Runtime error: “must construct a QApplication before a QWidget” when launching a QDialog from a dll built with static Qt

awyarbr

I built a static version of Qt from the VS2015 command line with the following:

..\Src\configure -prefix E:\Qt\Qt5.9.1\5.9.1\QtStatic -static -static-runtime -nomake tests -nomake examples -debug-and-release -developer-build -opensource -confirm-license -opengl desktop

I use Cmake to build my main application and a dll, linking (in debug mode) all the "*d.lib" files in the Qt /lib and /plugins/platforms directories and the C:/Program Files (x86)/Microsoft SDKs/Windows/v7.1A/Lib/x64 folder.

My main function:

#include "myWindowClass.h"                                       
#include <QApplication.h>
int main(int argc, char **argv) {
	QApplication app(argc, argv);
	myWindowClass *mwc = new myWindowClass();
	mwc->show();
	return app.exec();
}

myWindowClass.h (STATIC_BUILD is defined in Cmake):

#pragma once                                                                
#include <QMainWindow.h>
#include "ui_mainWindow.h"
#include <QtPlugin>
#include <QLibrary>
#include "myDllClass.h"
#ifdef STATIC_BUILD
Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin)
#endif
typedef MyDllClass* (*MyDll)();
namespace Ui {
	class myWindowClass;
}
class myWindowClass : public QMainWindow {
	Q_OBJECT
public:
	myWindowClass(QWidget *parent = 0);
	~myWindowClass() {
		delete ui;
	}
public slots:
	void launchMyDialog();
	void loadMyDll();
private:
	Ui::MainWindow *ui;
	QLibrary *lib;
	MyDllClass *myc;
};

myWindowClass.cpp:

#include "myWindowClass.h"

myWindowClass::myWindowClass(QWidget *parent) :
	QMainWindow(parent),
	ui(new Ui::MainWindow)
{
	ui->setupUi(this);
}
	void myWindowClass::launchMyDialog() {
	if (lib)
		if (lib->isLoaded())
			myc->launchDialog();
}

myWindowClass::loadMyDll() {
	lib = new QLibrary("MyDLL.dll");
	lib->load();
	if (lib->isLoaded()) { // Library is found to be loaded
		auto create = (MyDll)lib.resolve("Create");
		if (create)  // Create resolves successfully
			myc = create(); // create executes successfully
	}
}

I constructed a mainWindow.ui with Qt Designer that has buttons connected to the loadMyDll() and launchMyDialog() functions.

In myDllClass.h (BUILD_SHARED_LIBS defined in Cmake):

#include "myDllClass.h"
#include "MyDialog.h"

#if defined (WIN32) && defined (BUILD_SHARED_LIBS)
#if defined (_MSC_VER)
#pragma warning(disable: 4251)
#endif
#if defined(MyDLL_EXPORT)
#define  MYLIB_EXPORT __declspec(dllexport)
#else
#define  MYLIB_EXPORT __declspec(dllimport)
#endif
#else
#define MYLIB_EXPORT
#endif

struct MyDllClass {
	virtual void launchDialog() = 0;
};

class MyClass : public MyDllClass
{
public:
	MyClass() {}
	~MyClass() {}
	void launchDialog();
};


extern "C" MYLIB_EXPORT MyDllClass * Create() {
	return new MyClass();
}

In myDllClass.cpp:

#include "myDllClass.h"
#include "MyDialog.h"

MyClass::launchDialog() {
	myDialogClass *pDllDialog = new myDialogClass(); // Runtime error  "QWidget: must construct a QApplication before a QWidget"
	pDllDialog->raise();
	pDllDialog->activateWindow();
	pDllDialog->exec();
}

In myDialog.h:

#pragma once                                              
#include <QDialog>
#include "ui_MyDialog.h"

class myDialogClass : public QDialog {
	Q_OBJECT
public:
	myDialogClass(QWidget *parent = 0) :
		QDialog(parent), ui(new Ui::Dialog) {
		ui->setupUi(this);
	}

	~myDialogClass() {
		delete ui;
	}

private:
	Ui::Dialog *ui;
};

Also with Qt Designer, I constructed a myDialog.ui with only the OK and Cancel buttons.

The error appears at the point indicated. Setting breakpoints inside the constructor of myDialogClass shows that the first line, ui->setupUi(this), is never reached; the failure is happening before that.

Note that this code, built with the /MDd option and using the Qt dlls, runs okay. Building MyDLL as a static library and linking it also runs okay (by calling Create() directly rather than resolving it first). It is only in using a static build of Qt to build MyDLL as a dll that I run into problems.

Is this something that can be done? And if so, where am I going wrong?

Things that I tried that did not work, or would not build: passing "this" from myWIndowClass to the myDialog constructor via myDllClass; including the Q_IMPORT_PLUGIN macro in myDialog.h; trying to construct a new QApplication or QCoreApplication inside the dll.

Things that I haven't tried: mixing static and shared versions of the Qt builds between the main app and MyDLL; mixing static and shared versions of Qt and the runtime libraries. That wouldn't really interest me: the point is to have an application that doesn't rely on Qt or runtime dlls.

SGaist

Hi and welcome to devnet,

Do you have any static QWidget based class in your library/application ?