Qt - Dll related problem

Question

I am just learning about the dll's. I tried that in Qt. First I am posting dll- related files.

First dll - pro file

TEMPLATE = lib

SOURCES += \
    check.cpp

HEADERS += \
    check.h

This one is dll- header file "check.h"

#ifndef CHECK_H
#define CHECK_H

#include <iostream>

extern "C++" __declspec(dllexport) std::string check();

#endif // CHECK_H

This one is dll - source file "check.cpp"

#include <iostream>
#include "check.h"

extern "C++" __declspec(dllexport) std::string check()
{
    return "dll applied";
}

I compiled the above project file, and got the dll . The dll's name is "dll.dll"

Now comes the main file. Here I tried to access the "check" function through dll.

#include "check.h"
#include <iostream>
#include "MyMessageBox.h"
#include <QApplication>
#include <QLibrary>
#include <QMessageBox>

typedef std::string (*CheckType) (void);

class MyMessageBox:public QMessageBox
{
public:
    MyMessageBox(std::string message,QWidget*parent=0):
       QMessageBox(QMessageBox::NoIcon,QString("ErrorMessage"),QString(message.c_str()),QMessageBox::Ok,parent,Qt::Widget)
    {
    }
};

int main(int argc,char * argv[])
{
    QApplication app(argc,argv);
    CheckType myCheck;
    QLibrary myLib("dll");
    myLib.load();

    bool ok = myLib.load();
    if(ok)
    {
        MyMessageBox mm("Load is done");
        mm.exec();
    }
    ok = myLib.isLoaded();
    if(ok)
    {
        MyMessageBox mm("Loaded");
        mm.exec();
    }
    myCheck = (CheckType) (myLib.resolve("check"));
    if(!myCheck)
    {
        MyMessageBox m0("Resolving isn't happened");
        m0.exec();
    }
    std::string result = myCheck();

    MyMessageBox mm(result);
    mm.exec();
    return app.exec();
}

But when I ran the above application, I got "Resolving isn't happened". That means, the function pointer became NULL. I don't know which part is wrong. Somebody help me?

Answer 1

You will need to __declspec(dllimport) when compiling the program using the DLL. A common approach is this:

// Windows DLL magic
#if defined(USE_DLL)
# if defined(BUILD_DLL)
#  define DLL_EXPORT  __declspec(dllexport)
# else // BUILD_DLL
#  define DLL_EXPORT  __declspec(dllimport)
# endif // BUILD_DLL
#else // USE_DLL
# define DLL_EXPORT
#endif // USE_DLL

You then replace the __declspec(dllexport) in your example with DLL_EXPORT, and be sure that the above code is included before any exported symbol.

You then add

DEFINES += USE_DLL BUILD_DLL

to the dll's project file (only when building as dll of course!) and

DEFINES += USE_DLL

to any project using the dll exported functions. It's quite ugly, but idiomatic and it works.