CMake/Help/guide/tutorial/A Basic Starting Point.rst

Step 1: A Basic Starting Point
==============================

Where do I start with CMake? This step will provide an introduction to some of
CMake's basic syntax, commands, and variables. As these concepts are
introduced, we will work through three exercises and create a simple CMake
project.

Each exercise in this step will start with some background information. Then, a
goal and list of helpful resources are provided. Each file in the
``Files to Edit`` section is in the ``Step1`` directory and contains one or
more ``TODO`` comments. Each ``TODO`` represents a line or two of code to
change or add. The ``TODO`` s are intended to be completed in numerical order,
first complete  ``TODO 1`` then ``TODO 2``, etc. The ``Getting Started``
section will give some helpful hints and guide you through the exercise. Then
the ``Build and Run`` section will walk step-by-step through how to build and
test the exercise. Finally, at the end of each exercise the intended solution
is discussed.

Also note that each step in the tutorial builds on the next. So, for example,
the starting code for ``Step2`` is the complete solution to ``Step1``.

Exercise 1 - Building a Basic Project
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The most basic CMake project is an executable built from a single source code
file. For simple projects like this, a ``CMakeLists.txt`` file with three
commands is all that is required.

**Note:** Although upper, lower and mixed case commands are supported by CMake,
lower case commands are preferred and will be used throughout the tutorial.

Any project's top most CMakeLists.txt must start by specifying a minimum CMake
version using the :command:`cmake_minimum_required` command. This establishes
policy settings and ensures that the following CMake functions are run with a
compatible version of CMake.

To start a project, we use the :command:`project` command to set the project
name. This call is required with every project and should be called soon after
:command:`cmake_minimum_required`. As we will see later, this command can
also be used to specify other project level information such as the language
or version number.

Finally, the :command:`add_executable` command tells CMake to create an
executable using the specified source code files.

Goal
----

Understand how to create a simple CMake project.

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

* :command:`add_executable`
* :command:`cmake_minimum_required`
* :command:`project`

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

* ``CMakeLists.txt``

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

The source code for ``tutorial.cxx`` is provided in the
``Help/guide/tutorial/Step1`` directory and can be used to compute the square
root of a number. This file does not need to be edited in this step.

In the same directory is a ``CMakeLists.txt`` file which you will complete.
Start with ``TODO 1`` and work through ``TODO 3``.

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

Once ``TODO 1`` through ``TODO 3`` have been completed, we are ready to build
and run our project! First, 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.

For example, from the command line we could navigate to the
``Help/guide/tutorial`` directory of the CMake source code tree and create a
build directory:

.. code-block:: console

  mkdir Step1_build

Next, navigate to that build directory and run
:manual:`cmake <cmake(1)>` to configure the project and generate a native build
system:

.. code-block:: console

  cd Step1_build
  cmake ../Step1

Then call that build system to actually compile/link the project:

.. code-block:: console

  cmake --build .

Finally, try to use the newly built ``Tutorial`` with these commands:

.. code-block:: console

  Tutorial 4294967296
  Tutorial 10
  Tutorial

Solution
--------

As mentioned above, a three line ``CMakeLists.txt`` is all that we need to get
up and running. The first line is to use :command:`cmake_minimum_required` to
set the CMake version as follows:

.. raw:: html

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

.. literalinclude:: Step2/CMakeLists.txt
  :caption: TODO 1: CMakeLists.txt
  :name: CMakeLists.txt-cmake_minimum_required
  :language: cmake
  :end-before: # set the project name and version

.. raw:: html

  </details>

The next step to make a basic project is to use the :command:`project`
command as follows to set the project name:

.. raw:: html

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

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

  project(Tutorial)

.. raw:: html

  </details>

The last command to call for a basic project is
:command:`add_executable`. We call it as follows:

.. raw:: html

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

.. literalinclude:: Step2/CMakeLists.txt
  :caption: TODO 3: CMakeLists.txt
  :name: CMakeLists.txt-add_executable
  :language: cmake
  :start-after: # add the executable
  :end-before: # TODO 9:

.. raw:: html

  </details>

Exercise 2 - Specifying the C++ Standard
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

CMake has some special variables that are either created behind the scenes or
have meaning to CMake when set by project code. Many of these variables start
with ``CMAKE_``. Avoid this naming convention when creating variables for your
projects. Two of these special user settable variables are
:variable:`CMAKE_CXX_STANDARD` and :variable:`CMAKE_CXX_STANDARD_REQUIRED`.
These may be used together to specify the C++ standard needed to build the
project.

Goal
----

Add a feature that requires C++11.

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

* :variable:`CMAKE_CXX_STANDARD`
* :variable:`CMAKE_CXX_STANDARD_REQUIRED`
* :command:`set`

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

* ``CMakeLists.txt``
* ``tutorial.cxx``

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

Continue editing files in the ``Step1`` directory. Start with ``TODO 4`` and
complete through ``TODO 6``.

First, edit ``tutorial.cxx`` by adding a feature that requires C++11. Then
update ``CMakeLists.txt`` to require C++11.

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

Let's build our project again. Since we already created a build directory and
ran CMake for Exercise 1, we can skip to the build step:

.. code-block:: console

  cd Step1_build
  cmake --build .

Now we can try to use the newly built ``Tutorial`` with same commands as
before:

.. code-block:: console

  Tutorial 4294967296
  Tutorial 10
  Tutorial

Solution
--------

We start by adding some C++11 features to our project by replacing
``atof`` with ``std::stod`` in ``tutorial.cxx``. This looks like
the following:

.. raw:: html

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

.. literalinclude:: Step2/tutorial.cxx
  :caption: TODO 4: tutorial.cxx
  :name: tutorial.cxx-cxx11
  :language: c++
  :start-after: // convert input to double
  :end-before: // TODO 12:

.. raw:: html

  </details>

To complete ``TODO 5``, simply remove ``#include <cstdlib>``.

