Linking C++ Dll containing class in QT Application

[ujwala]

Hi,

I am new to QT development. I have below query.

I am writing a simple dll in C++ which has a class with member function.

#DllProg.h

class __declspec(dllexport) Funcs
{
public:
   Funcs();
   void Print();
};

class __declspec(dllexport) Funcs
{
public:
   Funcs();
   void Print();
};

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#DllProg

#include "DllProg.h"
 
#include <stdexcept>
#include <iostream>
#include <string>
 
using namespace std;
 
__declspec(dllexport) Funcs::Funcs()
{
	cout<<"Funcs : In Constructor"<<endl;
}
 
__declspec(dllexport) void Funcs::Print()
{
	cout<<"Print : In DLL";
}
 
extern "C" __declspec(dllexport) Funcs* getObject()
{
	cout << "In getObject() "<<endl;
	return new Funcs();
}

#include "DllProg.h"

#include <stdexcept>
#include <iostream>
#include <string>

using namespace std;

__declspec(dllexport) Funcs::Funcs()
{
	cout<<"Funcs : In Constructor"<<endl;
}

__declspec(dllexport) void Funcs::Print()
{
	cout<<"Print : In DLL";
}

extern "C" __declspec(dllexport) Funcs* getObject()
{
	cout << "In getObject() "<<endl;
	return new Funcs();
}

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#QT Application

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
 
    typedef Funcs* (*pf)();
 
    QLibrary myLib("D:\\Practice\\QT\\QTClassApp-build-desktop\\debug\\DllProg.dll");
    myLib.load();
 
    if(myLib.isLoaded())
    {
        pf fptr = (pf)myLib.resolve("getObject");
        Funcs *s =  fptr();
 
         printf("getObject Returned -  %p" , s);
         s->Print();
    }
    return a.exec();
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    typedef Funcs* (*pf)();

    QLibrary myLib("D:\\Practice\\QT\\QTClassApp-build-desktop\\debug\\DllProg.dll");
    myLib.load();

    if(myLib.isLoaded())
    {
        pf fptr = (pf)myLib.resolve("getObject");
        Funcs *s =  fptr();

         printf("getObject Returned -  %p" , s);
         s->Print();
    }
    return a.exec();
}

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Here I am getting Error on statement s->Print();
getObject() is properly getting called and returning instance of the class. But if I put call to Print its giving Linker Error - undefined reference to `Funcs::Print()'

#.pro
win32: LIBS += -L$$PWD/../QTClassApp-build-desktop/debug/ -lDllProg
INCLUDEPATH += $$PWD/../QTClassApp-build-desktop/debug
DEPENDPATH += $$PWD/../QTClassApp-build-desktop/debug


I found the ablove code on QT forum. I am not sure how correct is the code.

Thanks in Advance.

[mcosta]

The error is to use hardcoded __declspec(dllexport)

When you import the library you have to pass __declspec(dllimport).

Read here for information.

[ujwala]

Thanks for the reply.

Could you please explain where to use dllimport?

I can successfully call the dll functions (non-members) from my QT application.

But I want to call class members presents in dll.

[mcosta]

have you read the link I posted?

It is explained

[ujwala]

Hi,
I have gone thru the link you have provided. But I think that describes the behaviour of libraries created in QT. But I have C++ Library compiled in MSVS 2010.

Correct me If m wrong.

Thanks.