build - CMake 将来自不同 CMakeLists.txt 的对象附加到一个库中

Question

我想从多个子目录中的对象创建一个库，每个子目录都包含自己的 CMakeLists.txt 和 OBJECT 库技巧，以使多个目标具有不同的编译选项。

以下是文件:

project_dir
|--- subdir1
|    |--- src1.c
|    |--- CMakeLists.txt
|--- subdir2
|    |--- src2.c
|    |--- CMakeLists.txt
|--- CMakeLists.txt

所有 CMakeLists.txt 的内容

// project_dir/CMakeLists.txt
// what to put here? to make one single library (mainLib)


// project_dir/subdir1/CMakeLists.txt
add_library(lib11 OBJECT src1.c)
set_target_properties(lib11 PROPERTIES COMPILE_FLAGS "some-flags11")

add_library(lib12 OBJECT src1.c)
set_target_properties(lib12 PROPERTIES COMPILE_FLAGS "some-flags12")
// here I would like to add lib11:objects and lib12:objects to mainLib
// how should it be done?

// project_dir/subdir2/CMakeLists.txt
// *** similar to subdir1 but with src2.c that creates lib21 and lib22
// here I would like to add lib21:objects and lib22:objects to mainLib
// how should it be done?

可以独立做平台吗？谢谢。

编辑:
基于 CMake: how create a single shared library from all static libraries of subprojects? ，我可以将cmake文件更改为以下内容，但仍然不能解决我的问题。

// project_dir/subdir1/CMakeLists.txt
add_library(lib11 OBJECT src1.c)
set_target_properties(lib11 PROPERTIES COMPILE_FLAGS "some-flags11")

add_library(lib12 OBJECT src1.c)
set_target_properties(lib12 PROPERTIES COMPILE_FLAGS "some-flags12")

add_library(lib1 STATIC $<TARGET_OBJECTS:lib11> $<TARGET_OBJECTS:lib12>)

// Above add_library cannot be OBJECT which would fix my problem.
// how to append the lib1 (lib11 and lib12) to mainLib?

编辑:
更新我的帖子，尝试按照答案中的建议链接接口(interface)库。

add_library(lib11 OBJECT test1/sub1/src1.c)
set_target_properties(lib11 PROPERTIES COMPILE_FLAGS "-DF1")

add_library(lib12 OBJECT test1/sub1/src1.c)
set_target_properties(lib12 PROPERTIES COMPILE_FLAGS "-DF2")

add_library(lib1 INTERFACE)
target_sources(lib1 INTERFACE $<TARGET_OBJECTS:lib11>)
target_sources(lib1 INTERFACE $<TARGET_OBJECTS:lib12>)

add_library(lib21 OBJECT test1/sub2/src2.c)
set_target_properties(lib21 PROPERTIES COMPILE_FLAGS "-DF1")

add_library(lib22 OBJECT test1/sub2/src2.c)
set_target_properties(lib22 PROPERTIES COMPILE_FLAGS "-DF2")

add_library(lib2 INTERFACE)
target_sources(lib2 INTERFACE $<TARGET_OBJECTS:lib21>)
target_sources(lib2 INTERFACE $<TARGET_OBJECTS:lib22>)

add_library(PARENT INTERFACE)

target_link_libraries(PARENT INTERFACE lib1)
target_link_libraries(PARENT INTERFACE lib2)

add_library(CORE OBJECT src.c)
add_library(GPARENT STATIC $<TARGET_OBJECTS:CORE>)

target_link_libraries(GPARENT INTERFACE PARENT)

Answer 1

多个 OBJECT 库可以合并到一个 INTERFACE 库中:

# Create lib1 OBJECT library with some sources, compile flags and so.
add_library(lib1 OBJECT ...)

# Create lib2 OBJECT library with some sources, compile flags and so.
add_library(lib2 OBJECT ...)

# Create INTERFACE library..
add_library(libs INTERFACE)
# .. which combines OBJECT libraries
target_sources(libs INTERFACE $<TARGET_OBJECTS:lib1> $<TARGET_OBJECTS:lib2>)

生成的库可以与标准 target_link_libraries 一起使用:

add_library(mainLib <some-source>)
target_link_libraries(mainLib libs)

可以进一步组合 INTERFACE 库:

# Create libs_another INTERFACE library like 'libs' one
add_library(libs_another INTERFACE)
..

# Combine 'libs' and 'libs_another' together
add_library(upper_level_libs INTERFACE)
target_link_libraries(upper_level_libs INTERFACE libs libs_another)

生成的库的“链”可以是任意长度。

虽然这种组合 OBJECT 库的方式看起来很老套，但实际上 CMake documentation 允许这样做。 :

Although object libraries may not be named directly in calls to the target_link_libraries() command, they can be "linked" indirectly by using an Interface Library whose INTERFACE_SOURCES target property is set to name $<TARGET_OBJECTS:objlib>.