cmake，迷失在全局变量的概念中(以及 PARENT_SCOPE 或 add_subdirectory 替代方案) [英] cmake, lost in the concept of global variables (and PARENT_SCOPE or add_subdirectory alternatives)

问题描述

我有一个 cmake 项目，其中有一些模块，我正在使用 Find-*.cmake 将共享模块包含在应用程序中.由于没有考虑到我添加的每个模块，我为链接器定义了一种全局 LIB 变量:

I have a cmake project in which I have some modules and I'm using Find-*.cmake for including the shared modules in the application. For not taking in account every module that I add, I have defined a kind of global LIB variables tor the linker:

# inside a Find-*.cmake or in the CMakeLists.txt of the modules:
set(LIB ${LIB} ...)

所以在使用某些模块的最终应用程序之一中，我可以这样做:

so in one of the final applications that uses some modules I can just do:

target_link_libraries(${APP_NAME} ${LIB})

然后，我想在 /project_path/modules/foo/build 中包含已编译的模块，这样如果一个模块真的很大而无法编译，它可以为所有应用程序编译一次使用它.我实现这一点的方式是从 Find-*.cmake 中加载模块的 CMakeLists.txt:

Then, I'd like to have the compiled modules in the /project_path/modules/foo/build so that if a module is really big to compile it can be compiled once for all the application that use it. The way I'm achieving this is to load the CMakeLists.txt of the module from the Find-*.cmake in this way:

# Inside FindFoo.cmake, for loading /project_path/modules/foo/CMakeLists.txt
# and compile it in /project_path/modules/foo/build
add_subdirectory(${CMAKE_CURRENT_LIST_DIR}/../modules/${PACKAGE_NAME} 
                 ${CMAKE_CURRENT_LIST_DIR}/../modules/${PACKAGE_NAME}/build
)
include_directories(${CMAKE_CURRENT_LIST_DIR}/../modules/${PACKAGE_NAME}/include)

但有时会发生某些模块需要另一个模块，以便 add_subdirectory 创建新范围并可以正确加载 LIB 但无法编写它(当我使用 设置它在更深的范围内，而不是改变上层的范围).为了绕过这个，我必须在 set 中添加 PARENT_SCOPE).. 所以我尝试将它添加到一些我认为可以嵌套并隐藏在某些依赖项中的模块中，但是编译我突然遇到的所有应用程序:

But it happened sometimes that some module require another modules so that the add_subdirectory creates new scopes and can correctly load LIB but cannot write it (when I use set it is in the deeper scope and not changes the upper scope). For bypass this I have to add PARENT_SCOPE in the set).. So I have tried to add it in some module that I think could be nested and hidden in some dependencies but compiling all the application I suddenly faced with:

CMake Warning (dev) at /path_to_repo/cmake/FindFooX.cmake:6 (set):
  Cannot set "LIB": current scope has no parent.
Call Stack (most recent call first):
  CMakeLists.txt:14 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

我担心这可能会因我需要的模块或模块本身的依赖树而因应用程序而异，所以我正在寻找更简洁的解决方案.

I am afraid that this can change from app to app in respect to which module I need or in respect of the dependecies tree in the modules itself so I'm looking for a cleaner solution.

推荐答案

默认情况下，CMake 中的所有变量都是本地变量.虽然您可以使用 PARENT_SCOPE 参数将局部变量的范围增加一层，但这对于 函数.

All variables in CMake are local by default. While you can use the PARENT_SCOPE parameter to increase the scope of a local variable by one layer, that mostly makes sense for return values of functions.

另一方面，对于查找脚本，您通常需要全局变量的行为:一旦任何人调用查找脚本，您希望结果在任何地方都可用.特别是，对同一个 find 脚本的第二次调用应该只是重用第一次调用的结果.在 CMake 中，这是通过将变量存储到缓存中来实现的.各种 find_* 调用已经自动执行此操作，因此您应该更喜欢在适用的情况下使用这些调用.对于任何其他自定义变量，set 还提供存储到缓存的功能:

For find scripts on the other hand you usually want the behavior of a global variable: Once the find script is called by anyone, you want the results to be available everywhere. In particular, a second call to the same find script should just reuse the results of the first call. In CMake this is achieved by storing variables to the cache. The various find_* calls already do this automatically, so you should prefer using those where applicable. For any additional custom variables, set offers the capability to store to the cache as well:

set(MY_GLOBAL_VARIABLE "Some value" CACHE STRING "Description")

请注意，局部变量可以在其作用域内隐藏缓存的同名变量.

Note that local variables can hide cached variables of the same name in their scope.

这篇关于cmake，迷失在全局变量的概念中(以及 PARENT_SCOPE 或 add_subdirectory 替代方案)的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！