CMake Notes: function, include, and FetchContent

1. function

Purpose:

Defines reusable blocks of CMake code. It creates a local scope, meaning variables defined inside a function won't affect the global scope unless explicitly modified.

Syntax:

function(FunctionName arg1 arg2)
    # Body of the function
    message("Argument 1: ${arg1}")
    message("Argument 2: ${arg2}")
endfunction()

Example:

function(print_message msg)
    message("Message: ${msg}")
endfunction()

print_message("Hello, CMake!")

Output:

Message: Hello, CMake!

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2. include

Purpose:

Includes another CMake script file to reuse its logic. Useful for modularizing CMake code.

Syntax:

include(FileName.cmake [OPTIONAL] [RESULT_VARIABLE var])

Example:

Main CMakeLists.txt

include(helper.cmake)
hello_world()

helper.cmake

function(hello_world)
    message("Hello from helper.cmake!")
endfunction()

Output:

Hello from helper.cmake!

---

3. FetchContent

Purpose:

Fetches and integrates external dependencies directly into the build process.

Syntax:

include(FetchContent)
FetchContent_Declare(
    <Name>
    GIT_REPOSITORY <url>
    GIT_TAG <commit/tag/branch>
)
FetchContent_MakeAvailable(<Name>)

Example:

include(FetchContent)

FetchContent_Declare(
    json
    GIT_REPOSITORY https://github.com/nlohmann/json.git
    GIT_TAG v3.11.2
)

FetchContent_MakeAvailable(json)

add_executable(my_app main.cpp)
target_link_libraries(my_app PRIVATE nlohmann_json::nlohmann_json)

This fetches the nlohmann/json library and links it to my_app.

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Summary Table:

Feature	Purpose	Key Point
function	Defines reusable logic in local scope	Use for modular code blocks
include	Includes another CMake script	Helps in splitting CMake logic
FetchContent	Fetches external dependencies dynamically	Simplifies dependency management