Setup linking between shared and static lib in cmake - c++

Question

After reading some articles on static and shared libs I think I know the difference but I still cannot figure out a way to fix my use case. I have the following project structure
Project
├── A
│   ├── A.cpp
│   ├── A.h
│   └── CMakeLists.txt
├── B
│   ├── CMakeLists.txt
│   └── B.cpp
├── CMakeLists.txt
in folder A I have:
//content of A.h
#include <opencv2/opencv.hpp>
cv::Mat A_load_image(std::string file_path);
//content of A.cpp
#include "A.h"
cv::Mat A_load_image(std::string file_path) {
return cv::imread(file_path);
}
// content of CMakelists.txt in A
set(TARGET A)
add_library( ${TARGET} STATIC A.cpp )
target_include_directories(${TARGET} PUBLIC
${CMAKE_CURRENT_SOURCE_DIR}
/path/to/opencv/include/folder )
link_directories( /path/to/opencv/lib/folder)
target_link_libraries( ${TARGET} PUBLIC libopencv_core.a )
then in my folder B I have:
//content of B.cpp
#include "A.h"
cv::Mat B_load_image() {
return A_load_image("img.bmp");
}
// content of CMakelists.txt in B
set(TARGET B)
add_library(A STATIC IMPORTED)
set_target_properties(A PROPERTIES IMPORTED_LOCATION /PATH/TO/libA.a)
add_library(${TARGET} SHARED B.cpp)
target_include_directories(${TARGET} PUBLIC
${CMAKE_CURRENT_SOURCE_DIR}/../A
/path/to/opencv/include/folder )
target_link_libraries(${TARGET} PUBLIC A libopencv_core.a)
And of course I have the CMakelists.txt file in the project root has:
cmake_minimum_required(VERSION 3.4.1)
add_subdirectory(B)
add_subdirectory(A)
I have questions below regarding this project.
How to tell cmake to compile B first so that when I import B for A, it is already updated if any changes
The above setup does not work as I got error when linking B: "cannot find -lopencv_core", I already used PUBLIC for linking A, I also tried to add link_directories( /path/to/opencv/lib/folder) to the CMakelists.txt for B, but still not working.
I believe "cannot find -lopencv_core" failed because it is looking for dynamic lib, e.g., libopencv_core.so rather than the static one. But why is that and how I force to link to the static lib?

Answer 1

Try an object library:
cmake_minimum_required(VERSION 3.11)
add_subdirectory(A)
add_subdirectory(B)
# A/CMakeLists.txt
find_package(OpenCV REQUIRED)
add_library(A OBJECT A.cpp)
target_include_directories(A PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
target_link_libraries(A PUBLIC ${OpenCV_LIBS})
# B/CMakeLists.txt
add_library(B SHARED B.cpp)
target_include_directories(B PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
target_link_libraries(B PUBLIC A)
# also try
target_link_options(B PRIVATE "-Wl,--whole-archive")
# or maybe
target_link_options(B PRIVATE "-Wl,--export-all-symbols")
# or maybe also
target_sources(B $<TARGET_OBJECTS:A>)