Step 2: Adding a Library
========================

At this point, we have seen how to create a basic project using CMake. In this
step, we will learn how to create and use a library in our project. We will
also see how to make the use of our library optional.

Exercise 1 - Creating a Library
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To add a library in CMake, use the :command:`add_library` command and specify
which source files should make up the library.

Rather than placing all of the source files in one directory, we can organize
our project with one or more subdirectories. In this case, we will create a
subdirectory specifically for our library. Here, we can add a new
``CMakeLists.txt`` file and one or more source files. In the top level
``CMakeLists.txt`` file, we will use the :command:`add_subdirectory` command
to add the subdirectory to the build.

Once the library is created, it is connected to our executable target with
:command:`target_include_directories` and :command:`target_link_libraries`.

Goal
----

Add and use a library.

Helpful Resources
-----------------

* :command:`add_library`
* :command:`add_subdirectory`
* :command:`target_include_directories`
* :command:`target_link_libraries`
* :variable:`PROJECT_SOURCE_DIR`

Files to Edit
-------------

* ``CMakeLists.txt``
* ``tutorial.cxx``
* ``MathFunctions/CMakeLists.txt``

Getting Started
---------------

In this exercise, we will add a library to our project that contains 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 the header files
``MathFunctions.h`` and ``mysqrt.h``. Their respective source files
``MathFunctions.cxx`` and ``mysqrt.cxx`` are also provided. We will not need
to modify any of these files. ``mysqrt.cxx`` has one function called
``mysqrt`` that provides similar functionality to the compiler's ``sqrt``
function. ``MathFunctions.cxx`` contains one function ``sqrt`` which serves
to hide the implementation details of ``sqrt``.

From the ``Help/guide/tutorial/Step2`` directory, start with ``TODO 1`` and
complete through ``TODO 6``.

First, fill in the one line ``CMakeLists.txt`` in the ``MathFunctions``
subdirectory.

Next, edit the top level ``CMakeLists.txt``.

Finally, use the newly created ``MathFunctions`` library in ``tutorial.cxx``

Build and Run
-------------

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.

Below is a refresher of what that looks like from the command line:

.. code-block:: console

  mkdir Step2_build
  cd Step2_build
  cmake ../Step2
  cmake --build .

Try to use the newly built ``Tutorial`` and ensure that it is still
producing accurate square root values.

Solution
--------

In the ``CMakeLists.txt`` file in the ``MathFunctions`` directory, we create
a library target called ``MathFunctions`` with :command:`add_library`. The
source files for the library are passed as an argument to
:command:`add_library`. This looks like the following line:

.. raw:: html/

  <details><summary>TODO 1: Click to show/hide answer</summary>

.. code-block:: cmake
  :caption: TODO 1: MathFunctions/CMakeLists.txt
  :name: MathFunctions/CMakeLists.txt-add_library

  add_library(MathFunctions MathFunctions.cxx mysqrt.cxx)

.. raw:: html

  </details>

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.

.. raw:: html

  <details><summary>TODO 2: Click to show/hide answer</summary>

.. code-block:: cmake
  :caption: TODO 2: CMakeLists.txt
  :name: CMakeLists.txt-add_subdirectory

  add_subdirectory(MathFunctions)

.. raw:: html

  </details>

Next, the new library target is linked to the executable target using
:command:`target_link_libraries`.

.. raw:: html

  <details><summary>TODO 3: Click to show/hide answer</summary>

.. code-block:: cmake
  :caption: TODO 3: CMakeLists.txt
  :name: CMakeLists.txt-target_link_libraries

  target_link_libraries(Tutorial PUBLIC MathFunctions)

.. raw:: html

  </details>

Finally we need to specify the library's header file location. Modify
:command:`target_include_directories` to add the ``MathFunctions`` subdirectory
as an include directory so that the ``MathFunctions.h`` header file can be
found.

