how to fix linker error in eclipse on linux

[micha_eleric]

using eclipse on linux
error, i am mostly sure is an eclipse issue, and not a programming issue.

g++ -L/usr/include/qt4 -L/usr/include/qt4/QtSerialPort -L/usr/include/qt4/QtGui -o "GUI2" ./src/CGUI.o ./src/Main.o
/usr/bin/ld: ./src/CGUI.o: in function CGUI::CGUI()': /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/CGUI.cpp:10: undefined reference to QMainWindow::QMainWindow(QWidget*, QFlagsQt::WindowType)'
/usr/bin/ld: /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/CGUI.cpp:10: undefined reference to vtable for CGUI' /usr/bin/ld: /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/CGUI.cpp:10: undefined reference to vtable for CGUI'
/usr/bin/ld: ./src/CGUI.o: in function CGUI::~CGUI()': /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/CGUI.cpp:15: undefined reference to vtable for CGUI'
/usr/bin/ld: /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/CGUI.cpp:15: undefined reference to vtable for CGUI' /usr/bin/ld: /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/CGUI.cpp:15: undefined reference to QMainWindow::~QMainWindow()'
/usr/bin/ld: ./src/Main.o: in function main': /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/Main.cpp:14: undefined reference to QApplication::QApplication(int&, char**, int)'
/usr/bin/ld: /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/Main.cpp:17: undefined reference to QApplication::exec()' /usr/bin/ld: /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/Main.cpp:14: undefined reference to QApplication::~QApplication()'
/usr/bin/ld: /home/michaeleric/eclipse-workspace/GUI2/Debug/../src/Main.cpp:14: undefined reference to `QApplication::~QApplication()'
collect2: error: ld returned 1 exit status
make: *** [makefile:47: GUI2] Error 1

based on https://forum.qt.io/topic/101400/qt-with-eclipse-c/5 , i added QtCore and QtWidgets, even though there were no errors with #include<>. same errors from compile.

https://stackoverflow.com/questions/6478317/eclipse-undefined-reference says it is a linker error

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->command: g++

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Libraries->Libraries (-l) [nothing]

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Libraries->Libraries search path (-L) /usr/include/qt4 /usr/include/qt4/QtWidgets /usr/include/qt4/QtCore /usr/include/qt4/QtSerialPort /usr/include/qt4/QtGui

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Miscellaneous->Linker flags [nothing]

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Miscellaneous->Other options (-Xlinker [option]) [nothing]

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Miscellaneous->Other objects [nothing]

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Shared Library Settings [nothing][nothing][nothing][nothing]

what is suppose to be added, and where?

if:
under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Libraries->Libraries (-l) /usr/include/qt4/QtGui
error:
/usr/bin/ld: cannot find -l/usr/include/qt4/QtGui

if:
under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Miscellaneous->Other options (-Xlinker [option]) /usr/include/qt4/QtGui
error:
/usr/bin/ld: cannot find /usr/include/qt4/QtGui: file format not recognized

if:
under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Miscellaneous->Other objects /usr/include/qt4/QtGui
error:
/usr/bin/ld: cannot find /usr/include/qt4/QtGui: file format not recognized

if it is a virtual function that has to be defined, then what would it be, other than constructor/destructor?

---

answer:
deleting:
under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Libraries->Libraries search path (-L) /usr/include/qt4 /usr/include/qt4/QtWidgets /usr/include/qt4/QtCore /usr/include/qt4/QtSerialPort /usr/include/qt4/QtGui, did nothing, no new errors, nothing with previous errors

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Miscellaneous->Other objects /usr/lib/x86_64-linux-gnu/libQtGui.so
fixed errors, exposed new errors

[jsulm]

@micha_eleric said in how to fix linker error in eclipse on linux:

under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Libraries->Libraries (-l) /usr/include/qt4/QtGui

/usr/include/qt4/QtGui contains header files, not libraries!
You need to specify the folder where Qt libraries are located.
Thoese should be somewhere in /usr/lib

[micha_eleric]

@jsulm well. /usr/lib/qt5 only contains dir bin, which contains fixqt4headers.pl, moc, qdbus, qdbuscpp2xml, qdbusxml2cpp, qlalr, qmake, qvkgen, rcc, syncqt.pl, uic

if:
under Properties->C/C++ Build->Settings->Tool Settings->GCC C++ Linker->Libraries->Libraries (-l) /usr/lib/qt5/bin
error:
/usr/bin/ld: cannot find -l/usr/lib/qt5/bin

/usr/lib/ only has qt5 dir, that may look like it has anything to do with qt

[jsulm]

@micha_eleric said in how to fix linker error in eclipse on linux:

/usr/lib/qt5/bin

You are setting include folder for Qt4, but here you're trying to add Qt5 lib folder!
What Qt version do you actually want to use?
What Qt version did you install and how?

