Flecs is a fast and lightweight Entity Component System for C89 / C99 / C++11 that packs a lot of punch in a small footprint:

• Blazing fast iteration speeds with direct access to raw C arrays across multiple components
• Built-in support for entity hierarchies, prefabs, nested prefabs and prefab variants
• An efficient lock-free architecture allows for modifying entities across multiple threads
• Expressive entity queries with support for and, or, not and optional operators
• Systems that are time triggered, rate triggered, run every frame or run only when needed
• A customizable core that lets you include only the features you need

This is Flecs v2, which is a breaking change from v1. For the last v1 release, see: https://github.com/SanderMertens/flecs/releases/tag/v1.3

What is an Entity Component System?

ECS (Entity Component System) is a design pattern often found in gaming and simulation which produces code that is fast and reusable. Dynamic omposition is a first-class citizen in ECS, and there is a strict separation between data and behavior. A framework is an Entity Component System if it:

• Has entities that are unique identifiers
• Has components that are plain data types
• Has systems which are behavior matched with entities based on their components

Documentation

• Quickstart
• Manual
• Migration guide
• ECS FAQ
• C examples
• C++ examples

Example

This is a simple flecs example in the C++11 API:

struct Position {
    float x;
    float y;
};

struct Speed {
    float value;
};

int main(int argc, char *argv[]) {
    flecs::world world;

    flecs::component<Position>(world, "Position");
    flecs::component<Speed>(world, "Speed");

    flecs::system<Position, Speed>(world)
        .each([](flecs::entity e, Position& p, Speed& s) {
            p.x += s.value * e.delta_time();
            p.y += s.value * e.delta_time();
        });

    flecs::entity(world, "MyEntity")
        .set<Position>({0, 0})
        .set<Speed>({1});

    while (world.progress()) { }
}

Building

The easiest way to add Flecs to a project is to add flecs.c and flecs.h to your source code. These files can be added to both C and C++ projects (the C++ API is embedded in flecs.h). Alternatively you can also build Flecs as a library by using the cmake, meson, bazel or bake buildfiles.

Custom builds

Whether you're looking for a minimal ECS library or a full-fledged system runtime, customizable builds let you remove Flecs features you don't need. By default all features are included. To customize a build, follow these steps:

• define FLECS_CUSTOM_BUILD. This removes all optional features from the build.
• define constants for the features you want to include (see below)
• remove the files of the features you don't need

Features are split up in addons and modules. Addons implement a specific Flecs feature, like snapshots. Modules are like addons but register their own components and systems, and therefore need to be imported.

Addons

Addons are located in the src/addons and include/addons folders. The following addons are available:

Addon	Description	Constant
Bulk	Efficient operations that run on many entities	FLECS_BULK
Dbg	Debug API for inspection of internals	FLECS_DBG
Module	Organize components and systems in modules	FLECS_MODULE
Queue	A queue data structure	FLECS_QUEUE
Reader_writer	Serialize components to series of bytes	FLECS_READER_WRITER
Snapshot	Take a snapshot that can be restored afterwards	FLECS_SNAPSHOT

Builtin modules

Addons are located in the src/modules and include/modules folders. The following modules are available:

Module	Description	Constant
System	Support for systems, monitors and triggers	FLECS_SYSTEMS
Pipeline	Run systems each frame and/or multithreaded	FLECS_PIPELINE
Timer	Run systems at intervals, timeouts or fixed rate	FLECS_TIMER
Stats	Collect statistics on entities and systems	FLECS_STATS

Modules

The following modules are available in flecs-hub and are compatible with v2:

Module	Description
flecs.meta	Reflection for Flecs components
flecs.json	JSON serializer for Flecs components
flecs.rest	A REST interface for introspecting & editing entities
flecs.player	Play, stop and pause simulations
flecs.dash	Web-frontend for remote monitoring and debugging of Flecs apps
flecs.components.geometry	Components that describe geometry
flecs.components.graphics	Components used for computer graphics
flecs.components.gui	Components used to describe GUI components
flecs.components.http	Components describing an HTTP server
flecs.components.input	Components that describe keyboard and mouse input
flecs.components.physics	Components that describe physics and movement
flecs.systems.civetweb	A civetweb-based implementation of flecs.components.http