godot-docs/development/cpp/custom_modules_in_cpp.rst

.. _doc_custom_modules_in_c++:

Custom modules in C++
=====================

Modules
-------

Godot allows extending the engine in a modular way. New modules can be
created and then enabled/disabled. This allows for adding new engine
functionality at every level without modifying the core, which can be
split for use and reuse in different modules.

Modules are located in the ``modules/`` subdirectory of the build system.
By default, many different modules exist, such as GDScript (which, yes,
is not part of the base engine), the Mono runtime, a regular expressions
module, and others. As many new modules as desired can be
created and combined, and the SCons build system will take care of it
transparently.

What for?
---------

While it's recommended that most of a game be written in scripting (as
it is an enormous time saver), it's perfectly possible to use C++
instead. Adding C++ modules can be useful in the following scenarios:

-  Binding an external library to Godot (like PhysX, FMOD, etc).
-  Optimize critical parts of a game.
-  Adding new functionality to the engine and/or editor.
-  Porting an existing game.
-  Write a whole, new game in C++ because you can't live without C++.

Creating a new module
---------------------

Before creating a module, make sure to download the source code of Godot
and manage to compile it. There are tutorials in the documentation for this.

To create a new module, the first step is creating a directory inside
``modules/``. If you want to maintain the module separately, you can checkout
a different VCS into modules and use it.

The example module will be called "summator", and is placed inside the
Godot source tree (``C:\godot`` refers to wherever the Godot sources are
located):

::

    C:\godot> cd modules
    C:\godot\modules> mkdir summator
    C:\godot\modules> cd summator
    C:\godot\modules\summator>

Inside we will create a simple summator class:

.. code:: cpp

    /* summator.h */

    #ifndef SUMMATOR_H
    #define SUMMATOR_H

    #include "core/reference.h"

    class Summator : public Reference {
        GDCLASS(Summator, Reference);

        int count;

    protected:
        static void _bind_methods();

    public:
        void add(int p_value);
        void reset();
        int get_total() const;

        Summator();
    };

    #endif // SUMMATOR_H

And then the cpp file.

.. code:: cpp

    /* summator.cpp */

    #include "summator.h"

    void Summator::add(int p_value) {
        count += p_value;
    }

    void Summator::reset() {
        count = 0;
    }

    int Summator::get_total() const {
        return count;
    }

    void Summator::_bind_methods() {
        ClassDB::bind_method(D_METHOD("add", "value"), &Summator::add);
        ClassDB::bind_method(D_METHOD("reset"), &Summator::reset);
        ClassDB::bind_method(D_METHOD("get_total"), &Summator::get_total);
    }

    Summator::Summator() {
        count = 0;
    }

Then, the new class needs to be registered somehow, so two more files
need to be created:

::

    register_types.h
    register_types.cpp

With the following contents:

.. code:: cpp

    /* register_types.h */

    void register_summator_types();
    void unregister_summator_types();
    /* yes, the word in the middle must be the same as the module folder name */

.. code:: cpp

    /* register_types.cpp */

    #include "register_types.h"

    #include "core/class_db.h"
    #include "summator.h"

    void register_summator_types() {
        ClassDB::register_class<Summator>();
    }

    void unregister_summator_types() {
       // Nothing to do here in this example.
    }

Next, we need to create a ``SCsub`` file so the build system compiles
this module:

.. code:: python

    # SCsub

    Import('env')

    env.add_source_files(env.modules_sources, "*.cpp") # Add all cpp files to the build

With multiple sources, you can also add each file individually to a Python
string list:

.. code:: python

    src_list = ["summator.cpp", "other.cpp", "etc.cpp"]
    env.add_source_files(env.modules_sources, src_list)

This allows for powerful possibilities using Python to construct the file list
using loops and logic statements. Look at some of the other modules that ship
with Godot by default for examples.

