How to do it

Let us start with the same source code as for the previous recipe. We want to be able to toggle between two behaviors:

1. Build Message.hpp and Message.cpp into a library, static or shared, and then link the resulting library into the hello-world executable.
2. Build Message.hpp, Message.cpp, and hello-world.cpp into a single executable, without producing the library.

Let us construct CMakeLists.txt to achieve this:

1. We start out by defining the minimum CMake version, project name, and supported language:

cmake_minimum_required(VERSION 3.5 FATAL_ERROR)

project(recipe-04 LANGUAGES CXX)

2. We introduce a new variable, USE_LIBRARY. This is a logical variable and its value will be set to OFF. We also print its value for the user:

set(USE_LIBRARY OFF)

message(STATUS "Compile sources into a library? ${USE_LIBRARY}")

3. Set the BUILD_SHARED_LIBS global variable, defined in CMake, to OFF. Calling add_library and omitting the second argument will build a static library:

set(BUILD_SHARED_LIBS OFF)

4. We then introduce a variable, _sources, listing Message.hpp and Message.cpp:

list(APPEND _sources Message.hpp Message.cpp)

5. We then introduce an if-else statement based on the value of USE_LIBRARY. If the logical toggle is true, Message.hpp and Message.cpp will be packaged into a library:

if(USE_LIBRARY)
  # add_library will create a static library
  # since BUILD_SHARED_LIBS is OFF
  add_library(message ${_sources})

  add_executable(hello-world hello-world.cpp)

  target_link_libraries(hello-world message)
else()
  add_executable(hello-world hello-world.cpp ${_sources})
endif()

6. We can again build with the same set of commands. Since USE_LIBRARY is set to OFF, the hello-world executable will be compiled from all sources. This can be verified by running the objdump -x command on GNU/Linux.