cmake_language¶

New in version 3.18.

Call meta-operations on CMake commands.

Synopsis¶

cmake_language(CALL <command> [<arg>...])
cmake_language(EVAL CODE <code>...)
cmake_language(DEFER <options>... CALL <command> [<arg>...])
cmake_language(SET_DEPENDENCY_PROVIDER <command> SUPPORTED_METHODS <methods>...)

Introduction¶

This command will call meta-operations on built-in CMake commands or those created via the macro() or function() commands.

cmake_language does not introduce a new variable or policy scope.

Calling Commands¶

cmake_language(CALL <command> [<arg>...])

Calls the named <command> with the given arguments (if any). For example, the code:

set(message_command "message")
cmake_language(CALL ${message_command} STATUS "Hello World!")

is equivalent to

message(STATUS "Hello World!")

Note

To ensure consistency of the code, the following commands are not allowed:

if / elseif / else / endif
while / endwhile
foreach / endforeach
function / endfunction
macro / endmacro

Evaluating Code¶

cmake_language(EVAL CODE <code>...)

Evaluates the <code>... as CMake code.

For example, the code:

set(A TRUE)
set(B TRUE)
set(C TRUE)
set(condition "(A AND B) OR C")

cmake_language(EVAL CODE "
  if (${condition})
    message(STATUS TRUE)
  else()
    message(STATUS FALSE)
  endif()"
)

is equivalent to

set(A TRUE)
set(B TRUE)
set(C TRUE)
set(condition "(A AND B) OR C")

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/eval.cmake "
  if (${condition})
    message(STATUS TRUE)
  else()
    message(STATUS FALSE)
  endif()"
)

include(${CMAKE_CURRENT_BINARY_DIR}/eval.cmake)

Deferring Calls¶

New in version 3.19.

cmake_language(DEFER <options>... CALL <command> [<arg>...])

Schedules a call to the named <command> with the given arguments (if any) to occur at a later time. By default, deferred calls are executed as if written at the end of the current directory's CMakeLists.txt file, except that they run even after a return() call. Variable references in arguments are evaluated at the time the deferred call is executed.

The options are:

DIRECTORY <dir>

Schedule the call for the end of the given directory instead of the current directory. The <dir> may reference either a source directory or its corresponding binary directory. Relative paths are treated as relative to the current source directory.

The given directory must be known to CMake, being either the top-level directory or one added by add_subdirectory(). Furthermore, the given directory must not yet be finished processing. This means it can be the current directory or one of its ancestors.

ID <id>

Specify an identification for the deferred call. The <id> may not be empty and may not begin with a capital letter A-Z. The <id> may begin with an underscore (_) only if it was generated automatically by an earlier call that used ID_VAR to get the id.

ID_VAR <var>

Specify a variable in which to store the identification for the deferred call. If ID <id> is not given, a new identification will be generated and the generated id will start with an underscore (_).

The currently scheduled list of deferred calls may be retrieved:

cmake_language(DEFER [DIRECTORY <dir>] GET_CALL_IDS <var>)

This will store in <var> a semicolon-separated list of deferred call ids. The ids are for the directory scope in which the calls have been deferred to (i.e. where they will be executed), which can be different to the scope in which they were created. The DIRECTORY option can be used to specify the scope for which to retrieve the call ids. If that option is not given, the call ids for the current directory scope will be returned.

Details of a specific call may be retrieved from its id:

cmake_language(DEFER [DIRECTORY <dir>] GET_CALL <id> <var>)

This will store in <var> a semicolon-separated list in which the first element is the name of the command to be called, and the remaining elements are its unevaluated arguments (any contained ; characters are included literally and cannot be distinguished from multiple arguments). If multiple calls are scheduled with the same id, this retrieves the first one. If no call is scheduled with the given id in the specified DIRECTORY scope (or the current directory scope if no DIRECTORY option is given), this stores an empty string in the variable.

