c – 如何使用Git子模块和CMake处理传递依赖冲突？

  executable_A
    |     |
    |     v
    |  library_B
    |     |
    v     v
   library_C

因此,可执行文件对library_C有两个依赖关系,一个是direct,一个是传递.我希望使用Git子模块和CMake将它们组合在一起,因此简化的目录结构如下所示：

executable_A/
  CMakeListst.txt
  library_B/
    CMakeLists.txt
    library_C/
      CMakeLists.txt
  library_C/
    CMakeLists.txt

如您所见,library_C存储库作为子模块包含两次.让我们假设两个子模块都指向同一个提交(关于如何强制执行的任何想法都是受欢迎的,但不是这个问题的主题).

我们使用add_subdirectory,target_link_libraries和target_include_directories来管理这些相互依赖性.很标准.

问题是如果你创建一个具有相同名称的目标两次,CMake不喜欢它,所以它会抱怨：

CMake Error at library_C/CMakeLists.txt:13 (add_library):
add_library cannot create target “library_C” because another target
with the same name already exists. The existing target is a static
library created in source directory “…/library_B/library_C”.
See documentation for policy CMP0002 for more details.

我宁愿不删除executable_A对library_C的直接依赖,因为它是通过library_B引入的,这是library_B的一个实现细节,不应该依赖它.此外,一旦我们添加了另一个依赖项,例如executable_A – >,这种方法就会崩溃. library_D – > library_C.

(This question是我能找到的最接近的,但是有点一般,但无论如何都没有答案.)

检查项目的目标存在

单个包含子项目的最简单模式是检查某个子项目的目标是否存在：

# When include 'C' subproject
if(NOT TARGET library_C)
    add_subdirectory(C)
endif()

(这里我们假设项目C定义了目标库_C.)

在有条件的包含之后,所有子项目的目标和功能将立即为呼叫者提供保证.

最好在所有地方(在executable_A和library_B中)使用此模式.这样改变executable_A中library_B和library_C的顺序并不会破坏正确性.

这个模式可以重新设计,供子项目本身使用：

# At the beginning of 'C' project
cmake_minimum_required(...)
if(TARGET library_C)
    return() # The project has already been built.
endif()

project(C)
...

检查项目是否存在

创建项目时,CMake为其定义了几个变量,其中包含<PROJECT-NAME>_BINARY_DIR个变量.请注意,此变量是缓存的,因此当第二次调用cmake时(例如,如果某些CMakeLists.txt已更改),该变量一开始就存在.

# When include 'C' subproject
if(NOT C_BINARY_DIR # Check that the subproject has never been included
    OR C_BINARY_DIR STREQUAL "${CMAKE_CURRENT_BINARY_DIR}/C" # Or has been included by us.
)
    add_subdirectory(C)
endif()

这个模式可以重新设计,供子项目本身使用：

# At the beginning of 'C' project
cmake_minimum_required(...)
if(NOT C_BINARY_DIR # Check that the project has never been created
    OR C_BINARY_DIR STREQUAL "${CMAKE_CURRENT_BINARY_DIR}" # Or has been created by us.
    project(C)
else()
    return() # The project has already been built
endif()