QT QLibrary "Cannot load library *.so: (*.so: undefined symbol: staticMetaObject)" - c++

Question

I am working on a big project where I am trying to load dynamically a shared library with QLibrary, I was able to reproduce the run-time link error (undefined symbol: staticMetaObject) in the following example:
Folder structure:
root\
-- src\
---- TestLib\
------ TestLib.pro
------ Derived.h
------ Derived.cpp
------ TestLibModuleBridge.h
------ TestLibModuleBridge.cpp
---- TestLibCommon\
------ IBase.h
---- TestLibManager\
------ TestLibManager.pro
------ main.cpp
--lib\
TestLib files:
# TestLib.pro
QT -= gui
TARGET = TestLib
TEMPLATE = lib
QMAKE_CXXFLAGS += -Wall
DEFINES += TESTLIB_LIBRARY
SOURCES += Derived.cpp \
TestLibModuleBridge.cpp
HEADERS += Derived.h \
TestLibModuleBridge.h
INCLUDEPATH += ../TestLibCommon
unix {
target.path = ../../lib
INSTALLS += target
}
-
// Derived.h
#ifndef DERIVED_H
#define DERIVED_H
#include "IBase.h"
#include <iostream>
class Derived : public IBase
{
Q_OBJECT
public:
Derived();
virtual ~Derived();
public:
virtual void methodA();
virtual void methodB();
};
#endif // DERIVED_H
-
// Derived.cpp
#include "Derived.h"
Derived::Derived()
{
}
Derived::~Derived()
{
}
void Derived::methodA()
{
std::cout << "methodA()" << std::endl;
}
void Derived::methodB()
{
std::cout << "methodB()" << std::endl;
}
-
// TestLibModuleBridge.h
#ifndef TESTLIBMODULEBRIDGE_H
#define TESTLIBMODULEBRIDGE_H
#include "IBase.h"
#ifdef __cplusplus
extern "C" {
#endif
IBase* getModuleInterface();
#ifdef __cplusplus
}
#endif
#endif // TESTLIBMODULEBRIDGE_H
-
// TestLibModuleBridge.cpp
#include "TestLibModuleBridge.h"
#include "Derived.h"
IBase* getModuleInterface()
{
return new Derived();
}
TestLibManager Files:
// TestLibManager.pro
QT += core
QT -= gui
TARGET = TestLibManager
CONFIG += console
CONFIG -= app_bundle
QMAKE_CXXFLAGS += -Wall
TEMPLATE = app
SOURCES += main.cpp
INCLUDEPATH += ../TestLibCommon
-
// main.cpp
#include <QCoreApplication>
#include <QLibrary>
#include <QDebug>
#include "IBase.h"
typedef IBase* (*ModuleGetterFunction) (void);
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QLibrary testLib("../../lib/libTestLib.so");
if (!testLib.load())
{
qDebug() << "Error : " << testLib.errorString();
exit(EXIT_FAILURE);
}
else
{
ModuleGetterFunction getModuleInterfaceFunc = (ModuleGetterFunction) testLib.resolve("getModuleInterface");
if (getModuleInterfaceFunc)
{
IBase* obj = getModuleInterfaceFunc();
obj->methodA();
obj->methodB();
}
}
return a.exec();
}
TestLibCommon Files
// IBase.h
#ifndef IBASE_H
#define IBASE_H
#include <QObject>
class IBase : public QObject
{
Q_OBJECT
protected:
virtual ~IBase() {}
public:
virtual void methodA() = 0;
virtual void methodB() = 0;
};
#endif // IBASE_H
The testLib.load() fails with Error : "Cannot load library ../../lib/libTestLib.so: (../../lib/libTestLib.so: undefined symbol: _ZN5IBase16staticMetaObjectE)"
How can I fix this, removing Q_OBJECT macro from IBase.h will fix the error but in the production project that interface contains signals and slots, and it is from a project we're not allowed to change.

Answer 1

It looks like moc has not been run before compiling the testlib.
moc is responsible for generating the staticMetaObject for classes which contain the Q_OBJECT Macro and are listed under the HEADERS section in the pro file.
Adding TestLibCommon\IBase.h to the HEADERS in TestLib.pro should probably fix this problem. (Untested).
Possible improvement of your solution:
Instead of using QLibrary take a look at QPluginLoader.
QPluginLoader will give you a QObject which you can then cast to any interface you want using qobject_cast<T*>(pluginloader.instance())
Here's an example using QPluginLoader:
http://doc.qt.io/qt-5/qtwidgets-tools-plugandpaint-example.html