QObject::connect error: function is used but not defined in this translation unit...

[clostridium_difficile]

I try to connect signal in MainWindow class with slot in GraphicsScene, but during compilation occurs an error: function 'GraphicsScene::ToolSelected' is used but not defined in this translation unit, and cannot be defined in any other translation unit because its type does not have linkage. Here is simplified code:

#include <QtWidgets>
#include "Globals.h"
#include "ImageLayer.h"

class GraphicsScene : public QGraphicsScene
{
Q_OBJECT

	friend class MainWindow;

public slots:
	void ToolSelected(ActiveTool activeTool);

};

#include <QtWidgets>
#include "Canvas.h"
#include "LayerPreview.h"
#include "Globals.h"
#include "GraphicsScene.h"

namespace Ui
{
	class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

signals:
	void ToolSelected(ActiveTool activeTool);
};

Some MainWindow's function:

...
graphicsScene = QSharedPointer<GraphicsScene>(new GraphicsScene(this));
graphicsScene->setSceneRect(0, 0, width, height);
connect(this, &MainWindow::ToolSelected, graphicsScene, &GraphicsScene::ToolSelected);
...

How to fix it?

[jsulm]

@clostridium_difficile said in QObject::connect error: function is used but not defined in this translation unit...:

How to fix it?

Define void ToolSelected(ActiveTool activeTool)
So, do you have

void GraphicsScene::ToolSelected(ActiveTool activeTool)
{
}

somewhere?

Also, why do you define MainWindow as friend in GraphicsScene?

[clostridium_difficile]

It is defined already. Error still occurs. MainWindow is friend of GraphicsScene since... well, it needs access to private member functions of GraphicsScene.

[jsulm]

@clostridium_difficile said in QObject::connect error: function is used but not defined in this translation unit...:

It is defined already

Can you show the definition? And do you actually build/link the cpp files where the definition is?

[KroMignon]

@clostridium_difficile Maybe a silly question, but is ActiveTool a registered type?

[clostridium_difficile]

@KroMignon

Maybe a silly question, but is ActiveTool a registered type?

What does it exacly mean? ActiveTool's declaration is in file "Globals.h":

#pragma once

namespace
{
	enum class ActiveTool
	{
		None,
		PenTool,
		MoveTool
	};
}

[KroMignon]

@clostridium_difficile said in QObject::connect error: function is used but not defined in this translation unit...:

What does it exacly mean? ActiveTool's declaration is in file "Globals.h":

I you want to use QObject::connect() all arguments type must be registred to Qt with qRegisterMetaType<>()
A far as I know, this is mandatory.

[kshegunov]

@KroMignon said in QObject::connect error: function is used but not defined in this translation unit...:

A far as I know, this is mandatory.

For queued connections, not for direct connections.

@clostridium_difficile said in QObject::connect error: function is used but not defined in this translation unit...:

What does it exacly mean? ActiveTool's declaration is in file "Globals.h":

Remove the anonymous namespace, and try again.

[KroMignon]

@kshegunov said in QObject::connect error: function is used but not defined in this translation unit...:

For queued connections, not for direct connections.

Yes, but as far as I can see, the connect is not forced as Qt::DirectConnection.

If you don't want to have trouble, my suggestion would be to change the function signature to int and cast the int to your enum inside the function:

class MainWindow : public QMainWindow
{
Q_OBJECT

signals:
	void ToolSelected(int);
};

void GraphicsScene::ToolSelected(int activeTool)
{
    ActiveTool at = ActiveTool(activeTool);
    ...
}

[kshegunov]

@KroMignon said in QObject::connect error: function is used but not defined in this translation unit...:

Yes, but as far as I can see, the connect is not forced as Qt::DirectConnection.

True enough. However the connect is between widgets, and these always must live in the main thread, so the connection is a direct one.

If you don't want to have trouble, my suggestion would be to change the function signature to int and cast the int to your enum inside the function:

I disagree. This breaks type-safety for no good reason.

[JKSH]

@KroMignon said in QObject::connect error: function is used but not defined in this translation unit...:

as far as I can see, the connect is not forced as Qt::DirectConnection

Qt::DirectConnection is used by default when connecting two objects that live in the same thread: https://doc.qt.io/qt-5/qt.html#ConnectionType-enum

my suggestion would be to change the function signature to int and cast the int to your enum inside the function

void GraphicsScene::ToolSelected(int activeTool)
{
    ActiveTool at = ActiveTool(activeTool);
    ...
}

I would not do this, for many reasons:

• It breaks type safety as @kshegunov said. In other words, it defeats the whole purpose of enum class which is to improve type safety in enums.
• It is good practice to avoid C-style casts whenever possible. Let's take extra care to avoid introducing bad practices to new C++ programmers, especially when more robust alternatives exist.
• There is a solution that does not require breaking type safety, does not require casting, and ultimately cleans up the code: Just remove the namespace as @kshegunov said. It is a good idea to remove the namespace anyway because anonymous namespaces should usually not be used in headers.