What's the current way to handle linking Qt as a dll to my native C++ app?

[dogoooooo]

I'm trying just to load in the libs into my C++ project but can't seem to do it, please help me out, it's been a few days of banging my head on the wall:

#include <QtCore\QLibrary>
// #include <QtWidgets\QWidget>
int main()
{
    QLibrary myLibrary("./lib/mylib-frontend.dll");

properties.json

{
    "configurations": [
        {
            "name": "Win32",
            "includePath": [
                "${default}",
                "C:/Qt/6.5.1/msvc2019_64/include",
                "${workspaceFolder}/lib"
            ],
            "defines": [
                "_DEBUG",
                "UNICODE",
                "_UNICODE"
            ],
            "windowsSdkVersion": "10.0.22000.0",
            "cStandard": "c17",
            "cppStandard": "c++17",
            "intelliSenseMode": "windows-msvc-x64"
        }
    ],
    "version": 4
}

tasks.json:

{
    "tasks": [
        {
            "type": "cppbuild",
            "label": "C/C++: gcc.exe build active file",
            "command": "C:\\msys64\\mingw64\\bin\\gcc.exe",
            "args": [
                "-I",
                "C:/Qt/6.5.1/msvc2019_64/include",
                "-fdiagnostics-color=always",
                "-g",
                "${file}",
                "-o",
                "${workspaceFolder}\\build\\${fileBasenameNoExtension}.exe",
                "-lstdc++",
                "-L C:/Qt/6.5.1/msvc2019_64/lib/ -lQt6Core"
            ],
            "options": {
                "cwd": "${fileDirname}"
            },
            "problemMatcher": [
                "$gcc"
            ],
            "group": {
                "kind": "build",
                "isDefault": true
            },
            "detail": "Task generated by Debugger."
        }
    ],
    "version": "2.0.0"
}

Error:

 Executing task: C/C++: gcc.exe build active file 

Starting build...
C:\msys64\mingw64\bin\gcc.exe -I C:/Qt/6.5.1/msvc2019_64/include -fdiagnostics-color=always -g C:\Users\xxxxx\Documents\Development\Cpp\cpp-xxxxxa-frontend\src\main.cpp -o C:\Users\xxxxx\Documents\Development\Cpp\cpp-xxxxxa-frontend\build\main.exe -lstdc++ "-L C:/Qt/6.5.1/msvc2019_64/lib/ -lQt6Core"
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\xxxxx\AppData\Local\Temp\ccDqkaZL.o: in function `main':
C:/Users/xxxxx/Documents/Development/Cpp/cpp-xxxxxa-frontend/src/main.cpp:5: undefined reference to `__imp__ZN8QLibraryC1ERK7QStringP7QObject'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/Users/xxxxx/Documents/Development/Cpp/cpp-xxxxxa-frontend/src/main.cpp:52: undefined reference to `__imp__ZN8QLibraryD1Ev'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\xxxxx\AppData\Local\Temp\ccDqkaZL.o:C:/Qt/6.5.1/msvc2019_64/include/QtCore/qversiontagging.h:125: undefined reference to `__imp_qt_version_tag_6_5'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\xxxxx\AppData\Local\Temp\ccDqkaZL.o: in function `QString::QString(char const*)':
C:/Qt/6.5.1/msvc2019_64/include/QtCore/qstring.h:1029: undefined reference to `__imp__ZN7QString8fromUtf8E14QByteArrayView'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\xxxxx\AppData\Local\Temp\ccDqkaZL.o: in function `QByteArrayView::QByteArrayView<char, true>(char const*, long long)':
C:/Qt/6.5.1/msvc2019_64/include/QtCore/qbytearrayview.h:137: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/Qt/6.5.1/msvc2019_64/include/QtCore/qbytearrayview.h:137: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\xxxxx\AppData\Local\Temp\ccDqkaZL.o: in function `QtPrivate::QPodArrayOps<char16_t>::destroyAll()':
C:/Qt/6.5.1/msvc2019_64/include/QtCore/qarraydataops.h:95: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/Qt/6.5.1/msvc2019_64/include/QtCore/qarraydataops.h:96: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\xxxxx\AppData\Local\Temp\ccDqkaZL.o: in function `QTypedArrayData<char16_t>::deallocate(QArrayData*)':
C:/Qt/6.5.1/msvc2019_64/include/QtCore/qarraydata.h:124: undefined reference to `__imp__ZN10QArrayData10deallocateEPS_xx'
collect2.exe: error: ld returned 1 exit status

Build finished with error(s).

[jeremy_k]

@dogoooooo said in What's the current way to handle linking Qt as a dll to my native C++ app?:

I'm trying just to load in the libs into my C++ project but can't seem to do it, please help me out, it's been a few days of banging my head on the wall:

tasks.json:

{
    "tasks": [
        {
            "type": "cppbuild",
            "label": "C/C++: gcc.exe build active file",
            "command": "C:\\msys64\\mingw64\\bin\\gcc.exe",
            "args": [
                "-I",
                "C:/Qt/6.5.1/msvc2019_64/include",

It looks like this is attempting to use the MinGW compiler with libraries built for MSVC 2019. I believe that the two toolchains are incompatible.

