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Value Type Components

In fennecs, components are typically defined as value types (e.g., structs). Value type components are stored directly within the memory of each entity, providing excellent performance and memory efficiency.

Neofox: packed TIGHTLY PACKED

By default, components in fennecs are value types. This means that each entity has its own unique copy of the component.

Their data is always tightly packed within the Archetype that houses the Entity.

Defining Value Type Components

To define a value type component, simply create a struct:

public struct Position
    public float X;
    public float Y;

public struct Velocity
    public float X;
    public float Y;

A special case are Tags - empty structs. These use no memory at all for storage, and carry their meaning just through their presence.

public struct PlayerTag;

Adding Value Type Components to Entities

When you add a value type component to an entity, the component's data is stored directly within the entity's memory:

var entity = world.Spawn()
    .Add(new Position { X = 0, Y = 0 })
    .Add(new Velocity { X = 1, Y = 1 });

Each entity has its own unique instance of the Position and Velocity components, with the data stored contiguously in memory.

Efficient Iteration with Runners

One of the key benefits of using value type components is the favorable cache locality when iterating over entities using Runners like For and Job.

Neofox: rocket CACHE LOCALITY FTW!

Since value type components are stored contiguously in memory, iterating over entities with the same component layout (i.e., within the same archetype) is extremely efficient. The CPU cache can effectively prefetch and cache the component data, minimizing costly memory accesses.

var query = world.Query<Position, Velocity>().Compile();

query.For((ref Position position, ref Velocity velocity) =>
    position.X += velocity.X;
    position.Y += velocity.Y;

In this example, the For runner efficiently iterates over entities with Position and Velocity components, taking advantage of the cache locality provided by the contiguous memory layout.


  1. Value type components are the default and recommended choice for most scenarios in fennecs.

  2. Value types provide the best performance and memory efficiency due to their contiguous storage and cache-friendly iteration.

  3. Each entity has its own unique instance of a value type component, allowing for independent modification without affecting other entities.

  4. When an entity's component layout changes (e.g., adding or removing components), the entity is moved to a different archetype, and the value type components are copied to the new memory location.


Value type components are the foundation of fennecs, providing excellent performance and memory efficiency. By storing components directly within entities and enabling cache-friendly iteration, value types unlock the full potential of the ECS architecture.

When defining components, consider using value types (structs) by default, and leverage the power of Runners like For and Job to efficiently process entities and their components.

fennecs is released under the MIT License. Neofox is released under the CC BY-NC-SA 4.0 License.