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 is 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 Bullet, 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 "reference.h"

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

        int count;

    protected:
        static void _bind_methods();

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

        Summator();
    };

    #endif

And then the cpp file.

.. code:: cpp

    /* summator.cpp */

    #include "summator.h"

    void Summator::add(int value) {

        count += 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 "class_db.h"
    #include "summator.h"

    void register_summator_types() {

            ClassDB::register_class<Summator>();
    }

    void unregister_summator_types() {
       //nothing to do here
    }

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") # just 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 contruct 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(CXXFLAGS=['-O2', '-std=c++11'])

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(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
----------------

Using your newly created module is very easy, from any script you can
now do:

::

    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``.

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-of 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(CXXFLAGS='-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 you 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(CXXFLAGS=['-O2', '-std=c++11'])

    if ARGUMENTS.get('summator_shared', 'no') == 'yes':
        # Shared lib compilation
        module_env.Append(CXXFLAGS='-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 bin/summator.x11.tools.64.so


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

As you see, it's really easy to develop Godot in C++. Just write your
stuff normally and remember to:

-  use ``OBJ_TYPE`` 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 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.