To add include directories for the compiler to look at you can append it to the
environment's paths:

.. code:: python

    env.Append(CPPPATH=["mylib/include"]) # this is a relative path
    env.Append(CPPPATH=["#myotherlib/include"]) # this is an 'absolute' path

If you want to add custom compiler flags when building your module, you need to clone
`env` first, so it won't add those flags to whole Godot build (which can cause errors).
Example `SCsub` with custom flags:

.. code:: python

    # SCsub

    Import('env')

    module_env = env.Clone()
    module_env.add_source_files(env.modules_sources, "*.cpp")
    module_env.Append(CCFLAGS=['-O2']) # Flags for C and C++ code
    module_env.Append(CXXFLAGS=['-std=c++11']) # Flags for C++ code only

And finally, the configuration file for the module, this is a simple
python script that must be named ``config.py``:

.. code:: python

    # config.py

    def can_build(env, platform):
        return True

    def configure(env):
        pass

The module is asked if it's OK to build for the specific platform (in
this case, ``True`` means it will build for every platform).

And that's it. Hope it was not too complex! Your module should look like
this:

::

    godot/modules/summator/config.py
    godot/modules/summator/summator.h
    godot/modules/summator/summator.cpp
    godot/modules/summator/register_types.h
    godot/modules/summator/register_types.cpp
    godot/modules/summator/SCsub

You can then zip it and share the module with everyone else. When
building for every platform (instructions in the previous sections),
your module will be included.

Using the module
----------------

You can now use your newly created module from any script:

::

    var s = Summator.new()
    s.add(10)
    s.add(20)
    s.add(30)
    print(s.get_total())
    s.reset()

And the output will be ``60``.

.. seealso:: The previous Summator example is great for small, custom modules,
  but what if you want to use a larger, external library?  Refer to
  :ref:`doc_binding_to_external_libraries` for details about binding to
  external libraries.

Improving the build system for development
------------------------------------------

So far we defined a clean and simple SCsub that allows us to add the sources
of our new module as part of the Godot binary.

This static approach is fine when we want to build a release version of our
game given we want all the modules in a single binary.

However the trade-off is every single change means a full recompilation of the
game. Even if SCons is able to detect and recompile only the file that have
changed, finding such files and eventually linking the final binary is a
long and costly part.

The solution to avoid such a cost is to build our own module as a shared
library that will be dynamically loaded when starting our game's binary.

.. code:: python

    # SCsub

    Import('env')

    sources = [
        "register_types.cpp",
        "summator.cpp"
    ]

    # First, create a custom env for the shared library.
    module_env = env.Clone()
    module_env.Append(CCFLAGS=['-fPIC'])  # Needed to compile shared library
    # We don't want godot's dependencies to be injected into our shared library.
    module_env['LIBS'] = []

    # Now define the shared library. Note that by default it would be built
    # into the module's folder, however it's better to output it into `bin`
    # next to the Godot binary.
    shared_lib = module_env.SharedLibrary(target='#bin/summator', source=sources)

    # Finally notify the main env it has our shared lirary as a new dependency.
    # To do so, SCons wants the name of the lib with it custom suffixes
    # (e.g. ".x11.tools.64") but without the final ".so".
    # We pass this along with the directory of our library to the main env.
    shared_lib_shim = shared_lib[0].name.rsplit('.', 1)[0]
    env.Append(LIBS=[shared_lib_shim])
    env.Append(LIBPATH=['#bin'])

Once compiled, we should end up with a ``bin`` directory containing both the
``godot*`` binary and our ``libsummator*.so``. However given the .so is not in
a standard directory (like ``/usr/lib``), we have to help our binary find it
during runtime with the ``LD_LIBRARY_PATH`` environ variable:

::

    user@host:~/godot$ export LD_LIBRARY_PATH=`pwd`/bin/
    user@host:~/godot$ ./bin/godot*

**note**: Pay attention you have to ``export`` the environ variable otherwise
you won't be able to play your project from within the editor.

