cmake link xlib directories c++ - c++

Question

I'm trying to compile a c++ program that uses xlib with cmake. However, I'm having a problem including and linking xlib libraries in cmake file.
This is the error that I'm getting.
main.cpp:378: undefined reference to `XClearWindow'
collect2: error: ld returned 1 exit status
CMakeFiles/project1.dir/build.make:94: recipe for target 'project1' failed
make[2]: *** [project1] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/project1.dir/all' failed
make[1]: *** [CMakeFiles/project1.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
And when I use just the command line to compile, it works just fine.
I use this command (g++ main.cpp -L/usr/X11R6/lib -lX11)
and this is my cmake file.
cmake_minimum_required(VERSION 3.6)
project(project1)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
link_directories(/usr/X11R6/lib)
include_directories(/usr/share/X11)
set(SOURCE_FILES main.cpp)
add_executable(project1 ${SOURCE_FILES})

Answer 1

In your case, you forgot to specify the libraries that cmake should use to link your application (target_link_libraries or link_libraries).
But, why not just let cmake find the required path, libraries and includes by itself? I suggest you to use find_package(X11). In your case, you can try:
cmake_minimum_required(VERSION 3.6)
project(project1)
set(CMAKE_CXX_STANDARD 11) # for c++11
find_package(X11 REQUIRED)
link_libraries(${X11_LIBRARIES})
include_directories(${X11_INCLUDE_DIR})
set(SOURCE_FILES main.cpp)
add_executable(project1 ${SOURCE_FILES})

Answer 2

I won't be able to explain why (if you have some idea, don't hesitate to comment my answer), but, in my case, I had to link X11 library using this command:
target_link_libraries( DisplayImage "-lX11" )
It works, at least, for the 3.5.1 version of cmake.