I'm making a simple animation software, cutting it short, basically, i have made few classes out of which
FrameManager class manages animation frames (inside a QMap<int, Frame*> )
FramesEditor class inherits QGraphicsView and is used to view current active frame ( QGraphicsScene)

These classes are all implemented using singleton design pattern, however, im facing issue when defining connection only between FrameManager and FramesEditor instances

in FrameManager class
Qt Code: 
  1. void FrameManager::setCurrentActiveFrame(int frameID)
  2. {
  3.     qDebug()<<">>> frameManager.cpp : currentActiveFrame ID : "<<currentActiveFrame;
  4.     Frame *activeFrame = frameBank[keyStartFrame];
  5.  
  6.     if(frameBank.contains(frameID)){
  7.         currentActiveFrame = frameID;
  8.         activeFrame = frameBank[currentActiveFrame];
  9.     }
  10.     qDebug()<<">>> frameManager.cpp : currentActiveFrame ID updated: "<<currentActiveFrame;
  11.  
  12.     emit (FramesEditor::getInstance())->setNewActiveFrame(activeFrame); //causing error
  13. }
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the last line where im emitting the signal, it is causing the issue
Qt Code: 
  1. E:\GITHUB_REPOSITORIES\ProtoAnimator\protoanimator\framemanager.cpp:74: error: undefined reference to `FramesEditor::setNewActiveFrame(Frame*)'
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I've checked everything such as including frameseditor.h header and related files but could not figure out why is this happening

in case you need more code, here is how im defining connections

I have 3 classes (singleton)
1. Manager (this class is acting as a communication handler between other 2 manager classes)
2. FrameManager
3. SpriteManager

the manager class is acting as a central class that is initialized first
inside it's constructor, i have one of the connection
Qt Code: 
  1. connect(FramesEditor::getInstance(), SIGNAL(setNewActiveFrame(Frame*)), FramesEditor::getInstance(), SLOT(renderFrame(Frame*)));
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note that renderFrame(Frame*) simply calls setScene() method