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, dozens of modules are enabled, 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. 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 to Godot.

  • 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 compile it.

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" (godot/modules/summator). Inside we will create a summator class:

/* summator.h */

#ifndef SUMMATOR_H
#define SUMMATOR_H

#include "core/object/ref_counted.h"

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

    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.

/* 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

Important

These files must be in the top-level folder of your module (next to your SCsub and config.py files) for the module to be registered properly.

These files should contain the following:

/* register_types.h */

#include "modules/register_module_types.h"

void initialize_summator_module(ModuleInitializationLevel p_level);
void uninitialize_summator_module(ModuleInitializationLevel p_level);
/* yes, the word in the middle must be the same as the module folder name */
/* register_types.cpp */

#include "register_types.h"

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

void initialize_summator_module(ModuleInitializationLevel p_level) {
    if (p_level != MODULE_INITIALIZATION_LEVEL_SCENE) {
            return;
    }
    ClassDB::register_class<Summator>();
}

void uninitialize_summator_module(ModuleInitializationLevel p_level) {
    if (p_level != MODULE_INITIALIZATION_LEVEL_SCENE) {
            return;
    }
   // Nothing to do here in this example.
}

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

# 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:

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 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:

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:

# SCsub

Import('env')

module_env = env.Clone()
module_env.add_source_files(env.modules_sources, "*.cpp")
# Append CCFLAGS flags for both C and C++ code.
module_env.Append(CCFLAGS=['-O2'])
# If you need to, you can:
# - Append CFLAGS for C code only.
# - Append CXXFLAGS for C++ code only.

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

# 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.

Note

There is a parameter limit of 5 in C++ modules for things such as subclasses. This can be raised to 13 by including the header file core/method_bind_ext.gen.inc.

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()

The output will be 60.

See also

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

Warning

If your module is meant to be accessed from the running project (not just from the editor), you must also recompile every export template you plan to use, then specify the path to the custom template in each export preset. Otherwise, you'll get errors when running the project as the module isn't compiled in the export template. See the Compiling pages for more information.

Compiling a module externally

Compiling a module involves moving the module's sources directly under the engine's modules/ directory. While this is the most straightforward way to compile a module, there are a couple of reasons as to why this might not be a practical thing to do:

  1. Having to manually copy modules sources every time you want to compile the engine with or without the module, or taking additional steps needed to manually disable a module during compilation with a build option similar to module_summator_enabled=no. Creating symbolic links may also be a solution, but you may additionally need to overcome OS restrictions like needing the symbolic link privilege if doing this via script.

  2. Depending on whether you have to work with the engine's source code, the module files added directly to modules/ changes the working tree to the point where using a VCS (like git) proves to be cumbersome as you need to make sure that only the engine-related code is committed by filtering changes.

So if you feel like the independent structure of custom modules is needed, lets take our "summator" module and move it to the engine's parent directory:

mkdir ../modules
mv modules/summator ../modules

Compile the engine with our module by providing custom_modules build option which accepts a comma-separated list of directory paths containing custom C++ modules, similar to the following:

scons custom_modules=../modules

The build system shall detect all modules under the ../modules directory and compile them accordingly, including our "summator" module.

Warning

Any path passed to custom_modules will be converted to an absolute path internally as a way to distinguish between custom and built-in modules. It means that things like generating module documentation may rely on a specific path structure on your machine.

Customizing module types initialization

Modules can interact with other built-in engine classes during runtime and even affect the way core types are initialized. So far, we've been using register_summator_types as a way to bring in module classes to be available within the engine.

A crude order of the engine setup can be summarized as a list of the following type registration methods:

preregister_module_types();
preregister_server_types();
register_core_singletons();
register_server_types();
register_scene_types();
EditorNode::register_editor_types();
register_platform_apis();
register_module_types();
initialize_physics();
initialize_navigation_server();
register_server_singletons();
register_driver_types();
ScriptServer::init_languages();

Our Summator class is initialized during the register_module_types() call. Imagine that we need to satisfy some common module run-time dependency (like singletons), or allow us to override existing engine method callbacks before they can be assigned by the engine itself. In that case, we want to ensure that our module classes are registered before any other built-in type.

This is where we can define an optional preregister_summator_types() method which will be called before anything else during the preregister_module_types() engine setup stage.

We now need to add this method to register_types header and source files:

/* register_types.h */

#define MODULE_SUMMATOR_HAS_PREREGISTER
void preregister_summator_types();

void register_summator_types();
void unregister_summator_types();

Note

Unlike other register methods, we have to explicitly define MODULE_SUMMATOR_HAS_PREREGISTER to let the build system know what relevant method calls to include at compile time. The module's name has to be converted to uppercase as well.

/* register_types.cpp */

#include "register_types.h"

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

void preregister_summator_types() {
    // Called before any other core types are registered.
    // Nothing to do here in this example.
}

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

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

Improving the build system for development

Warning

This shared library support is not designed to support distributing a module to other users without recompiling the engine. For that purpose, use a GDExtension instead.

So far, we defined a clean 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 that every single change requires a full recompilation of the game. Even though SCons is able to detect and recompile only the file that was changed, finding such files and eventually linking the final binary takes a long time.

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.

# SCsub

Import('env')

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

# First, create a custom env for the shared library.
module_env = env.Clone()

# Position-independent code is required for a shared library.
module_env.Append(CCFLAGS=['-fPIC'])

# Don't inject Godot's dependencies into our shared library.
module_env['LIBS'] = []

# Define the shared library. By default, it would be built in 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 build environment it now has our shared library
# as a new dependency.

# LIBPATH and LIBS need to be set on the real "env" (not the clone)
# to link the specified libraries to the Godot executable.

env.Append(LIBPATH=['#bin'])

# SCons wants the name of the library with it custom suffixes
# (e.g. ".linuxbsd.tools.64") but without the final ".so".
shared_lib_shim = shared_lib[0].name.rsplit('.', 1)[0]
env.Append(LIBS=[shared_lib_shim])

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 environment variable:

export LD_LIBRARY_PATH="$PWD/bin/"
./bin/godot*

Note

You have to export the environment variable. Otherwise, you won't be able to run your project from 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:

# SCsub

Import('env')

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

module_env = env.Clone()
module_env.Append(CCFLAGS=['-O2'])

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('.'