Library path is definitely correct and can create an instance of said library, but get "Undefined reference" when calling any function

Question

I'm using Qt Creator to create a new project, and I have a test "MathLibrary" that I created in Visual Studio. I want to use this library in my Qt project.

I have searched for many hours for an answer to my solution and in almost all cases the answer was simply that the library had not been added to the PATH in the .pro file. I'm 99% sure that I have done everything correct, but something is causing me to get an Undefined Reference error when I try calling any function in this library. Here's what I have so far.

The library -

MathLibraryH.h:

#pragma once

namespace MathLibrary
{
    class Functions
    {
    public:
        // Returns a + b  
        double Add(double a, double b);

        // Returns a * b  
        double Multiply(double a, double b);

        // Returns a + (a * b)  
        double AddMultiply(double a, double b);
    };
}

MathLibrary.cpp:

#include "stdafx.h"
#include "MathLibraryH.h"

namespace MathLibrary
{
    double Functions::Add(double a, double b)
    {
        return a + b;
    }

    double Functions::Multiply(double a, double b)
    {
        return a * b;
    }

    double Functions::AddMultiply(double a, double b)
    {
        return a + (a * b);
    }
}

The QT project -

TestQTProject.pro:

    QT       += core gui \
            network

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = TestQTProject
TEMPLATE = app

DEFINES += QT_DEPRECATED_WARNINGS

#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0


SOURCES += \
        main.cpp \
        mainwindow.cpp \

HEADERS += \
        mainwindow.h \

FORMS += \
        mainwindow.ui

DISTFILES += \
    com_github_msorvig_s3.pri

LIBS += -L$$PWD/../Libs -lMathLibrary

INCLUDEPATH += $$PWD/../Incs

mainwindow.cpp:

#include "MathLibraryH.h"

// .... other stuff ....

void MainWindow::on_btnStage1_clicked()
{
    MathLibrary::Functions lib; // This is just fine
    lib.Add(5, 9); // The "Add" function (or any other function in the library)
                       causes an undefined reference error
}

I'm still new to Qt but I can't see what's wrong with any of this code.

Other things I've tried based on answers from searching:

Adding the following code to MathLibrary.h:

#ifdef MATHLIBRARY_EXPORTS  
#define MATHLIBRARY_API __declspec(dllexport)   
#else  
#define MATHLIBRARY_API __declspec(dllimport)   
#endif  

Changing the format of the LIBS declaration in the .pro file to all of the following:

Multi-line:

LIBS += -L$$PWD/../Libs
LIBS += -lMathLibrary

Hard coded single line:

LIBS += -LC:\svn\software\WIP\TestQTProject\Libs -lMathsLibrary

Nothing I've done works and I have no other ideas left.

For what it's worth, the library works fine in any project created using visual studio, and I've tried creating both a static and dynamic library.

Answer 1

Add

MATHLIBRARY_API

infront of your class name to export all public members in your dll. During export the dllexport part should be set and during import the dllimport part of course.

Here's a link that explains how to export classes from a dll: https://www.codeproject.com/Articles/28969/HowTo-Export-C-classes-from-a-DLL