These two new files will also have to be compiled and we have to modify CMakeLists.txt accordingly. However, in this example we want to compile them first into a library, and not directly into the executable:

1. Create a new target, this time a static library. The name of the library will be the name of the target and the sources are listed as follows:

add_library(message 
  STATIC
    Message.hpp
    Message.cpp
  )

2. The creation of the target for the hello-world executable is unmodified:

add_executable(hello-world hello-world.cpp) 

3. Finally, tell CMake that the library target has to be linked into the executable target:

target_link_libraries(hello-world message)

4. We can configure and build with the same commands as before. This time a library will be compiled, alongside the hello-world executable:

$ mkdir -p build
$ cd build
$ cmake ..
$ cmake --build .

Scanning dependencies of target message
[ 25%] Building CXX object CMakeFiles/message.dir/Message.cpp.o
[ 50%] Linking CXX static library libmessage.a
[ 50%] Built target message
Scanning dependencies of target hello-world
[ 75%] Building CXX object CMakeFiles/hello-world.dir/hello-world.cpp.o
[100%] Linking CXX executable hello-world
[100%] Built target hello-world

$ ./hello-world

This is my very nice message: 
Hello, CMake World!
This is my very nice message: 
Goodbye, CMake World