.. raw:: html

  <details><summary>TODO 4: Click to show/hide answer</summary>

.. code-block:: cmake
  :caption: TODO 4: CMakeLists.txt
  :name: CMakeLists.txt-target_include_directories-step2

  target_include_directories(Tutorial PUBLIC
                            "${PROJECT_BINARY_DIR}"
                            "${PROJECT_SOURCE_DIR}/MathFunctions"
                            )

.. raw:: html

  </details>

Now let's use our library. In ``tutorial.cxx``, include ``MathFunctions.h``:

.. raw:: html

  <details><summary>TODO 5: Click to show/hide answer</summary>

.. literalinclude:: Step3/tutorial.cxx
  :caption: TODO 5: tutorial.cxx
  :name: CMakeLists.txt-include-MathFunctions.h
  :language: cmake
  :start-after: #include <string>
  :end-before: #include "TutorialConfig.h"

.. raw:: html

  </details>

Lastly, replace ``sqrt`` with our library function ``mathfunctions::mysqrt``.

.. raw:: html

  <details><summary>TODO 6: Click to show/hide answer</summary>

.. literalinclude:: Step3/tutorial.cxx
  :caption: TODO 6: tutorial.cxx
  :name: CMakeLists.txt-option
  :language: cmake
  :start-after:   const double inputValue = std::stod(argv[1]);
  :end-before: std::cout

.. raw:: html

  </details>

Exercise 2 - Adding an Option
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Now let us add an option in the MathFunctions library to allow developers to
select either the custom square root implementation or the built in standard
implementation. While for the tutorial
there really isn't any need to do so, for larger projects this is a common
occurrence.

CMake can do this using the :command:`option` command. This gives users a
variable which they can change when configuring their cmake build. 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.

Goal
----

Add the option to build without ``MathFunctions``.


Helpful Resources
-----------------

* :command:`if`
* :command:`option`
* :command:`target_compile_definitions`

Files to Edit
-------------

* ``MathFunctions/CMakeLists.txt``
* ``MathFunctions/MathFunctions.cxx``

Getting Started
---------------

Start with the resulting files from Exercise 1. Complete ``TODO 7`` through
``TODO 14``.

First create a variable ``USE_MYMATH`` using the :command:`option` command
in ``MathFunctions/CMakeLists.txt``. In that same file, use that option
to pass a compile definition to the ``MathFunctions`` library.

Then, update ``MathFunctions.cxx`` to redirect compilation based on
``USE_MYMATH``.

Lastly, prevent ``mysqrt.cxx`` from being compiled when ``USE_MYMATH`` is on
by making it its own library inside of the ``USE_MYMATH`` block of
``MathFunctions/CMakeLists.txt``.

Build and Run
-------------

Since we have our build directory already configured from Exercise 1, we can
rebuild by simply calling the following:

.. code-block:: console

  cd ../Step2_build
  cmake --build .

Next, run the ``Tutorial`` executable on a few numbers to verify that it's
still correct.

Now let's update the value of ``USE_MYMATH`` to ``OFF``. 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

Now, rebuild the code with the following:

.. code-block:: console

  cmake --build .

Then, run the executable again to ensure that it still works with
``USE_MYMATH`` set to ``OFF``. Which function gives better results, ``sqrt``
or ``mysqrt``?

Solution
--------

The first step is to add an option to ``MathFunctions/CMakeLists.txt``.
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.

.. raw:: html

  <details><summary>TODO 7: Click to show/hide answer</summary>

.. literalinclude:: Step3/MathFunctions/CMakeLists.txt
  :caption: TODO 7: MathFunctions/CMakeLists.txt
  :name: CMakeLists.txt-option-library-level
  :language: cmake
  :start-after: # should we use our own math functions
  :end-before: if (USE_MYMATH)

.. raw:: html

  </details>

Next, make building and linking our library with ``mysqrt`` function
conditional using this new option.

Create an :command:`if` statement which checks the value of
``USE_MYMATH``. Inside the :command:`if` block, put the
:command:`target_compile_definitions` command with the compile
definition ``USE_MYMATH``.

.. raw:: html

  <details><summary>TODO 8: Click to show/hide answer</summary>

.. code-block:: cmake
  :caption: TODO 8: MathFunctions/CMakeLists.txt
  :name: CMakeLists.txt-USE_MYMATH

  if (USE_MYMATH)
    target_compile_definitions(MathFunctions PRIVATE "USE_MYMATH")
  endif()

.. raw:: html

  </details>

When ``USE_MYMATH`` is ``ON``, the compile definition ``USE_MYMATH`` will
be set. We can then use this compile definition to enable or disable
sections of our source code.

