How to include static libraries in a static library?

Question

I have 2 static libraries in two different folders: libA and libB

• libB must include libA

My main CMakeLists.txt is:

add_subdirectory(libA)
add_subdirectory(libB)

My first mistake was to think linking libA in libB would include it but it isn't:

target_link_libraries(${PROJECT_NAME} PUBLIC libA::libA)

I get undefined reference to some libA's functions when I try to use libB in an app.

• How can I tell CMake to include libA as part of libB?
• What's the best practice for that?
  • I'd like to avoid any extra step (How to merge two "ar" static libraries into one?)

Answer 1

If you control the builds for both libA and libB, you can solve this by creating OBJECT libraries libA-obj and libB-obj. You would then link libA-obj to libA and then link both object libraries to libB.

---

Here's a more detailed sketch

cmake_minimum_required(VERSION 3.22)
project(example)

# ...

# Can be in subdirectory
add_library(libA-obj ${libA-srcs})
add_library(libA)
target_link_libraries(libA PUBLIC libA-obj)

# Can be in subdirectory
add_library(libB-obj ${libB-srcs})
add_library(libB)
target_link_libraries(libB PUBLIC libB-obj libA-obj)

# Can be in parent directory
add_executable(app ${app-srcs})
target_link_libraries(app PRIVATE libB)

Answer 2

You can add the object files that comprise libA to the sources of libB to include them:

target_sources(LibB PRIVATE $<TARGET_OBJECTS:LibA>)