How to define the option() command in CMake
The option() command is used to define an option variable that can be toggled by the user when configuring the project. It provides a way to enable or disable certain features or behaviors of the project based on the user's preference.
Here is a list of features or behaviors of the options() command in CMake:
User-configurable options: The primary purpose of the
options()command is to provide a way for users to customize the behavior of the project by enabling or disabling specific options.CMakeCache.txt Entries: When anoption()command is defined, it creates an entry in theCMakeCache.txtfile, allowing users to modify the value of the option without reconfiguring the project from scratch.Cache variables: For each option defined using the
options()command, a corresponding cache variable is created. This variable can be accessed and modified by developers in theirCMakeLists.txtfiles.Default values: Using the
option(<option_name> "option description" [default_value])syntax, we can specify a default value for each option. If no default value is provided, the option will default toOFF.User interaction: When users run the CMake configuration step, the
options()command prompts them to enable or disable the options defined using a user-friendly interface.CMake GUI support: The
options()command is particularly useful when using the CMake GUI tools because they provide checkboxes that users can toggle to enable or disable options.Conditional compilation: The values of options can be used to conditionally include or exclude certain parts of the project's build process or source code, enabling fine-grained customization.
Control over build configuration: By allowing users to choose options during configuration, the
options()command gives control over the build configuration without the need to edit CMake files.
Syntax
The syntax for the option() command is as follows:
option(<variable> "help string" [initial_value])
Parameters
Here’s what each parameter represents:
<variable>: This is the name of theoptionvariable. This variable is created and can be used throughout theCMakeLists.txtfile to control the project’s behavior conditionally."help string": This is a brief description of theoption. This string is displayed to users when they run CMake and configure the project.[initial_value]: This optional initial value is assigned to theoptionvariable. By default, the initial value isOFF. If the user doesn’t specify the value, theoptionvariable will have this initial value.
After defining an option using the option() command, we can check its value using an if() statement to conditionally enable or disable certain project parts based on the user’s choice.
Example
Here’s an example option.cmake file that demonstrates the usage of the option() command. Click the “Run” button and execute the cmake -P option.cmake command in the following terminal:
cmake_minimum_required(VERSION 3.25.1)
message("BUILD_TESTS before: ${BUILD_TESTS}")
option(BUILD_TESTS "Build tests" ON)
message("BUILD_TESTS After: ${BUILD_TESTS}")Explanation
Line 1: The CMake script establishes the minimum required version as
3.25.1.Line 3: We print the value of the
BUILD_TESTSvariable before it is defined as an option. The value will be printed if the variable has been previously set or initialized.Line 5: We define the
BUILD_TESTSvariable as an option. The help string"Build tests"will be displayed to the user during configuration, providing information about the option. The initial value of theoptionis set toON, meaning the tests will be built by default.Line 7: We print the value of the
BUILD_TESTSvariable after it has been defined as an option. The updated value, which the user may have changed during configuration, will be displayed.
Overall, this code establishes an option called BUILD_TESTS that enables users to control whether tests should be built. It shows the value of this option both before and after defining it, offering insights into the variable’s state.
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