FetchContent

FetchContent is a CMake module that provides a lightweight mechanism to acquire external dependencies by downloading their sources during the configure step, then integrating them into the current build. It was designed to simplify dependency management by avoiding separate configure/build steps typical of traditional ExternalProject usage, and by letting downloaded content be used as part of the primary CMake project.

The core API consists of a few key commands. include(FetchContent) loads the module, then FetchContent_Declare(NAME …) defines a dependency, with options such as URL or GIT_REPOSITORY, GIT_TAG, and SOURCE_SUBDIR. After declaration, you can either call FetchContent_Populate(NAME) to download and prepare the content, or use the convenience FetchContent_MakeAvailable(NAME), which performs population and, if the content provides a CMakeLists.txt, adds it to the build via add_subdirectory. The populated content exposes properties such as NAME_SOURCE_DIR and NAME_POPULATED, which can be queried with FetchContent_GetProperties(NAME).

Typical usage involves declaring a dependency and then making it available, letting CMake fetch and incorporate it automatically. For example, a project can declare a library's source, then call FetchContent_MakeAvailable for that library, after which targets from the external content can be linked as usual.

FetchContent is especially useful for reproducible builds and for projects that want deterministic, self-contained dependency management without separate external projects. It integrates well with modern CMake workflows and is commonly preferred over more complex packaging or manual downloads. Limitations include potential download overhead and less suitability for very large or private dependencies, where alternative strategies may be warranted. FetchContent became available in CMake 3.11 and remains a core tool in contemporary CMake usage.