Using signals in Qt breaks DLL - c++

Question

I'm running a simulation program in Visual Studio 2013 for which I wanted a simple GUI to output/input data.
Since I know some Qt I decided to write a small Qt5 program in Qt Creator, build it as a .dll and link this .dll in my program. The program then calls an initialization function to start the GUI.
Overall this works quite well. The GUI works just like a stand-alone Qt program would. However, once I added a custom signal to my Qt .dll like this:
//File.h
class MainGui : public QMainWindow
{
Q_OBJECT
public:
explicit MainGui(QWidget *parent = 0);
~MainGui();
signals:
void addItemSignal(QGraphicsView* it);
private slots:
void addItemImpl(QGraphicsView* it);
private:
Ui::MainGui *ui;
};
//File.cpp
void MainGui::addItemImpl(QGraphicsView *it)
{
//do anything
}
MainGui::MainGui(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainGui)
{
QObject::connect(this, &MainGui::addItemSignal,
this, &MainGui::addItemImpl);
}
MainGui::~MainGui()
{
delete ui;
}
I ended up getting the following error message when trying to start my main program:
The procedure entry point could not be located in the dynamic link library e:\...\...\MyQtLibrary.dll.
The two spaces between "point" and "could" are not a mistake - it seems like, for some reason, the entry point is not set to anything at all.
After some testing I discovered that the issue lies with using Qt classes within the signals. The following works fine:
//File.h
class MainGui : public QMainWindow
{
Q_OBJECT
public:
explicit MainGui(QWidget *parent = 0);
~MainGui();
signals:
void addItemSignal(void* it);
private slots:
void addItemImpl(void* it);
private:
Ui::MainGui *ui;
};
//File.cpp
void MainGui::addItemImpl(void*it)
{
//do anything
}
MainGui::MainGui(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainGui)
{
QObject::connect(this, &MainGui::addItemSignal,
this, &MainGui::addItemImpl);
}
MainGui::~MainGui()
{
delete ui;
}
This doesn't just affect custom signals but built-in ones too:
QObject::connect(models, &QStandardItemModel::dataChanged,[this](a b, x y){
//do something
});
This breaks the .dll as well.
I also noticed that I had to completely rebuild the .dll in Qt Creator to fix the problem. Removing any problematic signals and just building didn't fix the issue.
The error only happens when the Qt .dll is built in Debug configuration. Release config works well. Whether the MSVC program is Debug or Release seems to have no effect. I haven't changed any of the default settings for either of those configurations (other than a few things that definitely have nothing to do with it).
The only difference I found in Qt Creator between the two configurations is the call of qmake:
qmake.exe "D:\Dev\Qt Workspace\ArduGui\ArduGui.pro" -r -spec win32-msvc2013 "CONFIG+=debug" "CONFIG+=qml_debug"
This is the call for the Debug configuration. In Release the two debug config flags are missing. But when I messed around with the qmake arguments the result did not change. Debug config would always cause the entry point error, regardless of the presence of CONFIG+=debug or CONFIG+=qml_debug. Likewise, Release would always work even if the two flags are added.
So at this point I'm running against a wall. Does anyone have experience with this or can suggest options on how to debug the problem?
Some more info:
I'm using Windows 10, MSVC 2013 and Qt Creator 3.6 with Qt 5.5.1. Both the .exe and .dll are compiled with the MSVC++ 12.0 compiler through their respective IDEs.

Answer 1

If it works fine with void* but doesn't work with QT class pointer passed as argument, I guess you have not registered the class to be able to use it in signal/slot mechanism:
qRegisterMetaType<QGraphicsView *>("QGraphicsView*");
Be sure to call this function before your first use of this signal.
Though I'm not sure this is the cause of all your issues.