Setup linking between shared and static lib in cmake

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.

1. How to tell cmake to compile B first so that when I import B for A, it is already updated if any changes
2. 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>)