[dogoooooo]

@jeremy_k

Thanks so much, I swaped over to the wingw64, but getting issues still:

{
    "tasks": [
        {
            "type": "cppbuild",
            "label": "C/C++: gcc.exe build active file",
            "command": "C:/msys64/mingw64/bin/gcc.exe",
            "args": [
                "-I",
                "C:/Qt/6.5.1/mingw_64/include",
                "-fdiagnostics-color=always",
                "-g",
                "${file}",
                "-o",
                "${workspaceFolder}\\build\\${fileBasenameNoExtension}.exe",
                "-lstdc++",
                "-L C:/Qt/6.5.1/mingw_64/lib/ -lQt6Core"
            ],
            "options": {
                "cwd": "${fileDirname}"
            },
            "problemMatcher": [
                "$gcc"
            ],
            "group": {
                "kind": "build",
                "isDefault": true
            },
            "detail": "Task generated by Debugger."
        }
    ],
    "version": "2.0.0"
}

Starting build...
C:/msys64/mingw64/bin/gcc.exe -I C:/Qt/6.5.1/mingw_64/include -fdiagnostics-color=always -g C:\Users\user\Documents\Development\Cpp\cpp-project-frontend\src\main.cpp -o C:\Users\user\Documents\Development\Cpp\cpp-project-frontend\build\main.exe -lstdc++ "-L C:/Qt/6.5.1/mingw_64/lib/ -lQt6Core"
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\user\AppData\Local\Temp\ccv1QKZq.o: in function `main':
C:/Users/user/Documents/Development/Cpp/cpp-project-frontend/src/main.cpp:5: undefined reference to `__imp__ZN8QLibraryC1ERK7QStringP7QObject'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/Users/user/Documents/Development/Cpp/cpp-project-frontend/src/main.cpp:52: undefined reference to `__imp__ZN8QLibraryD1Ev'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\user\AppData\Local\Temp\ccv1QKZq.o:C:/Qt/6.5.1/mingw_64/include/QtCore/qversiontagging.h:125: undefined reference to `__imp_qt_version_tag_6_5'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\user\AppData\Local\Temp\ccv1QKZq.o: in function `QString::QString(char const*)':
C:/Qt/6.5.1/mingw_64/include/QtCore/qstring.h:1029: undefined reference to `__imp__ZN7QString8fromUtf8E14QByteArrayView'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\user\AppData\Local\Temp\ccv1QKZq.o: in function `QByteArrayView::QByteArrayView<char, true>(char const*, long long)':
C:/Qt/6.5.1/mingw_64/include/QtCore/qbytearrayview.h:137: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/Qt/6.5.1/mingw_64/include/QtCore/qbytearrayview.h:137: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\user\AppData\Local\Temp\ccv1QKZq.o: in function `QtPrivate::QPodArrayOps<char16_t>::destroyAll()':
C:/Qt/6.5.1/mingw_64/include/QtCore/qarraydataops.h:95: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:/Qt/6.5.1/mingw_64/include/QtCore/qarraydataops.h:96: undefined reference to `__imp__Z9qt_assertPKcS0_i'
C:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/13.1.0/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\user\AppData\Local\Temp\ccv1QKZq.o: in function `QTypedArrayData<char16_t>::deallocate(QArrayData*)':
C:/Qt/6.5.1/mingw_64/include/QtCore/qarraydata.h:124: undefined reference to `__imp__ZN10QArrayData10deallocateEPS_xx'
collect2.exe: error: ld returned 1 exit status

Build finished with error(s).

[jeremy_k]

@dogoooooo said in What's the current way to handle linking Qt as a dll to my native C++ app?:

I'm not sure what build tool is in use here, so maybe this is ok, but it looks like the library path (-L) and library link (-l) flags are being combined into a single argument. They probably need to be separated into individual quoted values separated by a comma.

[...]
                "-lstdc++",
                "-L C:/Qt/6.5.1/mingw_64/lib/ -lQt6Core"

Starting build...
C:/msys64/mingw64/bin/gcc.exe [...] -lstdc++ "-L C:/Qt/6.5.1/mingw_64/lib/ -lQt6Core"

[SimonSchroeder]

@jeremy_k said in What's the current way to handle linking Qt as a dll to my native C++ app?:

They probably need to be separated into individual quoted values separated by a comma.

Good catch. Furthermore, there should be no space after -L.

[jeremy_k]

@SimonSchroeder said in What's the current way to handle linking Qt as a dll to my native C++ app?:

Good catch. Furthermore, there should be no space after -L.

Separating -L and the link path seems to be fine. I don't see it mentioned explicitly in the documentation like the two variations of -l, but it has worked for a significant length of time. That may be a general allowance for -<option><value> arguments. It can even be necessary if relying on a shell to convert ~ to $HOME. ie:
g++ -L ~/Qt/5.15.2/gcc_64/lib -l Qt5Core a.cpp works.
g++ -L~/Qt/5.15.2/gcc_64/lib -l Qt5Core a.cpp does not.