create and use shared library with qt

Question

I am new to the shared library stuff, so I have question about how to create/use a shared library, I am using Qt Creator with qt 5.4.2 with Microsoft Visual C++ 11.0 Compliler.

In my project, I will need to create a dll which call functions from an external library (there are .h, .lib, .dll to use from). To understand how export/import of functions from library work, I tried to create a simple library with one function and use this in another programm first. After reading different tutorials, I managed to create the library. In Qt Creator, New Project->Library(C++ Library)->Type(shared library)Name: sharedlib->Modules(QtCore)->Finish.

sharedlib.h:

#ifndef SHAREDLIB_H
#define SHAREDLIB_H

#include <QtCore/qglobal.h>

#if defined(SHAREDLIB_LIBRARY)
#  define SHAREDLIBSHARED_EXPORT Q_DECL_EXPORT
#else
#  define SHAREDLIBSHARED_EXPORT Q_DECL_IMPORT
#endif

extern "C" SHAREDLIBSHARED_EXPORT int add(int a, int b);

#endif // SHAREDLIB_H

sharedlib.cpp:

#include "sharedlib.h"
#include <stdio.h>

extern "C" SHAREDLIBSHARED_EXPORT int add(int a, int b)
{
    return a + b;
}

only added a simple function to add 2 numbers.

after build, I get sharedlib.dll and sharedlib.lib and some other files, (no .a file like in some tutorials, I thought its because I am using microsoft vc compiler which give the .lib file instead).

Now to create a second programm in which I want to use the library: New Project->Qt Console Application->Name(loadlib)->Finish, then I copied the sharedlib.lib, sharedlib.h, sharedlib.dll into the loadlib directory. (do I need them all? and where shall I put them exactly?) According to tutorial, right-click on the project->add library->external library->choose the .lib file inside the loadlib directory, uncheck the Linux and Mac under Platform and choose the Dynamic Linkage. this is my loadlib.pro:

QT       += core
QT       -= gui

TARGET = loadlib
CONFIG   += console
CONFIG   -= app_bundle

TEMPLATE = app
SOURCES += main.cpp

win32:CONFIG(release, debug|release): LIBS += -L$$PWD/ -lsharedlib
else:win32:CONFIG(debug, debug|release): LIBS += -L$$PWD/ -lsharedlib

INCLUDEPATH += $$PWD/
DEPENDPATH += $$PWD/

1. if I put .h and .dll/.lib file in subfolder like loadlib/include and loadlib/libs, it will change to INCLUDEPATH += $$PWD/include DEPENDPATH += $$PWD/include and LIBS += -L$$PWD/libs -lsharedlib, right?
2. do I need to copy all 3 files to my loadlib directory?

3. main.cpp:

   #include <QCoreApplication>
   #include <QDebug>
   
   int main(int argc, char *argv[])
   {
       QCoreApplication a(argc, argv);
       // simple Debug output to add 7 and 3
   
       return a.exec();
   }
   

   how do I actually use the add function here?

EDIT: I changed few things, got rid of the sharedlib_global.h and paste the content into sharedlib.h, and get rid of the class,can I call a function directly without wrap this into a class?

Answer 1

Everything you've done so far is correct. Now just include the library header file sharedlib.h in your main.cpp or whichever file and you can then use add() function there.

#include <QCoreApplication>
#include <QDebug>
#include "sharedlib.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    // simple Debug output to add 7 and 3

    SharedLib lib;
    int a = 5, b = 6;
    int sum = lib.add (a, b);

    return a.exec();
}

You need to pack sharedlib.dll in the same directory along with the executable file when deploying.

Answer 2

Try this (main.cpp):

#include "sharedlib.h"

#include <QCoreApplication>
#include <QDebug>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    // simple Debug output to add 7 and 3
    SharedLib sLib;
    qDebug() << sLib.add(7, 3); // should print 10

    return 0;   // just exit

//    return a.exec();  // you need to kill / force stop your app if you do ths.
}

If you can compile the above, then your library is working as expected.