How to use nested OBJECT libraries and how to pass the object lib dependencies?

Question

I have three libraries, each in a different folder.

LibParent   add_library(${PROJECT_NAME} STATIC LibParent.cpp $<TARGET_OBJECTS:LibChild-obj>))
LibChild    add_library(${PROJECT_NAME}-obj OBJECT LibChild.cpp $<TARGET_OBJECTS:LibBase-obj>))
LibBase     add_library(${PROJECT_NAME}-obj OBJECT LibBase.cpp)

The top CMakeLists.txt has:

add_subdirectory(LibBase)
add_subdirectory(LibChild)
add_subdirectory(LibParent)

The problem is that using the LibParent.a in an app, I get undefined reference to LibBase::....

How can I pass the LibBase's dependencies to every CMake target using LibChild-obj?

• LibChild-obj should include both the LibBase and the object of LibChild

• LibParent does not call directly LibBase, only LibChild's functions

• I'd like to avoid adding $<TARGET_OBJECTS:LibBase-obj> in the LibParent since it doesn't call it directly.

I also would like to stick with OBJECT libraries when I don't need a STATIC library.

The goal is to deliver only LibParent.a containing LibChild AND LibChild's dependency LibBase so that the LibChild's functions can call LibBase's functions.

I think it's important to mention the CMake version: >3.17

Answer 1

The goal is to deliver only LibParent.a containing LibChild AND LibChild's dependency LibBase so that the LibChild's functions can call LibBase's functions.

You're almost there... here are the build files:

# ./CMakeLists.txt
cmake_minimum_required(VERSION 3.22)
project(example)

add_subdirectory(LibBase)
add_subdirectory(LibChild)
add_subdirectory(LibParent)

add_executable(test main.cpp)
target_link_libraries(test PRIVATE LibParent)

# ./LibBase/CMakeLists.txt

add_library(LibBase OBJECT base.cpp)
target_include_directories(
  LibBase
  PUBLIC
  "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>"
)

# ./LibChild/CMakeLists.txt

add_library(LibChild OBJECT child.cpp)
target_include_directories(
  LibChild
  PUBLIC
  "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>"
)
target_link_libraries(LibChild PUBLIC LibBase)

# ./LibParent/CMakeLists.txt

add_library(LibParent parent.cpp)
target_include_directories(
  LibParent
  PUBLIC
  "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}>"
)
target_link_libraries(LibParent PRIVATE LibChild LibBase)

---

Here are some dummy source files:

// Lib{Base,Child,Parent}/{base,child,parent}.cpp
#include <cstdio>
#include "{base,child,parent}.h"  // only contains do_{..}  decl.

void do_{base,child,parent}() {
  do_{base,child}(); // parent calls child, child calls base
  puts("{base,child,parent}");
}

// main.cpp
#include <parent.h>

int main () {
  do_parent();
  return 0;
}

---

Console interaction:

$ cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
...
$ $ cmake --build build/ --verbose
[1/6] /usr/bin/c++  -I/home/alex/test/LibParent -I/home/alex/test/LibChild -I/home/alex/test/LibBase -O3 -DNDEBUG -MD -MT CMakeFiles/test.dir/main.cpp.o -MF CMakeFiles/test.dir/main.cpp.o.d -o CMakeFiles/test.dir/main.cpp.o -c /home/alex/test/main.cpp
[2/6] /usr/bin/c++  -I/home/alex/test/LibBase -O3 -DNDEBUG -MD -MT LibBase/CMakeFiles/LibBase.dir/base.cpp.o -MF LibBase/CMakeFiles/LibBase.dir/base.cpp.o.d -o LibBase/CMakeFiles/LibBase.dir/base.cpp.o -c /home/alex/test/LibBase/base.cpp
[3/6] /usr/bin/c++  -I/home/alex/test/LibChild -I/home/alex/test/LibBase -O3 -DNDEBUG -MD -MT LibChild/CMakeFiles/LibChild.dir/child.cpp.o -MF LibChild/CMakeFiles/LibChild.dir/child.cpp.o.d -o LibChild/CMakeFiles/LibChild.dir/child.cpp.o -c /home/alex/test/LibChild/child.cpp
[4/6] /usr/bin/c++  -I/home/alex/test/LibParent -I/home/alex/test/LibChild -I/home/alex/test/LibBase -O3 -DNDEBUG -MD -MT LibParent/CMakeFiles/LibParent.dir/parent.cpp.o -MF LibParent/CMakeFiles/LibParent.dir/parent.cpp.o.d -o LibParent/CMakeFiles/LibParent.dir/parent.cpp.o -c /home/alex/test/LibParent/parent.cpp
[5/6] : && /usr/bin/cmake -E rm -f LibParent/libLibParent.a && /usr/bin/ar qc LibParent/libLibParent.a  LibChild/CMakeFiles/LibChild.dir/child.cpp.o LibBase/CMakeFiles/LibBase.dir/base.cpp.o LibParent/CMakeFiles/LibParent.dir/parent.cpp.o && /usr/bin/ranlib LibParent/libLibParent.a && :
[6/6] : && /usr/bin/c++ -O3 -DNDEBUG  CMakeFiles/test.dir/main.cpp.o -o test  LibParent/libLibParent.a && :
$ ./build/test 
base
child
parent
$ nm build/LibParent/libLibParent.a

child.cpp.o:
                 U _GLOBAL_OFFSET_TABLE_
0000000000000000 r .LC0
                 U puts
                 U _Z7do_basev
0000000000000000 T _Z8do_childv

base.cpp.o:
                 U _GLOBAL_OFFSET_TABLE_
0000000000000000 r .LC0
                 U puts
0000000000000000 T _Z7do_basev

parent.cpp.o:
                 U _GLOBAL_OFFSET_TABLE_
0000000000000000 r .LC0
                 U puts
                 U _Z8do_childv
0000000000000000 T _Z9do_parentv

The final library indeed includes all objects.