The corresponding changes to the source code are fairly straightforward.
In ``MathFunctions.cxx``, we make ``USE_MYMATH`` control which square root
function is used:

.. raw:: html

  <details><summary>TODO 9: Click to show/hide answer</summary>

.. literalinclude:: Step3/MathFunctions/MathFunctions.cxx
  :caption: TODO 9: MathFunctions/MathFunctions.cxx
  :name: MathFunctions-USE_MYMATH-if
  :language: c++
  :start-after: which square root function should we use?
  :end-before: }

.. raw:: html

  </details>

Next, we need to include ``mysqrt.h`` if ``USE_MYMATH`` is defined.

.. raw:: html

  <details><summary>TODO 10: Click to show/hide answer</summary>

.. literalinclude:: Step3/MathFunctions/MathFunctions.cxx
  :caption: TODO 10: MathFunctions/MathFunctions.cxx
  :name: MathFunctions-USE_MYMATH-if-include
  :language: c++
  :start-after: include <cmath>
  :end-before: namespace mathfunctions

.. raw:: html

  </details>

Finally, we need to include ``cmath`` now that we are using ``std::sqrt``.

.. raw:: html

  <details><summary>TODO 11: Click to show/hide answer</summary>

.. code-block:: c++
  :caption: TODO 11 : MathFunctions/MathFunctions.cxx
  :name: tutorial.cxx-include_cmath

  #include <cmath>

.. raw:: html

  </details>

At this point, if ``USE_MYMATH`` is ``OFF``, ``mysqrt.cxx`` would not be used
but it will still be compiled because the ``MathFunctions`` target has
``mysqrt.cxx`` listed under sources.

There are a few ways to fix this. The first option is to use
:command:`target_sources` to add ``mysqrt.cxx`` from within the ``USE_MYMATH``
block. Another option is to create an additional library within the
``USE_MYMATH`` block which is responsible for compiling ``mysqrt.cxx``. For
the sake of this tutorial, we are going to create an additional library.

First, from within ``USE_MYMATH`` create a library called ``SqrtLibrary``
that has sources ``mysqrt.cxx``.

.. raw:: html

  <details><summary>TODO 12: Click to show/hide answer</summary>

.. literalinclude:: Step3/MathFunctions/CMakeLists.txt
  :caption: TODO 12 : MathFunctions/CMakeLists.txt
  :name: MathFunctions/CMakeLists.txt-add_library-SqrtLibrary
  :language: cmake
  :start-after: # library that just does sqrt
  :end-before: # TODO 7: Link

.. raw:: html

  </details>

Next, we link ``SqrtLibrary`` onto ``MathFunctions`` when ``USE_MYMATH`` is
enabled.

.. raw:: html

  <details><summary>TODO 13: Click to show/hide answer</summary>

.. literalinclude:: Step3/MathFunctions/CMakeLists.txt
  :caption: TODO 13 : MathFunctions/CMakeLists.txt
  :name: MathFunctions/CMakeLists.txt-target_link_libraries-SqrtLibrary
  :language: cmake
  :start-after: to tutorial_compiler_flags
  :end-before: endif()

.. raw:: html

  </details>

Finally, we can remove ``mysqrt.cxx`` from our ``MathFunctions`` library
source list because it will be pulled in when ``SqrtLibrary`` is included.

.. raw:: html

  <details><summary>TODO 14: Click to show/hide answer</summary>

.. literalinclude:: Step3/MathFunctions/CMakeLists.txt
  :caption: TODO 14 : MathFunctions/CMakeLists.txt
  :name: MathFunctions/CMakeLists.txt-remove-mysqrt.cxx-MathFunctions
  :language: cmake
  :end-before: # TODO 1:

.. raw:: html

  </details>

With these changes, the ``mysqrt`` function is now completely optional to
whoever is building and using the ``MathFunctions`` library. Users can toggle
``USE_MYMATH`` to manipulate what library is used in the build.
