QT creator and CMake, cannot link to external library

Question

Newbee here on QT creator and CMake and apologies for the basic question but struggling to link simple external library to my main project. Here are the project paths:

build-QMLTest-Desktop_Qt_6_3_1_MinGW_64_bit-Debug   // application binary path
QMLLib
    build-mylib-Desktop_Qt_6_3_1_MinGW_64_bit-Debug // lib binary path
    mylib
        CMakeLists.txt
        mylib.cpp   // lib source path
        mylib.h
QMLTest
    CMakeLists.txt
    main.cpp    // application source path

Here is my how I am trying to link the static library (snipped of QMLTest/CMakeLists.txt). I am using this thread as reference:

How do I explicitly specify an out-of-tree source in CMake?

add_subdirectory(../QMLLib/mylib ../QMLLib/build-mylib-Desktop_Qt_6_3_1_MinGW_64_bit-Debug)

target_link_libraries(mylib)

set(PROJECT_SOURCES
        main.cpp
        qml.qrc
)

main.cpp

#include "../QMLLib/mylib/mylib.h"

int main(int argc, char *argv[])
{
    Mylib mylib;

Yet, it won't resolve, here is the output

C:/Qt/Tools/mingw1120_64/bin/../lib/gcc/x86_64-w64-mingw32/11.2.0/../../../../x86_64-w64-mingw32/bin/ld.exe: CMakeFiles/QMLTest.dir/main.cpp.obj: in function `qMain(int, char**)':
C:/myroot/UITest/QMLTest/main.cpp:13: undefined reference to `Mylib::Mylib()'
collect2.exe: error: ld returned 1 exit status

I could not get an answer, so 50 points for grabs, since I cannot resolve this issue.

Ideally if you can point me out to recent project using CMake and QT Creator 8.0 making a simple project and library and link them together, it would be great.

score 0 · Answer 1 · answered Oct 03 '22 at 17:13

At first, try to separate the directories to add in the add_subdirectory command arguments to be sure that everything within the library subdirectories is compiled. Then use target_link_libraries command properly:

add_subdirectory(../QMLLib/mylib)
set(PROJECT_SOURCES
        main.cpp
        qml.qrc
)
include_directories(../QMLLib/mylib/include)
# Link subdir target AFTER definition of the project target
add_library(myproject ${PROJECT_SOURCES})
target_link_libraries(myproject mylib)

I suggest you refer to the actual CMakeLists.txt file I used when importing my library into the existing project (despite being ROS-based, CMake works the same way). Just extract the sequence of commands from there.

It did now work for me, the add_subdirectory is asking for two parameters. One where the source is and the second where the binary for the library. I will study your project more carefully. It might be something really simple. — gmmo, Oct 05 '22 at 16:40
@gmmo the second parameter is optional unless there is no `CMakeLists.txt` in the subdirectory. — Andrei Vukolov, Oct 05 '22 at 17:10

score 0 · Accepted Answer · answered Oct 07 '22 at 02:56

0

I ended up re-arranging the folders and the solution is here:

Cannot add library source to cmake project on QT

where I just include the sources from a separate library project

answered Oct 07 '22 at 02:56

gmmo

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QT creator and CMake, cannot link to external library

2 Answers2