a class A as follows:

class A
{
     protected:
     void contentsMousePressEvent (QMouseEvent *e);
     signals:
     void extractClicked();
}
void A::contentsMousePressEvent (QMouseEvent *e)
{
  .....
  emit extractClicked();
}

class A
{
     protected:
     void contentsMousePressEvent (QMouseEvent *e);
     signals:
     void extractClicked();
}
void A::contentsMousePressEvent (QMouseEvent *e)
{
  .....
  emit extractClicked();
}

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when compile,error:error LNK2019: unresolved external symbol ...
means that can not find the function extractClicked.
Why?