QPluginLoader question

derivedhttprequesthandler.cpp

#include <QtPlugin>

#include "squarehttprequesthandler.h"

Q_EXPORT_PLUGIN2(squarehttprequesthandler, SquareHTTPRequestHandler)

SquareHTTPRequestHandler::SquareHTTPRequestHandler()
{
}

void SquareHTTPRequestHandler::createResponse()
{
    HTTPResponse response;

    if("GET" != requestData.method || !requestData.url.hasQueryItem("a")){
        response.setStatusCode(400);
        response.setReasonPhrase("Bad Request");

        emit responseWritten(response);
        emit endOfWriting();
        return;
    }

    response.setStatusCode(200);
    response.setReasonPhrase("OK");

    QString numToSquare = requestData.url.queryItemValue("a");
    QString body;

    bool ok;
    double n = numToSquare.toDouble(&ok);

    if(!ok){
        response.setBody("a-ul trebuie sa fie numar!\n");
    }
    else{
        response.setBody(numToSquare + "^2 = " + QString::number(n*n) + "\n");
    }

    emit responseWritten(response);
    emit endOfWriting();
    return;
}

derivedhttprequesthandler.pro

#-------------------------------------------------
#
# Project created by QtCreator 2013-08-03T20:54:55
#
#-------------------------------------------------

QT       -= gui

TARGET = squarehttprequesthandler
TEMPLATE = lib

DEFINES += SQUAREHTTPREQUESTHANDLER_LIBRARY

SOURCES += squarehttprequesthandler.cpp

HEADERS += squarehttprequesthandler.h\
        squarehttprequesthandler_global.h

INCLUDEPATH += ../http-server

unix:!symbian {
    maemo5 {
        target.path = /opt/usr/lib
    } else {
        target.path = /usr/lib
    }
    INSTALLS += target
}

error

QLibraryPrivate::loadPlugin failed on "/path/to/libhandler.so.1.0.0" : "Cannot load library /path/to/libhandler.so.1.0.0: (/path/to/libhandler.so.1.0.0: undefined symbol: _ZNK18HTTPRequestHandler10metaObjectEv)" 

HTTPRequestHandler.hpp

#ifndef HTTPREQUESTHANDLER_H
#define HTTPREQUESTHANDLER_H

#include <QObject>

#include "httprequest.h"
#include "httpresponse.h"

class HTTPRequestHandler : public QObject
{
    Q_OBJECT
public:
    HTTPRequestHandler();
    HTTPRequestHandler(const HTTPRequest &r) : requestData(r) {}
    virtual ~HTTPRequestHandler() {}
    virtual void createResponse()=0;
    void setRequestData(const HTTPRequest &r){
        requestData = r;
    }

protected:
    HTTPRequest requestData;

signals:
    void responseWritten(HTTPResponse &response);
    void endOfWriting();
};

Q_DECLARE_INTERFACE(HTTPRequestHandler, "http-daemon.HTTPRequestHandler/1.0")

#endif // HTTPREQUESTHANDLER_H