Deferred calls may be canceled by their id:

cmake_language(DEFER [DIRECTORY <dir>] CANCEL_CALL <id>...)

This cancels all deferred calls matching any of the given ids in the specified DIRECTORY scope (or the current directory scope if no DIRECTORY option is given). Unknown ids are silently ignored.

Deferred Call Examples¶

For example, the code:

cmake_language(DEFER CALL message "${deferred_message}")
cmake_language(DEFER ID_VAR id CALL message "Canceled Message")
cmake_language(DEFER CANCEL_CALL ${id})
message("Immediate Message")
set(deferred_message "Deferred Message")

prints:

Immediate Message
Deferred Message

The Cancelled Message is never printed because its command is canceled. The deferred_message variable reference is not evaluated until the call site, so it can be set after the deferred call is scheduled.

In order to evaluate variable references immediately when scheduling a deferred call, wrap it using cmake_language(EVAL). However, note that arguments will be re-evaluated in the deferred call, though that can be avoided by using bracket arguments. For example:

set(deferred_message "Deferred Message 1")
set(re_evaluated [[${deferred_message}]])
cmake_language(EVAL CODE "
  cmake_language(DEFER CALL message [[${deferred_message}]])
  cmake_language(DEFER CALL message \"${re_evaluated}\")
")
message("Immediate Message")
set(deferred_message "Deferred Message 2")

also prints:

Immediate Message
Deferred Message 1
Deferred Message 2

Dependency Providers¶

New in version 3.24.

Note

A high-level introduction to this feature can be found in the Using Dependencies Guide.

cmake_language(SET_DEPENDENCY_PROVIDER <command>
               SUPPORTED_METHODS <methods>...)

When a call is made to find_package() or FetchContent_MakeAvailable(), the call may be forwarded to a dependency provider which then has the opportunity to fulfill the request. If the request is for one of the <methods> specified when the provider was set, CMake calls the provider's <command> with a set of method-specific arguments. If the provider does not fulfill the request, or if the provider doesn't support the request's method, or no provider is set, the built-in find_package() or FetchContent_MakeAvailable() implementation is used to fulfill the request in the usual way.

One or more of the following values can be specified for the <methods> when setting the provider:

FIND_PACKAGE: The provider command accepts find_package() requests.
FETCHCONTENT_MAKEAVAILABLE_SERIAL: The provider command accepts FetchContent_MakeAvailable() requests. It expects each dependency to be fed to the provider command one at a time, not the whole list in one go.

Only one provider can be set at any point in time. If a provider is already set when cmake_language(SET_DEPENDENCY_PROVIDER) is called, the new provider replaces the previously set one. The specified <command> must already exist when cmake_language(SET_DEPENDENCY_PROVIDER) is called. As a special case, providing an empty string for the <command> and no <methods> will discard any previously set provider.

The dependency provider can only be set while processing one of the files specified by the CMAKE_PROJECT_TOP_LEVEL_INCLUDES variable. Thus, dependency providers can only be set as part of the first call to project(). Calling cmake_language(SET_DEPENDENCY_PROVIDER) outside of that context will result in an error.

Note

The choice of dependency provider should always be under the user's control. As a convenience, a project may choose to provide a file that users can list in their CMAKE_PROJECT_TOP_LEVEL_INCLUDES variable, but the use of such a file should always be the user's choice.

Provider commands¶

Providers define a single <command> to accept requests. The name of the command should be specific to that provider, not something overly generic that another provider might also use. This enables users to compose different providers in their own custom provider. The recommended form is xxx_provide_dependency(), where xxx is the provider-specific part (e.g. vcpkg_provide_dependency(), conan_provide_dependency(), ourcompany_provide_dependency(), and so on).

xxx_provide_dependency(<method> [<method-specific-args>...])

Because some methods expect certain variables to be set in the calling scope, the provider command should typically be implemented as a macro rather than a function. This ensures it does not introduce a new variable scope.

