Multi-project

Learn multi-project structure with CMake.

Multi-project structure

Large projects split into multiple subdirectories: each has its own CMakeLists.txt. The top-level file includes subdirectories with the add_subdirectory command. This keeps the code modular.

Typical layout: a root CMakeLists.txt, a lib/ subdirectory for the library, an app/ subdirectory for the application. The root file calls add_subdirectory(lib) and add_subdirectory(app). Each subdirectory CMakeLists defines its own targets. Variable and target sharing between parent and child is automatic.

Library and application separation

In a modular project the lib/ directory defines a library with add_library. The app/ directory defines the application with add_executable and links to the library with target_link_libraries. This lets the library be reused in other projects.

Example: in lib/CMakeLists.txt write add_library(mylib utils.cpp). In app/CMakeLists.txt write add_executable(myapp main.cpp) and target_link_libraries(myapp PRIVATE mylib). The root file calls both with add_subdirectory. The dependency direction is clear: app depends on lib; lib is independent.

Exercise Write the multi-project root file

Write the root CMakeLists.txt. Add cmake_minimum_required(VERSION 3.20) and project(MyApp). Include two subdirectories with add_subdirectory(lib) and add_subdirectory(app).

Show example solution

cmake_minimum_required(VERSION 3.20)
project(MyApp)
add_subdirectory(lib)
add_subdirectory(app)

Test yourself

What does add_subdirectory do?
- Creates a new library
- Processes a subdirectory's CMakeLists
- Sets the output directory
- Changes the compiler
Show answer

Processes a subdirectory's CMakeLists — add_subdirectory brings a subdirectory's CMakeLists.txt into the build.
Match each piece to its role

add_subdirectory(lib)

Adds the lib directory
add_subdirectory(app)

Adds the app directory
lib/CMakeLists.txt

Library target
app/CMakeLists.txt

Application target
Complete the add_subdirectory command
```
____(lib)
```
Show answer
```
add_subdirectory(lib)
```

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