On top of that, it would be nice to be able to select whether to compile our
module as shared library (for development) or as a part of the Godot binary
(for release). To do that we can define a custom flag to be passed to SCons
using the `ARGUMENT` command:

.. code:: python

    # SCsub

    Import('env')

    sources = [
        "register_types.cpp",
        "summator.cpp"
    ]

    module_env = env.Clone()
    module_env.Append(CCFLAGS=['-O2'])
    module_env.Append(CXXFLAGS=['-std=c++11'])

    if ARGUMENTS.get('summator_shared', 'no') == 'yes':
        # Shared lib compilation
        module_env.Append(CCFLAGS=['-fPIC'])
        module_env['LIBS'] = []
        shared_lib = module_env.SharedLibrary(target='#bin/summator', source=sources)
        shared_lib_shim = shared_lib[0].name.rsplit('.', 1)[0]
        env.Append(LIBS=[shared_lib_shim])
        env.Append(LIBPATH=['#bin'])
    else:
        # Static compilation
        module_env.add_source_files(env.modules_sources, sources)

Now by default ``scons`` command will build our module as part of Godot's binary
and as a shared library when passing ``summator_shared=yes``.

Finally you can even speedup build further by explicitly specifying your
shared module as target in the scons command:

::

    user@host:~/godot$ scons summator_shared=yes platform=x11 bin/libsummator.x11.tools.64.so

Writing custom documentation
----------------------------

Writing documentation may seem like a boring task, but it is highly recommended
to document your newly created module in order to make it easier for users to
benefit from it. Not to mention that the code you've written one year ago may
become indistinguishable from the code that was written by someone else, so be
kind to your future self!

There are several steps in order to setup custom docs for the module:

1. Make a new directory in the root of the module. The directory name can be
   anything, but we'll be using the ``doc_classes`` name throughout this section.

2. Append the following code snippet to ``config.py``:

   .. code:: python

       def get_doc_classes():
           return [
               "ClassName",
           ]

       def get_doc_path():
           return "doc_classes"

The ``get_doc_classes()`` method is necessary for the build system to
know which documentation classes of the module must be merged, since the module
may contain several classes. Replace ``ClassName`` with the name of the class
you want to write documentation for. If you need docs for more than one class,
append those as well.

The ``get_doc_path()`` method is used by the build system to determine
the location of the docs. In our case, they will be located in the ``doc_classes``
directory.

3. Run command:

   ::

      godot --doctool <path>

This will dump the engine API reference to the given ``<path>`` in XML format.
Notice that you'll need to configure your ``PATH`` to locate Godot's executable,
and make sure that you have write access rights. If not, you might encounter an
error similar to the following:

.. code-block:: console

    ERROR: Can't write doc file: docs/doc/classes/@GDScript.xml
       At: editor/doc/doc_data.cpp:956

4. Get generated doc file from ``godot/doc/classes/ClassName.xml``

5. Copy this file to ``doc_classes``, optionally edit it, then compile the engine.

The build system will fetch the documentation files from the ``doc_classes`` directory
and merge them with the base types. Once the compilation process is finished,
the docs will become accessible within the engine's built-in documentation system.

In order to keep documentation up-to-date, all you'll have to do is simply modify
one of the ``ClassName.xml`` files and recompile the engine from now on.

Summing up
----------

Remember to:

-  use ``GDCLASS`` macro for inheritance, so Godot can wrap it
-  use ``_bind_methods`` to bind your functions to scripting, and to
   allow them to work as callbacks for signals.

But this is not all, depending what you do, you will be greeted with
some (hopefully positive) surprises.

-  If you inherit from :ref:`class_Node` (or any derived node type, such as
   Sprite), your new class will appear in the editor, in the inheritance
   tree in the "Add Node" dialog.
-  If you inherit from :ref:`class_Resource`, it will appear in the resource
   list, and all the exposed properties can be serialized when
   saved/loaded.
-  By this same logic, you can extend the Editor and almost any area of
   the engine.