CMake - Linking against a shared library using Qt6

[Loic B.]

Hello,

On a project using a shared library using Qt6, the compiler raise the LNK2001 error unresolved external symbol.
I am not a CMake expert so I start from the Qt example code here: https://doc.qt.io/qt-6/cmake-get-started.html that I modify to match my requirements.
I made a minimal project showing the problem.

Here is the tree view:

• helloworld
  • helloworld.hpp
• main
  • main.cpp
• CMakeLists.txt

The helloworld.cpp file:

#pragma once

#include <qobject.h>
#include <iostream>

class HelloWorld : public QObject {
    Q_OBJECT
public slots:
    __declspec(dllexport) void hello() { std::cout << "Hello world!\n"; }
};

The main.cpp file:

#include <helloworld.hpp>

int main(int argc, char** argv) {
    HelloWorld helloworld;
    helloworld.hello();
}

The CMakeLists.txt file:

cmake_minimum_required(VERSION 3.16.0)

project(helloworld VERSION 1.0.0 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(CMAKE_AUTOMOC ON)
set(CMAKE_INCLUDE_CURRENT_DIR ON)

add_library(helloworld SHARED helloworld/helloworld.hpp)

add_executable(main main/main.cpp)

find_package(Qt6 COMPONENTS Core REQUIRED)

target_link_libraries(helloworld PRIVATE Qt6::Core)

target_link_libraries(main PRIVATE helloworld)
target_include_directories(main PRIVATE "${helloworld_SOURCE_DIR}/helloworld")
target_link_libraries(main PRIVATE Qt6::Core)

To generate the project, I use this command:

cmake -G "Visual Studio 16 2019" -B .\build\ -S . -DCMAKE_PREFIX_PATH="C:\Qt\6.1.0\msvc2019_64\"

And this is the errors I get:

main.obj : error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl HelloWorld::metaObject(void)const " (?metaObject@HelloWorld@@UEBAPEBUQMetaObject@@XZ)
1>main.obj : error LNK2001: unresolved external symbol "public: virtual void * __cdecl HelloWorld::qt_metacast(char const *)" (?qt_metacast@HelloWorld@@UEAAPEAXPEBD@Z)
1>main.obj : error LNK2001: unresolved external symbol "public: virtual int __cdecl HelloWorld::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@HelloWorld@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z)
1>G:\dev\test-qt-cmake\build\Debug\main.exe : fatal error LNK1120: 3 unresolved externals

Do you have an idea from what it could come from ?

Thanks in advance !

Loïc

[Christian Ehrlicher]

@Loic-B said in CMake - Linking against a shared library using Qt6:

Do you have an idea from what it could come from ?

You don't export the library as needed and described here.

[Loic B.]

@Christian-Ehrlicher thanks for this quick reply, it works!