set variables in cmakelists.txt file,Set Variables in CMakeLists.txt File: A Detailed Guide

Set Variables in CMakeLists.txt File: A Detailed Guide

Managing variables in your CMakeLists.txt file is a crucial aspect of building and configuring your projects. By setting variables, you can streamline the build process, make it more flexible, and ensure that your project is portable across different environments. In this article, I’ll walk you through the ins and outs of setting variables in your CMakeLists.txt file, providing you with a comprehensive guide that will help you master this essential skill.

Understanding Variables in CMake

CMake uses variables to store information that can be used throughout the build process. These variables can be set at the beginning of your CMakeLists.txt file or within specific contexts, such as within a target definition. Variables can be of different types, including string, integer, and boolean, and they can be set using various syntaxes.

Setting Variables at the Project Level

At the project level, you can set variables that will be available throughout your entire project. To set a variable, use the following syntax:

set(VARIABLE_NAME value)

For example:

set(PROJECT_NAME "MyProject")

This sets a variable named PROJECT_NAME with the value “MyProject”. You can then use this variable throughout your CMakeLists.txt file by simply referencing it with the $ symbol, like so:

message("The project name is: $PROJECT_NAME")

Setting Variables within Target Definitions

Within a target definition, you can set variables that are specific to that target. This is useful for setting compiler flags, include directories, and other target-specific properties. To set a variable within a target definition, use the following syntax:

set(TARGET_VARIABLE_NAME value)

For example:

add_executable(MyExecutable main.cpp)set_property(TARGET MyExecutable PROPERTY CXX_STANDARD 11)

In this example, we set the C++ standard to 11 for the MyExecutable target.

Using List Variables

CMake also supports list variables, which can store sequences of values. To create a list variable, use the following syntax:

set(LIST_VARIABLE_NAME value1 value2 ...)

For example:

set(INCLUDE_DIRECTORIES /usr/local/include /usr/local/include/mylib)include_directories(${INCLUDE_DIRECTORIES})

This sets a list variable named INCLUDE_DIRECTORIES with two values and then includes those directories in the build.

Using String Variables

String variables are used to store text values. They are particularly useful for setting file paths, compiler flags, and other text-based information. To set a string variable, use the following syntax:

set(STRING_VARIABLE_NAME "value")

For example:

set(LIBRARY_PATH "lib/mylib.a")add_library(MyLibrary ${LIBRARY_PATH})

This sets a string variable named LIBRARY_PATH with the value “lib/mylib.a” and then adds a library with that path.

Using Boolean Variables

Boolean variables are used to store true or false values. They are useful for controlling the behavior of your build process based on certain conditions. To set a boolean variable, use the following syntax:

set(BOOLEAN_VARIABLE_NAME true|false)

For example:

set(BUILD_TESTS true)if(BUILD_TESTS)    add_test(MyTest test.cpp)endif()

This sets a boolean variable named BUILD_TESTS to true and then adds a test if the variable is true.

Using Function Variables

Function variables are a special type of variable that can be used to store the result of a function call. They are particularly useful for creating custom functions within your CMakeLists.txt file. To set a function variable, use the following syntax:

set(FUNCTION_VARIABLE_NAME () )

For example:

function(MY_FUNCTION variable)    set(${variable} "Hello, World!")endfunction()set(HELLO_MESSAGE )MY_FUNCTION(HELLO_MESSAGE)message("The message is: ${HELLO_MESSAGE}")

This defines a function named MY_FUNCTION that sets a variable to the string “Hello, World!” and