.. _packaging_getting_started:

Getting started
===============

To start learning about creating packages, we will create a package from an existing source code
repository: https://github.com/memsharded/hello. You can check that project, it is a very simple
"hello world" C++ library, using CMake as build system to build a library and an executable. It has
nothing related to conan in it.

We are using such github repository as an example, but the same process would apply to other source
code origins, like downloading a zip or tarball from the internet.

.. note::

    For this concrete example you will need, besides a C++ compiler, both *CMake* and *git*
    installed and in your path. They are not required by conan, you could use your own build system
    and version control instead.

Creating the package recipe
---------------------------

First, let's create a folder for our package recipe, and use the :command:`conan new` helper command that
will create a working package recipe for us:

.. code-block:: bash

    $ mkdir mypkg && cd mypkg
    $ conan new Hello/0.1 -t

This will generate the following files:

.. code-block:: text

    conanfile.py
    test_package
      conanfile.py
      CMakeLists.txt
      example.cpp

At the root level, there is a *conanfile.py* which is the main recipe file, the one actually
defining our package. Also there is a *test_package* folder, which contains a simple example
consuming project that will require and link with the created package. It is useful to make sure
that our package is correctly created.

Let's have a look to the root package recipe *conanfile.py*:

.. code-block:: python

    from conans import ConanFile, CMake, tools

    class HelloConan(ConanFile):
        name = "Hello"
        version = "0.1"
        settings = "os", "compiler", "build_type", "arch"
        options = {"shared": [True, False]}
        default_options = "shared=False"
        generators = "cmake"

        def source(self):
            self.run("git clone https://github.com/memsharded/hello.git")
            self.run("cd hello && git checkout static_shared")
            # This small hack might be useful to guarantee proper /MT /MD linkage in MSVC
            # if the packaged project doesn't have variables to set it properly
            tools.replace_in_file("hello/CMakeLists.txt", "PROJECT(MyHello)", '''PROJECT(MyHello)
    include(${CMAKE_BINARY_DIR}/conanbuildinfo.cmake)
    conan_basic_setup()''')

        def build(self):
            cmake = CMake(self)
            cmake.configure(source_folder="hello")
            cmake.build()

            # Explicit way:
            # self.run('cmake "%s/hello" %s' % (self.source_folder, cmake.command_line))
            # self.run("cmake --build . %s" % cmake.build_config)

        def package(self):
            self.copy("*.h", dst="include", src="hello")
            self.copy("*hello.lib", dst="lib", keep_path=False)
            self.copy("*.dll", dst="bin", keep_path=False)
            self.copy("*.so", dst="lib", keep_path=False)
            self.copy("*.dylib", dst="lib", keep_path=False)
            self.copy("*.a", dst="lib", keep_path=False)

        def package_info(self):
            self.cpp_info.libs = ["hello"]

This is a complete package recipe. Without worrying too much about every detail, these are the
basics:

- The ``settings`` field defines the configuration that defines the different binary packages. In
  this example we are defining that any change to the OS, compiler, architecture or build type will
  generate a different binary package. Remember, Conan generates different binary packages for
  different introduced configuration (in this case settings) for the same recipe.

  Note that the platform where the recipe is running and the package is being build can be different from
  the final platform where the code will be running (``self.settings.os`` and ``self.settings.arch``) if
  the package is being cross-built. So if you want to apply a different build depending on the current
  build machine, you need to check it:

  .. code-block:: python

         def build(self):
             if platform.system() == "Windows":
                 cmake = CMake(self)
                 cmake.configure(source_folder="hello")
                 cmake.build()
             else:
                 env_build = AutoToolsBuildEnvironment(self)
                 env_build.configure()
                 env_build.make()

  Learn more in the :ref:`Cross building <cross_building>` section.

- This package recipe is also able to create different binary packages for static and shared
  libraries with the ``shared`` option, which is defaulted to ``False`` (i.e. by default it will use
  static linkage).

- The ``source()`` method executes a :command:`git clone` to retrieve the sources from github. Other
  origins, as downloading a zip file are also available. As you can see, any manipulation of the
  code can be done, as checking out any branch or tag, or patching the source code. In this example,
  we are adding two lines to the existing CMake code, to ensure binary compatibility. Don't worry
  too much about it now, we'll visit it later.

- The ``build()`` first configures the project, then builds it, with standard CMake commands. The
  ``CMake`` object is just a helper to ease the translation of conan settings to CMake command line
  arguments. Also remember that **CMake is not strictly required**. You can build packages directly
  invoking **make**, **MSBuild**, **SCons** or any other build system.

  .. seealso:: Check the :ref:`existing build helpers <build_helpers>`.

- The ``package()`` method copies artifacts (headers, libs) from the build folder to the final
  package folder. 

- Finally, the ``package_info()`` method defines that consumer must link with the "hello" library
  when using this package. Other information as include or lib paths can be defined as well. This
  information is used for files created by generators to be used by consumers, as
  *conanbuildinfo.cmake*.

