MonkeroECS

A compact header-only, C++17 ECS (Entity Component System) gutted from a game engine project. I consider it feature-complete, so active development is sparse and limited to bug fixes.

Features:

• No entity count limits (other than 32-bit entity index)
• Only depends on standard library
• Component addresses never change during their lifetime
• Events
• Dependencies
  • Components can depend on other components which will be added automatically if not present already
• Flexible but simple-to-use foreach
• Very efficient multi-component iteration
• Memory-efficient handling of tag components
• Batched modification that lets you safely add & remove components while you iterate
• Unit tests included
• Constant-time component lookup and remove (& insert that is constant-time in practice, but not in theory ;))

In the interest of saving your time, here's a list of common game dev deal-breakers:

• Uses RTTI
  • Not in components themselves though, but used with type-erasure with internal component containers.
• Uses STL
  • std::vector for some mandatory parts
  • std::map temporarily for certain rare operations like concat and copy
• Lots of templates
• Some potentially slow-to-include standard library headers
• Thread-oblivious for now

Integration

Just copy monkeroecs.hh over and include it wherever you need ECS stuff.

The namespace name is quite long, so you may want to alias it to something shorter or put "using namespace monkero;" inside your own namespace.

Building docs & examples

To build the Doxygen documentation and examples:

meson build
ninja -C build # for examples
ninja -C build docs # for docs

Usage

1. Create your component types. Any type can be used as a component!

struct health
{
    int hit_points;
};

2. Create some event types. Any type can be used as an event!

struct hit_event
{
    monkero::entity damaged_entity;
    int damage;
};

3. Create a system & list emitted and received events!

class health_system: public monkero::receiver<hit_event>
{
public:
    void handle(monkero::scene& s, const hit_event& e)
    {
        s.get<health>(e.damaged_entity)->hit_points -= e.damage;
    }
};

class poison_gas_system
{
public:
    void tick(monkero::scene& s)
    {
        // Iterate all entities with health component
        s([&](monkero::entity id, health&){
            s.emit(hit_event{id, 10});
        });
    }
};

4. Add entities, components and systems to the ECS!

monkero::scene s;

health_system hs;
s.add_receiver(hs);
poison_gas_system gas;

// Adds entity with component health
s.add(health{30});
s.add(health{40});
s.add(health{50});

while(main_loop)
{
    //...
    gas.tick(s);
    //...
}

Advanced usage examples of components, systems and events can be found in the examples folder along with a complete usage example of the system as a whole.