We will use the example from Recipe 1, Code reuse with functions and macros and keep the C++ sources unchanged and the file tree essentially the same:

.
├── cmake
│   └── testing.cmake
├── CMakeLists.txt
├── src
│   ├── CMakeLists.txt
│   ├── main.cpp
│   ├── sum_integers.cpp
│   └── sum_integers.hpp

└── tests
    ├── catch.hpp
    ├── CMakeLists.txt
    └── test.cpp