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+Step 2: Adding a Library
+========================
+
+Now we will add a library to our project. This library will contain our own
+implementation for computing the square root of a number. The executable can
+then use this library instead of the standard square root function provided by
+the compiler.
+
+For this tutorial we will put the library into a subdirectory
+called ``MathFunctions``. This directory already contains a header file,
+``MathFunctions.h``, and a source file ``mysqrt.cxx``. The source file has one
+function called ``mysqrt`` that provides similar functionality to the
+compiler's ``sqrt`` function.
+
+Add the following one line ``CMakeLists.txt`` file to the ``MathFunctions``
+directory:
+
+.. literalinclude:: Step3/MathFunctions/CMakeLists.txt
+  :caption: MathFunctions/CMakeLists.txt
+  :name: MathFunctions/CMakeLists.txt
+  :language: cmake
+
+To make use of the new library we will add an :command:`add_subdirectory`
+call in the top-level ``CMakeLists.txt`` file so that the library will get
+built. We add the new library to the executable, and add ``MathFunctions`` as
+an include directory so that the ``mysqrt.h`` header file can be found. The
+last few lines of the top-level ``CMakeLists.txt`` file should now look like:
+
+.. code-block:: cmake
+        :caption: CMakeLists.txt
+        :name: CMakeLists.txt-add_subdirectory
+
+        # add the MathFunctions library
+        add_subdirectory(MathFunctions)
+
+        # add the executable
+        add_executable(Tutorial tutorial.cxx)
+
+        target_link_libraries(Tutorial PUBLIC MathFunctions)
+
+        # add the binary tree to the search path for include files
+        # so that we will find TutorialConfig.h
+        target_include_directories(Tutorial PUBLIC
+                                  "${PROJECT_BINARY_DIR}"
+                                  "${PROJECT_SOURCE_DIR}/MathFunctions"
+                                  )
+
+Now let us make the ``MathFunctions`` library optional. While for the tutorial
+there really isn't any need to do so, for larger projects this is a common
+occurrence. The first step is to add an option to the top-level
+``CMakeLists.txt`` file.
+
+.. literalinclude:: Step3/CMakeLists.txt
+  :caption: CMakeLists.txt
+  :name: CMakeLists.txt-option
+  :language: cmake
+  :start-after: # should we use our own math functions
+  :end-before: # add the MathFunctions library
+
+This option will be displayed in the :manual:`cmake-gui <cmake-gui(1)>` and
+:manual:`ccmake <ccmake(1)>`
+with a default value of ``ON`` that can be changed by the user. This setting
+will be stored in the cache so that the user does not need to set the value
+each time they run CMake on a build directory.
+
+The next change is to make building and linking the ``MathFunctions`` library
+conditional. To do this,  we will create an ``if`` statement which checks the
+value of the option.  Inside the ``if`` block, put the
+:command:`add_subdirectory` command from above with some additional list
+commands to store information needed to link to the library and add the
+subdirectory as an include directory in the ``Tutorial`` target.
+The end of the top-level ``CMakeLists.txt`` file will now look like the
+following:
+
+.. literalinclude:: Step3/CMakeLists.txt
+  :caption: CMakeLists.txt
+  :name: CMakeLists.txt-target_link_libraries-EXTRA_LIBS
+  :language: cmake
+  :start-after: # add the MathFunctions library
+
+Note the use of the variable ``EXTRA_LIBS`` to collect up any optional
+libraries to later be linked into the executable. The variable
+``EXTRA_INCLUDES`` is used similarly for optional header files. This is a
+classic approach when dealing with many optional components, we will cover
+the modern approach in the next step.
+
+The corresponding changes to the source code are fairly straightforward.
+First, in ``tutorial.cxx``, include the ``MathFunctions.h`` header if we
+need it:
+
+.. literalinclude:: Step3/tutorial.cxx
+  :caption: tutorial.cxx
+  :name: tutorial.cxx-ifdef-include
+  :language: c++
+  :start-after: // should we include the MathFunctions header
+  :end-before: int main
+
+Then, in the same file, make ``USE_MYMATH`` control which square root
+function is used:
+
+.. literalinclude:: Step3/tutorial.cxx
+  :caption: tutorial.cxx
+  :name: tutorial.cxx-ifdef-const
+  :language: c++
+  :start-after: // which square root function should we use?
+  :end-before: std::cout << "The square root of
+
+Since the source code now requires ``USE_MYMATH`` we can add it to
+``TutorialConfig.h.in`` with the following line:
+
+.. literalinclude:: Step3/TutorialConfig.h.in
+  :caption: TutorialConfig.h.in
+  :name: TutorialConfig.h.in-cmakedefine
+  :language: c++
+  :lines: 4
+
+**Exercise**: Why is it important that we configure ``TutorialConfig.h.in``
+after the option for ``USE_MYMATH``? What would happen if we inverted the two?
+
+Run the :manual:`cmake  <cmake(1)>` executable or the
+:manual:`cmake-gui <cmake-gui(1)>` to configure the project and then build it
+with your chosen build tool. Then run the built Tutorial executable.
+
+Now let's update the value of ``USE_MYMATH``. The easiest way is to use the
+:manual:`cmake-gui <cmake-gui(1)>` or  :manual:`ccmake <ccmake(1)>` if you're
+in the terminal. Or, alternatively, if you want to change the option from the
+command-line, try:
+
+.. code-block:: console
+
+  cmake ../Step2 -DUSE_MYMATH=OFF
+
+Rebuild and run the tutorial again.
+
+Which function gives better results, ``sqrt`` or ``mysqrt``?