The arguments CMake passes to the dependency provider depend on the type of request. The first argument is always the method, and it will only ever be one of the <methods> that was specified when setting the provider.

FIND_PACKAGE

The <method-specific-args> will be everything passed to the find_package() call that requested the dependency. The first of these <method-specific-args> will therefore always be the name of the dependency. Dependency names are case-sensitive for this method because find_package() treats them case-sensitively too.

If the provider command fulfills the request, it must set the same variable that find_package() expects to be set. For a dependency named depName, the provider must set depName_FOUND to true if it fulfilled the request. If the provider returns without setting this variable, CMake will assume the request was not fulfilled and will fall back to the built-in implementation.

If the provider needs to call the built-in find_package() implementation as part of its processing, it can do so by including the BYPASS_PROVIDER keyword as one of the arguments.

FETCHCONTENT_MAKEAVAILABE_SERIAL

The <method-specific-args> will be everything passed to the FetchContent_Declare() call that corresponds to the requested dependency, with the following exceptions:

If SOURCE_DIR or BINARY_DIR were not part of the original declared arguments, they will be added with their default values.
If FETCHCONTENT_TRY_FIND_PACKAGE_MODE is set to NEVER, any FIND_PACKAGE_ARGS will be omitted.
The OVERRIDE_FIND_PACKAGE keyword is always omitted.

The first of the <method-specific-args> will always be the name of the dependency. Dependency names are case-insensitive for this method because FetchContent also treats them case-insensitively.

If the provider fulfills the request, it should call FetchContent_SetPopulated(), passing the name of the dependency as the first argument. The SOURCE_DIR and BINARY_DIR arguments to that command should only be given if the provider makes the dependency's source and build directories available in exactly the same way as the built-in FetchContent_MakeAvailable() command.

If the provider returns without calling FetchContent_SetPopulated() for the named dependency, CMake will assume the request was not fulfilled and will fall back to the built-in implementation.

Note that empty arguments may be significant for this method (e.g. an empty string following a GIT_SUBMODULES keyword). Therefore, if forwarding these arguments on to another command, extra care must be taken to avoid such arguments being silently dropped.

If FETCHCONTENT_SOURCE_DIR_<uppercaseDepName> is set, then the dependency provider will never see requests for the <depName> dependency for this method. When the user sets such a variable, they are explicitly overriding where to get that dependency from and are taking on the responsibility that their overriding version meets any requirements for that dependency and is compatible with whatever else in the project uses it. Depending on the value of FETCHCONTENT_TRY_FIND_PACKAGE_MODE and whether the OVERRIDE_FIND_PACKAGE option was given to FetchContent_Declare(), having FETCHCONTENT_SOURCE_DIR_<uppercaseDepName> set may also prevent the dependency provider from seeing requests for a find_package(depName) call too.

Provider Examples¶

This first example only intercepts find_package() calls. The provider command runs an external tool which copies the relevant artifacts into a provider-specific directory, if that tool knows about the dependency. It then relies on the built-in implementation to then find those artifacts. FetchContent_MakeAvailable() calls would not go through the provider.

mycomp_provider.cmake¶

# Always ensure we have the policy settings this provider expects
cmake_minimum_required(VERSION 3.24)

set(MYCOMP_PROVIDER_INSTALL_DIR ${CMAKE_BINARY_DIR}/mycomp_packages
  CACHE PATH "The directory this provider installs packages to"
)
# Tell the built-in implementation to look in our area first, unless
# the find_package() call uses NO_..._PATH options to exclude it
list(APPEND CMAKE_MODULE_PATH ${MYCOMP_PROVIDER_INSTALL_DIR}/cmake)
list(APPEND CMAKE_PREFIX_PATH ${MYCOMP_PROVIDER_INSTALL_DIR})

macro(mycomp_provide_dependency method package_name)
  execute_process(
    COMMAND some_tool ${package_name} --installdir ${MYCOMP_PROVIDER_INSTALL_DIR}
    COMMAND_ERROR_IS_FATAL ANY
  )
endmacro()

cmake_language(
  SET_DEPENDENCY_PROVIDER mycomp_provide_dependency
  SUPPORTED_METHODS FIND_PACKAGE
)

The user would then typically use the above file like so:

cmake -DCMAKE_PROJECT_TOP_LEVEL_INCLUDES=/path/to/mycomp_provider.cmake ...

