CMP0168ΒΆ
Added 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 toFetchContent_Declare()orFetchContent_Populate().All
LOG_...andUSES_TERMINAL_...options, theQUIEToption, and theFETCHCONTENT_QUIETvariable are ignored.FetchContentoutput is always part of the main project's configure output. This also means it now respects the message logging level (seeCMAKE_MESSAGE_LOG_LEVELand--log-level). The default message log level should be comparable to usingQUIETwith theOLDpolicy setting, except that warnings will now be shown.The
PREFIX,TMP_DIR,STAMP_DIR,LOG_DIR, andDOWNLOAD_DIRoptions and their associated directory properties are ignored. TheFetchContentmodule controls those locations internally.cmake --freshwill 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 bycmake --fresh, so any previously downloaded files for theURLdownload method can still be reused.
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
QUIEToption is used, or theFETCHCONTENT_QUIETvariable is set to true, warnings will not be shown in the output.cmake --freshhas 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.