External Projects

Introduction

Sometimes, your project depends on external libraries or tools that aren’t part of your codebase. CMake provides two main ways to handle this:

1. ExternalProject_Add: Downloads, builds, and integrates an external project during the build process.
2. FetchContent: Fetches and integrates external projects at configure time (introduced in CMake 3.11).

Key Concepts

• FetchContent_Declare(): Declares an external project (e.g., a GitHub repository).
• FetchContent_MakeAvailable(): Downloads and makes the project available for use.
• Integration: The external project’s targets can be used in your CMakeLists.txt as if they were part of your project.

Code Sample

1. Let's start with a fresh CMakeLists.txt file to keep things simple. You can continue using the files from the previous chapter, but replace the entire contents of the CMakeLists.txt to the one below.

# ---- Replace everything with the text below -----
cmake_minimum_required(VERSION 3.15)
project(HelloWorld)

include(FetchContent)
# This declares the googletest repo but does not download it.
FetchContent_Declare(
    googletest
    GIT_REPOSITORY https://github.com/google/googletest.git
    GIT_TAG release-1.12.1
)

# This downloads the googletest repo when you run `cmake ..`
FetchContent_MakeAvailable(googletest)

add_executable(HelloWorld main.cpp)
target_link_libraries(HelloWorld PRIVATE gtest_main)

2. Run cmake. This will download the googletest repo when it encounters FetchContent_MakeAvailable.

cmake ..

3. Run the build

cmake --build .

Quiz