The next example demonstrates a provider that accepts both methods, but only handles one specific dependency. It enforces providing Google Test using FetchContent, but leaves all other dependencies to be fulfilled by CMake's built-in implementation. It accepts a few different names, which demonstrates one way of working around projects that hard-code an unusual or undesirable way of adding this particular dependency to the build. The example also demonstrates how to use the list() command to preserve variables that may be overwritten by a call to FetchContent_MakeAvailable().

mycomp_provider.cmake¶

cmake_minimum_required(VERSION 3.24)

# Because we declare this very early, it will take precedence over any
# details the project might declare later for the same thing
include(FetchContent)
FetchContent_Declare(
  googletest
  GIT_REPOSITORY https://github.com/google/googletest.git
  GIT_TAG        e2239ee6043f73722e7aa812a459f54a28552929 # release-1.11.0
)

# Both FIND_PACKAGE and FETCHCONTENT_MAKEAVAILABLE_SERIAL methods provide
# the package or dependency name as the first method-specific argument.
macro(mycomp_provide_dependency method dep_name)
  if("${dep_name}" MATCHES "^(gtest|googletest)$")
    # Save our current command arguments in case we are called recursively
    list(APPEND mycomp_provider_args ${method} ${dep_name})

    # This will forward to the built-in FetchContent implementation,
    # which detects a recursive call for the same thing and avoids calling
    # the provider again if dep_name is the same as the current call.
    FetchContent_MakeAvailable(googletest)

    # Restore our command arguments
    list(POP_BACK mycomp_provider_args dep_name method)

    # Tell the caller we fulfilled the request
    if("${method}" STREQUAL "FIND_PACKAGE")
      # We need to set this if we got here from a find_package() call
      # since we used a different method to fulfill the request.
      # This example assumes projects only use the gtest targets,
      # not any of the variables the FindGTest module may define.
      set(${dep_name}_FOUND TRUE)
    elseif(NOT "${dep_name}" STREQUAL "googletest")
      # We used the same method, but were given a different name to the
      # one we populated with. Tell the caller about the name it used.
      FetchContent_SetPopulated(${dep_name}
        SOURCE_DIR "${googletest_SOURCE_DIR}"
        BINARY_DIR "${googletest_BINARY_DIR}"
      )
    endif()
  endif()
endmacro()

cmake_language(
  SET_DEPENDENCY_PROVIDER mycomp_provide_dependency
  SUPPORTED_METHODS
    FIND_PACKAGE
    FETCHCONTENT_MAKEAVAILABLE_SERIAL
)

The final example demonstrates how to modify arguments to a find_package() call. It forces all such calls to have the QUIET keyword. It uses the BYPASS_PROVIDER keyword to prevent calling the provider command recursively for the same dependency.

mycomp_provider.cmake¶

cmake_minimum_required(VERSION 3.24)

macro(mycomp_provide_dependency method)
  find_package(${ARGN} BYPASS_PROVIDER QUIET)
endmacro()

cmake_language(
  SET_DEPENDENCY_PROVIDER mycomp_provide_dependency
  SUPPORTED_METHODS FIND_PACKAGE
)

cmake_language¶

Synopsis¶

Introduction¶

Calling Commands¶

Evaluating Code¶

Deferring Calls¶

Deferred Call Examples¶

Dependency Providers¶

Provider commands¶

Provider Examples¶

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