Crate gecs

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A generated entity component system 🦎

The gecs crate provides a compile-time generated, zero-overhead ECS for simulations on a budget. Unlike other ECS libraries, gecs takes a full ECS world structure definition from code and precompiles all queries to achieve better performance with no upfront cost or caching overhead. Queries in gecs can be inspected and checked at compile-time in order to catch what would otherwise be bugs presenting only in tests or execution. However, this comes at the cost of requiring all archetypes to be known and declared at compile-time, so that adding or removing components from entities at runtime isn’t currently possible – hybrid approaches could solve this in the future.

Archetypes in gecs can be set to contain a fixed or dynamic capacity of entities. If all of the archetypes in your ECS world declaration are set to a fixed capacity, gecs will perform zero allocations after startup. This guarantees that your ECS world will adhere to a known and predictable memory overhead for constrained environments (e.g. servers on cloud instances). Attempting to add an entity to a full archetype can either report failure or panic depending on the method you call to do so.

The goals for gecs are (in descending priority order):

  • Fast iteration and find queries
  • Fast entity creation and destruction
  • Low, predictable memory overhead
  • A user-friendly library interface
  • Simplicity and focus in features

All of the code that gecs generates in user crates is safe, and users of gecs can use #[deny(unsafe_code)] in their own crates. Note that gecs does use unsafe code internally to allow for compiler optimizations around known invariants. It is not a goal of this library to be written entirely in safe Rust.

Getting Started

See the ecs_world!, ecs_find!, and ecs_iter! macros for more information. The following example creates a world with three components and two archetypes:

use gecs::prelude::*;

// Components -- these must be pub because the world is exported as pub as well.
pub struct CompA(pub u32);
pub struct CompB(pub u32);
pub struct CompC(pub u32);

ecs_world! {
    // Declare two archetypes, ArchFoo and ArchBar.
    ecs_archetype!(ArchFoo, 100, CompA, CompB); // Fixed capacity of 100 entities.
    ecs_archetype!(ArchBar, dyn, CompA, CompC); // Dynamic (dyn) entity capacity.

fn main() {
    let mut world = EcsWorld::default(); // Initialize an empty new ECS world.

    // Add entities to the world by populating their components and receive their handles.
    let entity_a = world.create::<ArchFoo>((CompA(1), CompB(20)));
    let entity_b = world.create::<ArchBar>((CompA(3), CompC(40)));

    // Each archetype now has one entity.
    assert_eq!(world.archetype::<ArchFoo>().len(), 1);
    assert_eq!(world.archetype::<ArchBar>().len(), 1);

    // Look up each entity and check its CompB or CompC value.
    // We use the is_some() check here to make sure the entity was indeed found.
    assert!(ecs_find!(world, entity_a, |c: &CompB| assert_eq!(c.0, 20)).is_some());
    assert!(ecs_find!(world, entity_b, |c: &CompC| assert_eq!(c.0, 40)).is_some());

    // Add to entity_a's CompA value.
    ecs_find!(world, entity_a, |c: &mut CompA| { c.0 += 1; });

    // Sum both entities' CompA values with one iter despite being different archetypes.
    let mut sum = 0;
    ecs_iter!(world, |c: &CompA| { sum += c.0 });
    assert_eq!(sum, 5); // Adding 2 + 3 -- recall that we added 1 to entity_a's CompA.

    // Collect both entities that have a CompA component.
    let mut found = Vec::new();
    ecs_iter!(world, |entity: &EntityAny, _: &CompA| { found.push(*entity); });
    assert!(found == vec![entity_a.into(), entity_b.into()]);

    // Destroy both entities -- this will return an Option containing their components.

    // Try to look up a stale entity handle -- this will return None.
    assert!(ecs_find!(world, entity_a, |_: &Entity<ArchFoo>| { panic!() }).is_none());


  • Handles for accessing entity representations as component data.
  • Error reporting for ECS operation failure.
  • use gecs::prelude::*; to import common macros, traits, and types.
  • Traits for working with ECS types as generics.
  • A checked generational version.


  • Returns the compile-time ID of a given component in its archetype.
  • Finds a single entity in an ECS world and performs an operation on it, if found.
  • Variant of ecs_find! that runtime-borrows data, for use with a non-mut world reference.
  • Iterates over all entities across all archetypes that match the given component bounds.
  • Variant of ecs_iter! that runtime-borrows data, for use with a non-mut world reference.
  • Macro for declaring a new ECS world struct with archetype storage.


  • A special parameter type for ECS query closures to match one of multiple components.