We will need to explicitly state in the CMake code that it should use the
correct flags. One way to enable support for a specific C++ standard in CMake
is by using the :variable:`CMAKE_CXX_STANDARD` variable. For this tutorial, set
the :variable:`CMAKE_CXX_STANDARD` variable in the ``CMakeLists.txt`` file to
``11`` and :variable:`CMAKE_CXX_STANDARD_REQUIRED` to ``True``. Make sure to
add the :variable:`CMAKE_CXX_STANDARD` declarations above the call to
:command:`add_executable`.

.. raw:: html

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

.. literalinclude:: Step2/CMakeLists.txt
  :caption: TODO 6: CMakeLists.txt
  :name: CMakeLists.txt-CXX_STANDARD
  :language: cmake
  :start-after: # specify the C++ standard
  :end-before: # TODO 7:

.. raw:: html

  </details>

Exercise 3 - Adding a Version Number and Configured Header File
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Sometimes it may be useful to have a variable that is defined in your
``CMakelists.txt`` file also be available in your source code. In this case, we
would like to print the project version.

One way to accomplish this is by using a configured header file. We create an
input file with one or more variables to replace. These variables have special
syntax which looks like ``@VAR@``.
Then, we use the :command:`configure_file` command to copy the input file to a
given output file and replace these variables with the current value of ``VAR``
in the ``CMakelists.txt`` file.

While we could edit the version directly in the source code, using this
feature is preferred since it creates a single source of truth and avoids
duplication.

Goal
----

Define and report the project's version number.

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

* :variable:`<PROJECT-NAME>_VERSION_MAJOR`
* :variable:`<PROJECT-NAME>_VERSION_MINOR`
* :command:`configure_file`
* :command:`target_include_directories`

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

* ``CMakeLists.txt``
* ``tutorial.cxx``

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

Continue to edit files from ``Step1``. Start on ``TODO 7`` and complete through
``TODO 12``. In this exercise, we start by adding a project version number in
``CMakeLists.txt``. In that same file, use :command:`configure_file` to copy a
given input file to an output file and substitute some variable values in the
input file content.

Next, create an input header file ``TutorialConfig.h.in`` defining version
numbers which will accept variables passed from :command:`configure_file`.

Finally, update ``tutorial.cxx`` to print out its version number.

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

Let's build our project again. As before, we already created a build directory
and ran CMake so we can skip to the build step:

.. code-block:: console

  cd Step1_build
  cmake --build .

Verify that the version number is now reported when running the executable
without any arguments.

Solution
--------

In this exercise, we improve our executable by printing a version number.
While we could do this exclusively in the source code, using ``CMakeLists.txt``
lets us maintain a single source of data for the version number.

First, we modify the ``CMakeLists.txt`` file to use the
:command:`project` command to set both the project name and version number.
When the :command:`project` command is called, CMake defines
``Tutorial_VERSION_MAJOR`` and ``Tutorial_VERSION_MINOR`` behind the scenes.

.. raw:: html

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

.. literalinclude:: Step2/CMakeLists.txt
  :caption: TODO 7: CMakeLists.txt
  :name: CMakeLists.txt-project-VERSION
  :language: cmake
  :start-after: # set the project name and version
  :end-before: # specify the C++ standard

.. raw:: html

  </details>

Then we used :command:`configure_file` to copy the input file with the
specified CMake variables replaced:

.. raw:: html

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

.. literalinclude:: Step2/CMakeLists.txt
  :caption: TODO 8: CMakeLists.txt
  :name: CMakeLists.txt-configure_file
  :language: cmake
  :start-after: # to the source code
  :end-before: # TODO 8:

.. raw:: html

  </details>

Since the configured file will be written into the project binary
directory, we must add that directory to the list of paths to search for
include files.

**Note:** Throughout this tutorial, we will refer to the project build and
the project binary directory interchangeably. These are the same and are not
meant to refer to a `bin/` directory.

We used :command:`target_include_directories` to specify
where the executable target should look for include files.

.. raw:: html

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

.. literalinclude:: Step2/CMakeLists.txt
  :caption: TODO 9: CMakeLists.txt
  :name: CMakeLists.txt-target_include_directories
  :language: cmake
  :start-after: # so that we will find TutorialConfig.h

.. raw:: html

  </details>

``TutorialConfig.h.in`` is the input header file to be configured.
When :command:`configure_file` is called from our ``CMakeLists.txt``, the
values for ``@Tutorial_VERSION_MAJOR@`` and ``@Tutorial_VERSION_MINOR@`` will
be replaced with the corresponding version numbers from the project in
``TutorialConfig.h``.

.. raw:: html

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

.. literalinclude:: Step2/TutorialConfig.h.in
  :caption: TODO 10: TutorialConfig.h.in
  :name: TutorialConfig.h.in
  :language: c++
  :end-before: // TODO 13:

.. raw:: html

  </details>

Next, we need to modify ``tutorial.cxx`` to include the configured header file,
``TutorialConfig.h``.

.. raw:: html

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

.. code-block:: c++
  :caption: TODO 11: tutorial.cxx

  #include "TutorialConfig.h"

.. raw:: html

  </details>

Finally, we print out the executable name and version number by updating
``tutorial.cxx`` as follows:

.. raw:: html

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

.. literalinclude:: Step2/tutorial.cxx
  :caption: TODO 12 : tutorial.cxx
  :name: tutorial.cxx-print-version
  :language: c++
  :start-after: {
  :end-before: // convert input to double

.. raw:: html

  </details>