[micha_eleric]

@jsulm i installed qt4, but qt5 was the only qt dir made in /usr/lib

i am looking around in other dir for anything qt

[jsulm]

@micha_eleric said in how to fix linker error in eclipse on linux:

i installed qt4

How?
Search in /usr/lib, /usr/lib64 for files containing "qt4" in their names.

[micha_eleric]

@jsulm how i install qt4?
i did internet search for "sudo install qt4". i just redid search, but only came up with install qt5, however the numbers dont look the same.

under /usr/lib:
python3.9
qt5
recovery-mode

under usr/lib64:
ld-linux-x86-64.so.2

under /usr/include:
python3.8
qt4
rdma

[jsulm]

@micha_eleric said in how to fix linker error in eclipse on linux:

under /usr/lib:
python3.9
qt5
recovery-mode
under usr/lib64:
ld-linux-x86-64.so.2

There should be way more than that.
I suggested to search:

find /usr/lib -name *qt4*
find /usr/lib64 -name *qt4*

What Linux distribution do you use? Please also tell the version.

[micha_eleric]

@jsulm there is a lot more, i only listed what was before and after qt

looking back through command line, i wonder if i half installed

[micha_eleric]

@jsulm from https://ubuntuhandbook.org/index.php/2020/07/install-qt4-ubuntu-20-04/

i have:
sudo add-apt-repository ppa:rock-core/qt4
in command line.

??anything else should be installed?

[jsulm]

@micha_eleric said in how to fix linker error in eclipse on linux:

sudo add-apt-repository ppa:rock-core/qt4

This does not install anything!
This command only adds a new source for packages.
You need to install needed packages using "apt install PACKET_NAME".

One question: do you really need Qt4?

[micha_eleric]

@jsulm qt4 it is what i started with.

i also found:
sudo apt install qt4-dev-tools libqt4-dev libqt4-core libqt4-gui

which seems to be from:
https://askubuntu.com/questions/1274134/cannot-install-qt-4-on-ubuntu-20-04-quite-universal-circuit-simulator-qucs

this is last i found with qt*, if it is not installing qt4, then i guess my errors have been found

sudo apt-get install qt5-default
results:
qt5-default is already the newest version (5.12.8+dfsg-0ubuntu2.1).

however, there are no qt5 directory under /usr/include

[JonB]

@micha_eleric said in how to fix linker error in eclipse on linux:

@jsulm qt4 it is what i started with.

This is not a great/clear answer, as @jsulm is pointing out that Qt4 is very old. Do you have a choice to move to Qt5, or even (hesitantly!) Qt6?

[micha_eleric]

@JonB it seems i already installed qt5. just it is not showing up under /usr/include

[jsulm]

@micha_eleric said in how to fix linker error in eclipse on linux:

i already installed qt5

What exactly? Which qt5 packages?
" just it is not showing up under /usr/include" - as I already told you b efore the libraries will NOT show up in that directory. Search for the qt5 libs like I suggested before:

find /usr/lib -name *qt5*
find /usr/lib64 -name *qt5*

Don't forget to also change include folders in your set-up as currently you use qt4 include folder.

[micha_eleric]

@jsulm said in how to fix linker error in eclipse on linux:

find /usr/lib -name qt5

michaeleric@michaeleric-C7B250-CB-ML:/usr/lib$ find -name qt5
./qt5
./x86_64-linux-gnu/qt5
michaeleric@michaeleric-C7B250-CB-ML:/usr/lib$ cd /
michaeleric@michaeleric-C7B250-CB-ML:/$ cd /usr/lib64
michaeleric@michaeleric-C7B250-CB-ML:/usr/lib64$ find -name qt5
michaeleric@michaeleric-C7B250-CB-ML:/usr/lib64$

[jsulm]

@micha_eleric Please read again what I suggested (hint: * is important)...

[micha_eleric]

@jsulm the * was used, copied, pasted, but did not show on post.

[jsulm]

@micha_eleric Can you please answer questions?
I asked before: What exactly? Which qt5 packages?

[micha_eleric]

@jsulm sudo apt-get install qt5-default
from:
https://wiki.qt.io/Install_Qt_5_on_Ubuntu