The test_package folder
-----------------------

.. note::

    The **test_package** is different from the library unit or integration tests, which should be
    more comprehensive. These tests are "package" tests, and validate that the package is properly
    created, and that package consumers will be able to link against it and reuse it.

If you have a look to the ``test_package`` folder, you will realize that the ``example.cpp`` and the
``CMakeLists.txt`` files don't have anything special. The *test_package/conanfile.py* file is just
another recipe, you can think of it as the consumer *conanfile.txt* we have already seen in
previous sections:

.. code-block:: python

    from conans import ConanFile, CMake
    import os

    class HelloTestConan(ConanFile):
        settings = "os", "compiler", "build_type", "arch"
        generators = "cmake"

        def build(self):
            cmake = CMake(self)
            cmake.configure()
            cmake.build()

        def imports(self):
            self.copy("*.dll", dst="bin", src="bin")
            self.copy("*.dylib*", dst="bin", src="lib")

        def test(self):
            os.chdir("bin")
            self.run(".%sexample" % os.sep)

The main differences with the above *conanfile.py* are:

- It doesn't have a name and version, because we are not creating a package, so they are not
  necessary.
- The ``package()`` and ``package_info()`` methods are not required, since we are not creating a
  package.
- The ``test()`` method specifies which binaries have to be run.
- The ``imports()`` method is defined to copy shared libraries to the ``bin`` folder, so when
  dynamic linkage is used, and the ``test()`` method launches the ``example`` executable, they are
  found and ``example`` runs.

.. note::

    An important difference with respect to normal package recipes, is that this one does not need
    to declare a ``requires`` attribute, to depend on the ``Hello/0.1@demo/testing`` package we are
    testing. This ``requires`` will be automatically injected by conan while running. You can
    however declare it explicitely, it will work, but you will have to remember to bump the version,
    and possibly the user and channel if you change them.

.. _creating_and_testing_packages:

Creating and testing packages
-----------------------------

We can create and test the package with our default settings simply by:

.. code-block:: bash

    $ conan create . demo/testing
    ...
    Hello world!

If you see "Hello world!", it worked.

This will perform the following steps:

- Copy ("export" in conan terms) the *conanfile.py* from the user folder into the **local cache**.
- Install the package, forcing building it from sources.
- Move to the *test_package* folder, and create a temporary *build* folder.
- Execute there a :command:`conan install ..`, so it installs the requirements of the
  *test_package/conanfile.py*. Note that it will build "Hello" from sources.
- Build and launch the *example* consuming application, calling the *test_package/conanfile.py*
  ``build()`` and ``test()`` methods respectively.

Using conan commands, the :command:`conan create` command would be equivalent to:

.. code-block:: bash

    $ conan export . demo/testing
    $ conan install Hello/0.1@demo/testing --build=Hello
    # package is created now, use test to test it
    $ conan test test_package Hello/0.1@demo/testing

The :command:`conan create` command receives the same command line parameters as :command:`conan install` so
you can pass to it the same settings, options, and command line switches. If you want to create and
test packages for different configurations, you could:

.. code-block:: bash

    $ conan create . demo/testing -s build_type=Debug
    $ conan create . demo/testing -o Hello:shared=True -s arch=x86
    $ conan create . demo/testing -pr my_gcc49_debug_profile
    ...
    $ conan create ...


.. _settings_vs_options:

Settings vs. options
--------------------

We have used settings as ``os``, ``arch`` and ``compiler``. But the above package recipe also contains a
``shared`` option (defined as ``options = {"shared": [True, False]}``). What is the difference between
settings and options?

**Settings** are project-wide configuration, something that typically affect to the whole project that
is being built. For example the Operating System or the architecture would be naturally the same for all
packages in a dependency graph, linking a Linux library for a Windows app, or
mixing architectures is impossible.

Settings cannot be defaulted in a package recipe. A recipe for a given library cannot say that its default
``os=Windows``. The ``os`` will be given by the environment in which that recipe is processed. It is
a necessary input.

Settings are configurable. You can edit, add, remove settings or subsettings in your *settings.yml* file.
See :ref:`the settings.yml reference <settings_yml>`.

On the other hand, **options** are package-specific configuration. Being a static or shared library is not
something that applies to all packages. Some can be header only libraries. Other packages can be just data,
or package executables. Or packages can contain a mixture of different artifacts. ``shared`` is a common
option, but packages can define and use any options they want.

Options are defined in the package recipe, including their allowed values, and it can be defaulted by the package 
recipe itself. A package for a library can well define that by default it will be a static library (a typical default).
If no one else specifies something different, the package will be static.

There are some exceptions to the above, for example, settings can be defined per-package, like in command line:

.. code-block:: bash

    $ conan install . -s MyPkg:compiler=gcc -s compiler=clang ..

This will use ``gcc`` for MyPkg and ``clang`` for the rest of the dependencies (extremely unusual case)

Or you can have a very widely used option in many packages and set its value all at once with patterns, like:

.. code-block:: bash

    $ conan install . -o *:shared=True

Any doubts? Please check out our :ref:`FAQ section <faq>` or |write_us|.

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