CMP0168ΒΆ

New in version 3.30.

The FetchContent module implements steps directly instead of through a sub-build.

CMake 3.29 and below implement FetchContent as a separate sub-build. This required configuring that separate project and using a build tool. This approach can be very slow with some generators and operating systems. CMake 3.30 and above prefer to implement the download, update, and patch steps directly as part of the main project.

The NEW behavior has the following characteristics:

  • No sub-build is used. All operations are implemented directly from the main project's CMake configure step. When running in CMake script mode, no build tool needs to be available.

  • Generator expressions and GNU Make variables of the form $(SOMEVAR) are not supported. They should not be used in any argument to FetchContent_Declare() or FetchContent_Populate().

  • All LOG_... and USES_TERMINAL_... options, the QUIET option, and the FETCHCONTENT_QUIET variable are ignored. FetchContent output is always part of the main project's configure output. This also means it now respects the message logging level (see CMAKE_MESSAGE_LOG_LEVEL and --log-level). The default message log level should be comparable to using QUIET with the OLD policy setting, except that warnings will now be shown.

  • The PREFIX, TMP_DIR, STAMP_DIR, LOG_DIR, and DOWNLOAD_DIR options and their associated directory properties are ignored. The FetchContent module controls those locations internally.

  • cmake --fresh will remove the stamp and script files used for tracking and populating the dependency. This will force the dependency's download, update, and patch steps to be re-executed. The directory used for downloads is not affected by cmake --fresh, so any previously downloaded files for the URL download method can still be re-used.

The OLD behavior has the following characteristics:

  • A sub-build is always used to implement the download, update, and patch steps. A build tool must be available, even when using FetchContent_Populate() in CMake script mode.

  • Generator expressions and GNU Make variables of the form $(SOMEVAR) can be used, although such use is almost always inappropriate. They are evaluated in the sub-build, so they do not see any information from the main build.

  • All logging, terminal control, and directory options related to the download, update, or patch steps are supported.

  • If the QUIET option is used, or the FETCHCONTENT_QUIET variable is set to true, warnings will not be shown in the output.

  • cmake --fresh has no effect on the dependency's stamp or script files. Previously executed steps will only re-run if details about the dependency have changed.

There's a reasonably good chance that users can set the CMAKE_POLICY_DEFAULT_CMP0168 variable to NEW to globally switch to the NEW behavior while waiting for the project and its dependencies to be updated use the NEW policy setting by default. Projects don't typically make use of the features that the NEW behavior no longer supports, and even those projects that do will often still work fine when those options are ignored. Before setting this behavior globally, check whether any FetchContent_Declare() or FetchContent_Populate() calls use the ignored options in a way that would change observable behavior, other than putting temporary or internally-generated files in different locations.

This policy was introduced in CMake version 3.30. It may be set by cmake_policy() or cmake_minimum_required(). If it is not set, CMake does not warn, and uses OLD behavior.

Note

The OLD behavior of a policy is deprecated by definition and may be removed in a future version of CMake.