# 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>...])

## 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 "Cancelled 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 cancelled. 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