Issue
I have problems building a project with two external dependencies that define the same target.
My goal is to be able to compile the project’s tests along with the project’s dependencies
tests too. For that I was thinking of building the unit tests when compiling from source
with FetchContent. If you included include(CTest) any subsequent calls to add_test would
include that test so afterwards all the tests (project + dependencies’) can be run with
ctest on on the build dir (ctest --test-dir build):
$ ctest --test-dir build
Internal ctest changing into directory: /home/eulersson/cmake-fetchcontent-conflicting-targets/build
Test project /home/eulersson/cmake-fetchcontent-conflicting-targets/build
(FIRST IT RUNS THE TESTS FROM THIS PROJECT (TEST_FOO))
Start 1: test-foo
1/62 Test #1: test-foo .......................... Passed 0.02 sec
(NOW IT WOULD RUN THE TESTS FROM CJSON (DEPENDENCY))
Start 2: cJSON_test
2/62 Test #2: cJSON_test ........................ Passed 0.02 sec
Start 3: parse_examples
[...]
(NOW IT WOULD RUN THE TESTS FROM LIBZMQ (DEPENDENCY))
[...]
Does this make sense? Perhaps there’s another way that conform to best practices?
When adding various dependencies into a CMake project, if you want to also build the unit tests along them, I run into this problem where the target (in this case the unit testing library “unity”) is being added twice by the various dependencies you want to build tests for (add_library(unity STATIC [...])):
- add_library(unity [...]) in libzmq: https://github.com/zeromq/libzmq/blob/959a133520dfc80d29e83aa7ef762e1d0327f63b/tests/CMakeLists.txt#L206-L209
- add_library(unity [...]) in cJSON: https://github.com/DaveGamble/cJSON/blob/19ff92da79ee37c81a4cdf1f50a8dd1cbb02e84f/tests/CMakeLists.txt#L2
Reproducible exampe: https://github.com/eulersson/cmake-fetchcontent-conflicting-targets: Problem illustrating using FetchContent on projects defining the same un-namespaced target.
$ git clone https://github.com/eulersson/cmake-fetchcontent-conflicting-targets
$ cd cmake-conflicting-targets
$ cmake -S . -B build
[...]
CMake Error at build/_deps/cjson-src/tests/CMakeLists.txt:2 (add_library):
add_library cannot create target "unity" because another target with the
same name already exists. The existing target is a static library created
in source directory
"/Users/ramon/Devel/conflicting-targets/build/_deps/libzmq-src/tests". See
documentation for policy CMP0002 for more details.
This would not be an issue if those targets were namespaced.
What would be a good workaround?
Thanks in advance.
Solution
You can't use FetchContent in your situation. When you have multiple dependencies that want to define the same target name, they clash, as you've observed. In such cases, you can't bring the dependencies directly into your main project, but you can build them with ExternalProject since it creates its own separate sub-build.
In your scenario, you probably should use a superbuild based on ExternalProject. In the strictest sense, that means your main project is essentially nothing more than a bunch of ExternalProject_Add() calls. You must set up the dependencies between each of those calls to correctly express which ones depends on which others. If they are all CMake-based dependencies, you can set the CMAKE_PREFIX_PATH of each one to point at the location(s) the other ExternalProject_Add() calls install to. Some people choose to use a single, common install area for all of them to keep things simple. Others choose to install each one to its own separate install area. You can find a reasonable discussion of the superbuild pattern in Kitware's CMake Superbuilds and Git Submodules blog post, although it lacks examples.
Ultimately, what you probably want is the project-level namespaces feature I proposed in the CMake issue tracker. To my knowledge, no-one is working on it yet though, and I don't expect to have the time to dedicate to it myself in the medium term. It would be a pretty big task.
Answered By - Craig Scott Answer Checked By - Marilyn (WPSolving Volunteer)