Integrating External Libraries in CMake

Hello, today we will delve into a very practical and challenging topic—Integrating External Libraries in CMake: How to Manage Third-Party Libraries Using find_package and ExternalProject.

In most C++ projects, integrating external libraries is an indispensable part. Efficiently and elegantly managing these dependencies, especially during multi-platform builds, is a skill every developer needs to master.

1. Basic Methods for Integrating External Libraries in CMake

There are two common methods for integrating external libraries in CMake: find_package and ExternalProject.

1.1. Using find_package

find_package is the most commonly used command in CMake for integrating external libraries. It can automatically find libraries installed on the system and provide the associated build configuration. Most modern libraries (such as Boost, OpenCV, SDL, etc.) provide CMake support, allowing you to easily find and link these libraries using find_package.

Basic Syntax:

find_package(<package_name> [version] [EXACT] [QUIET] [REQUIRED])

• package_name: The name of the library.
• version: Specifies the version of the library (optional).
• EXACT: Requires the found version to match the specified version exactly.
• QUIET: Suppresses warnings if the library is not found.
• REQUIRED: If the library is not found, CMake will stop and report an error.

Example: Integrating OpenCV

Suppose we need to integrate the OpenCV library into our CMake project. OpenCV provides the FindOpenCV.cmake module in CMake, so we can use the find_package command to locate OpenCV and link it to our project.

1. Install OpenCV: If you haven’t installed OpenCV yet, you can use a package manager to install it (e.g., use apt-get on Ubuntu, or download a precompiled OpenCV library on Windows).

2. CMake Configuration File:

cmake_minimum_required(VERSION 3.10)
project(OpenCVExample)

find_package(OpenCV REQUIRED)

add_executable(main main.cpp)
target_link_libraries(main ${OpenCV_LIBS})

Explanation:

• find_package(OpenCV REQUIRED): CMake will search for the installed OpenCV library on the system, and if not found, it will report an error (due to using REQUIRED).
• ${OpenCV_LIBS}: This is the OpenCV library file found by CMake, automatically linked to our executable.

1. Benefits of Using find_package:

• Cross-Platform Support: Libraries like OpenCV typically provide CMake support for different platforms, so find_package can help you find the appropriate library version across Windows, Linux, macOS, and more.
• Flexibility: You can easily manage different versions of libraries using find_package and switch library versions as needed.

1.2. Using ExternalProject

ExternalProject is used to download, build, and install third-party libraries, usually for libraries that do not have CMake support or precompiled versions. It is suitable for cases where you need to compile from source and manage dependencies automatically.

Basic Syntax:

include(ExternalProject)
ExternalProject_Add(<name>
    PREFIX <prefix_dir>
    GIT_REPOSITORY <repository_url>
    GIT_TAG <commit_or_tag>
    CMAKE_ARGS <args_for_cmake>
)

• name: The project name.
• PREFIX: Specifies the prefix for the installation directory.
• GIT_REPOSITORY: Specifies the Git repository URL of the library.
• GIT_TAG: Specifies the Git version, tag, or commit hash.
• CMAKE_ARGS: Arguments passed to the external project CMake build.

Example: Integrating an External Project

Suppose we need to integrate a library that is not pre-installed on the system and does not have existing CMake module support. We can use ExternalProject to automatically download and compile it.

cmake_minimum_required(VERSION 3.10)
project(ExternalLibraryExample)

include(ExternalProject)

ExternalProject_Add(
    MyLibrary
    GIT_REPOSITORY https://github.com/someuser/somelibrary.git
    GIT_TAG master
    PREFIX ${CMAKE_BINARY_DIR}/_deps
    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}/_install
)

add_executable(main main.cpp)

# Wait for MyLibrary to finish building before linking to the project
add_dependencies(main MyLibrary)
target_include_directories(main PRIVATE ${CMAKE_BINARY_DIR}/_install/include)
target_link_libraries(main ${CMAKE_BINARY_DIR}/_install/lib/libsomelibrary.a)

Explanation:

• ExternalProject_Add: This command defines an external project, and CMake will automatically download and build the project from the specified Git repository.
• add_dependencies(main MyLibrary): Ensures that MyLibrary is built before compiling main.
• target_include_directories and target_link_libraries: Links the external library’s header files and library files to the project.

2. Comprehensive Example: How to Use find_package and ExternalProject Together

Integrating Multiple Libraries

Suppose you need to use both an installed external library (like Boost) and a library that needs to be built from source (like a library from a Git repository) in your project. We can manage these dependencies using both find_package and ExternalProject in CMake.

cmake_minimum_required(VERSION 3.10)
project(MixedLibraryExample)

# Find the installed Boost library
find_package(Boost REQUIRED)

# Use ExternalProject to integrate an external library
include(ExternalProject)
ExternalProject_Add(
    MyExternalLibrary
    GIT_REPOSITORY https://github.com/example/externalrepo.git
    GIT_TAG master
    PREFIX ${CMAKE_BINARY_DIR}/_deps
    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}/_install
)

# Create main executable
add_executable(main main.cpp)

# Add link to Boost library
 target_link_libraries(main Boost::Boost)

# Wait for MyExternalLibrary to finish building before linking to the main project
add_dependencies(main MyExternalLibrary)
target_include_directories(main PRIVATE ${CMAKE_BINARY_DIR}/_install/include)
target_link_libraries(main ${CMAKE_BINARY_DIR}/_install/lib/libexternal.a)

3. Summary and Best Practices

• **Use find_package**: When integrating a popular external library that already provides CMake support, find_package is the optimal choice. It simplifies dependency management and avoids manual path configuration.
• **Use ExternalProject**: For libraries that do not have CMake support or need to be built from source, ExternalProject provides automated build and installation capabilities.
• Cross-Platform Support: Whether using find_package or ExternalProject, CMake can work cross-platform, ensuring compatibility of libraries across different platforms like Windows, Linux, and macOS.
• Modular Management: Automating the management and build process of external dependencies can greatly enhance the maintainability and scalability of the project, especially in large projects.

Today’s lesson hopes to equip you with the knowledge of how to efficiently integrate external libraries in CMake projects, whether through find_package or ExternalProject, allowing you to quickly and flexibly handle third-party dependencies.

Practice is the best teacher, so get started and give it a try!