[CMake] Dependencies of an external project

[Marc CHEVRIER]

ExternalProject does its work at build time, so too late for find_package which is executed at config time.
Have a look at module FetchContent which does its work before config time.
Le 19 nov. 2019 à 16:13 +0100, hex <hex7c3 at gmail.com>, a écrit :
> I have an external CMake project that gets exported on install. The root project uses it as a package. The external project must be installed prior being used as a package.
> I can solve this problem by using `ExternalProject` for both projects and declare their dependency. My setup is different, though, as the root project depends on an external project. Since I am including the external project without it having installed on system I am not sure if it is correct to use the package as downstream or whether upstream should be applied instead.
>
> Consider the following project structure:
>
> my_test
> ├── cmake
> ├── extern
> │ └── mylibrary
> │ ├── cmake
> │ ├── include
> │ │ └── my_library
> │ └── src
> ├── include
> │ └── my_test
> └── src
>
> `mylibrary` is a standalone project with `export` on install to use its library as package.
>
> `my_test` (root) is another project that depends on `mylibrary`.
>
> CMakeLists.txt:
> ```
> # add dependencies
> add_subdirectory( extern )
> add_subdirectory( src )
> ```
>
> extern/CMakeLists.txt:
> ```
> ExternalProject_Add(mylibrary
> CMAKE_ARGS
> -D CMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}
> SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/mylibrary"
> )
> ```
>
> src/CMakeLists.txt:
> ```c
> list( APPEND CMAKE_PREFIX_PATH "${CMAKE_BINARY_DIR}" )
>
> find_package( my_library REQUIRED )
>
> #=============================
> # Define targets
> #=============================
> add_executable( MyTestProject test.cpp )
> add_dependencies(MyTestProject my_library-exportname)
> target_link_libraries( MyTestProject PUBLIC my_library-exportname)
> ```
>
> I added `add_dependencies` to have `my_library` be installed prior to building `my_test`. However, build fails even before that since I have `find_package()` in the same CMake txt file.
>
> How can I setup my install as dependency for a package?
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