VS Tools how to use a qt class library

[astoffregen]

I am trying to work on qt class libraries and i want to include a qt class library in a qt application. Platform is visual studio 2022, qt tools and qt 6.4.1. application and class library I created via project wizard.
I have included the class library via links and added the path to the header and library path in the QT project settings.
I can include the header file but nothing from the library is visible except the default constructor.

VS Project:

QT Project settings

Testlib.h

#pragma once
#include "testlib_global.h"

class TESTLIB_EXPORT TestLib
{
    public:
    TestLib();
    void SayHello();
};

Testlib_global.h

#pragma once

#include <QtCore/qglobal.h>

#ifndef BUILD_STATIC
# if defined(TESTLIB_LIB)
#  define TESTLIB_EXPORT Q_DECL_EXPORT
# else
#  define TESTLIB_EXPORT Q_DECL_IMPORT
# endif
#else
# define TESTLIB_EXPORT
#endif

Testlib.ccp

#include "TestLib.h"

TestLib::TestLib()
{
}

void TestLib::SayHello()
{
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtCore/qglobal.h>
#include "testlib.h"

int main(int argc, char *argv[])
{
#if defined(Q_OS_WIN)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);

    TestLib* lib = new TestLib();
    lib->SayHello();

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

lib->SayHello() gives an error because nothing is found in lib except the constructor

[JonB]

@astoffregen
The expansion of TESTLIB_EXPORT when Testlib.h is read depends on the existence/definition of TESTLIB_LIB at the time it is read (unless you are building with BUILD_STATIC which I imagine you are not). Isn't the idea that your Testlib.cpp (only this file) should have #define TESTLIB_LIB (to say this is the implementation) somewhere before it goes #include "TestLib.h"? Does that affect your situation?

[astoffregen]

@JonB I still changed the library type to static, because I don't need a dynamic library. The method can now be selected in the main.cpp. but when compiling comes these errors:

Severity Code Description Project File Line Suppression State
Error LNK2019 Reference to unresolved external symbol ""__declspec(dllimport) public: __cdecl TestLib::TestLib(void)" (__imp_??0TestLib@@QEAA@XZ)" in function "main".	TestApp C:\Users\arne\workspace4\TestApp\main.obj 1

Severity Code Description Project File Line Suppression state
Error LNK2019 Reference to unresolved external symbol ""__declspec(dllimport) public: void __cdecl TestLib::sayHello(void)" (__imp_?sayHello@TestLib@@QEAAXXZ)" in function "main".	TestApp C:\Users\arne\workspace4\TestApp\main.obj 1

Two unresolved external

Probably this is a beginner's problem - I'm just switching from c# to c++ due to the project and it's much more complex.

Since the application itself is already very complex i wanted to at least create the structures correctly right at the beginning and for that i need several libraries

Thanks for any support

[JonB]

@astoffregen Looks to me like you're not linking against TestLib.lib.