FetchContentDeclareNAME

FetchContentDeclare is a command provided by the CMake FetchContent module that declares an external content source to be downloaded and used in a build. It is part of a family of facilities designed to integrate third‑party libraries without requiring system-wide installation. The typical workflow is to declare a dependency by name and a set of retrieval options, such as a URL or a Git repository, along with a version specifier or tag.

The declared content can then be fetched and integrated into the build using subsequent commands like FetchContent_MakeAvailable or FetchContent_Populate. After declaration, CMake handles obtaining the sources and making them available to the project, exposing the downloaded content through standard variables related to the declared name, such as source and binary directories. By default, FetchContent downloads content into a base directory under the build tree (commonly ${CMAKE_BINARY_DIR}/_deps), although this location can be customized using the FETCHCONTENT_BASE_DIR variable.

Usage patterns typically involve including the module, declaring the content with options such as GIT_REPOSITORY, GIT_TAG, URL, or SOURCE_SUBDIR, and then invoking FetchContent_MakeAvailable for the declared item. This approach promotes reproducible builds by keeping dependencies within the project’s build system and avoids relying on system-provided installations.

FetchContent is often contrasted with ExternalProject_Add; FetchContent tends to be simpler for pulling in libraries that can be built alongside the main project, while ExternalProject_Add provides more explicit separation of build steps. FetchContent is supported by modern CMake versions and is commonly used to manage lightweight, header-only, or source-based dependencies without additional tooling.