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

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