As your project grows, placing all files in a single folder quickly becomes difficult to manage. Splitting code into subdirectories allows you to group related functionality together and keeps your project easy to navigate. CMake supports this by allowing each directory to have its own CMakeLists.txt file.

# MyProject/
# ├── CMakeLists.txt
# ├── src/
# │   ├── CMakeLists.txt
# │   └── main.c
# └── include/
#     ├── CMakeLists.txt
#     └── mylib.h

Each folder in this structure controls its own build configuration. The root project simply connects them.

cmake_minimum_required(VERSION 3.10)
project(MyProject)

add_subdirectory(include)
add_subdirectory(src)

When CMake runs, it enters the include directory first, processes its configuration, and then moves into src. This keeps your build logic modular and easier to update.

For bigger projects, code is often grouped by purpose using nested directories. Libraries, utilities, and the main application are kept separate.

# NestedProject/
# ├── CMakeLists.txt
# ├── lib/
# │   ├── CMakeLists.txt
# │   ├── math/
# │   │   ├── CMakeLists.txt
# │   │   ├── math.c
# │   │   └── math.h
# │   └── utils/
# │       ├── CMakeLists.txt
# │       ├── utils.c
# │       └── utils.h
# └── app/
#     ├── CMakeLists.txt
#     └── main.c

Each folder defines its own rules and dependencies. CMake brings everything together into a single build system while letting each part remain independent. This structure makes testing, reusing code, and scaling your project much easier.

cmake_minimum_required(VERSION 3.10)
project(NestedProject)

add_subdirectory(lib)
add_subdirectory(app)