15 files changed, 157 insertions, 160 deletions

diff --git a/src/crepe/system/AnimatorSystem.cpp b/src/crepe/system/AnimatorSystem.cpp
index bf45362..9d18873 100644
--- a/src/crepe/system/AnimatorSystem.cpp
+++ b/src/crepe/system/AnimatorSystem.cpp
@@ -1,27 +1,17 @@
-
 #include <cstdint>
 #include <functional>
 #include <vector>
 
 #include "api/Animator.h"
 #include "facade/SDLContext.h"
-#include "util/log.h"
 
 #include "AnimatorSystem.h"
 #include "ComponentManager.h"
 
 using namespace crepe;
 
-AnimatorSystem::AnimatorSystem() { dbg_trace(); }
-AnimatorSystem::~AnimatorSystem() { dbg_trace(); }
-
-AnimatorSystem & AnimatorSystem::get_instance() {
-	static AnimatorSystem instance;
-	return instance;
-}
-
 void AnimatorSystem::update() {
-	ComponentManager & mgr = ComponentManager::get_instance();
+	ComponentManager & mgr = this->component_manager;
 
 	std::vector<std::reference_wrapper<Animator>> animations
 		= mgr.get_components_by_type<Animator>();
diff --git a/src/crepe/system/AnimatorSystem.h b/src/crepe/system/AnimatorSystem.h
index 969e9d1..56cc7b3 100644
--- a/src/crepe/system/AnimatorSystem.h
+++ b/src/crepe/system/AnimatorSystem.h
@@ -17,28 +17,16 @@ namespace crepe {
 class AnimatorSystem : public System {
 
 public:
-	/**
-	 * \brief Retrieves the singleton instance of the AnimatorSystem.
-	 *
-	 * \return A reference to the single instance of the AnimatorSystem.
-	 *
-	 * This method ensures that there is only one instance of the AnimatorSystem, following the
-	 * singleton design pattern. It can be used to access the system globally.
-	 */
-	static AnimatorSystem & get_instance();
-
+	using System::System;
 	/**
 	 * \brief Updates the Animator components.
 	 *
 	 * This method is called periodically (likely every frame) to update the state of all
-	 * Animator components, moving the animations forward and managing their behavior (e.g., looping).
+	 * Animator components, moving the animations forward and managing their behavior (e.g.,
+	 * looping).
 	 */
 	void update() override;
-
-private:
-	// private because singleton
-	AnimatorSystem(); // dbg_trace
-	~AnimatorSystem(); // dbg_trace
+	// FIXME: never say "likely" in the documentation lmao
 };
 
 } // namespace crepe
diff --git a/src/crepe/system/CMakeLists.txt b/src/crepe/system/CMakeLists.txt
index 4c18b87..d658b25 100644
--- a/src/crepe/system/CMakeLists.txt
+++ b/src/crepe/system/CMakeLists.txt
@@ -1,4 +1,5 @@
 target_sources(crepe PUBLIC
+	System.cpp
 	ParticleSystem.cpp
 	ScriptSystem.cpp
 	PhysicsSystem.cpp
diff --git a/src/crepe/system/CollisionSystem.cpp b/src/crepe/system/CollisionSystem.cpp
index 55e0fdc..c74ca1d 100644
--- a/src/crepe/system/CollisionSystem.cpp
+++ b/src/crepe/system/CollisionSystem.cpp
@@ -2,6 +2,4 @@
 
 using namespace crepe;
 
-CollisionSystem::CollisionSystem() {}
-
 void CollisionSystem::update() {}
diff --git a/src/crepe/system/CollisionSystem.h b/src/crepe/system/CollisionSystem.h
index 1e9f1aa..c1a70d8 100644
--- a/src/crepe/system/CollisionSystem.h
+++ b/src/crepe/system/CollisionSystem.h
@@ -1,11 +1,13 @@
 #pragma once
 
+#include "System.h"
+
 namespace crepe {
 
-class CollisionSystem {
+class CollisionSystem : public System {
 public:
-	CollisionSystem();
-	void update();
+	using System::System;
+	void update() override;
 };
 
 } // namespace crepe
diff --git a/src/crepe/system/ParticleSystem.cpp b/src/crepe/system/ParticleSystem.cpp
index e7a3bec..7316309 100644
--- a/src/crepe/system/ParticleSystem.cpp
+++ b/src/crepe/system/ParticleSystem.cpp
@@ -13,20 +13,17 @@ using namespace crepe;
 
 void ParticleSystem::update() {
 	// Get all emitters
-	ComponentManager & mgr = ComponentManager::get_instance();
+	ComponentManager & mgr = this->component_manager;
 	std::vector<std::reference_wrapper<ParticleEmitter>> emitters
 		= mgr.get_components_by_type<ParticleEmitter>();
 
 	for (ParticleEmitter & emitter : emitters) {
 		// Get transform linked to emitter
 		const Transform & transform
-			= mgr.get_components_by_id<Transform>(emitter.game_object_id)
-				  .front()
-				  .get();
+			= mgr.get_components_by_id<Transform>(emitter.game_object_id).front().get();
 
 		// Emit particles based on emission_rate
-		int updates
-			= calculate_update(this->update_count, emitter.data.emission_rate);
+		int updates = calculate_update(this->update_count, emitter.data.emission_rate);
 		for (size_t i = 0; i < updates; i++) {
 			emit_particle(emitter, transform);
 		}
@@ -45,8 +42,7 @@ void ParticleSystem::update() {
 	this->update_count = (this->update_count + 1) % this->MAX_UPDATE_COUNT;
 }
 
-void ParticleSystem::emit_particle(ParticleEmitter & emitter,
-								   const Transform & transform) {
+void ParticleSystem::emit_particle(ParticleEmitter & emitter, const Transform & transform) {
 	constexpr double DEG_TO_RAD = M_PI / 180.0;
 
 	Vector2 initial_position = emitter.data.position + transform.position;
@@ -57,13 +53,13 @@ void ParticleSystem::emit_particle(ParticleEmitter & emitter,
 		= generate_random_speed(emitter.data.min_speed, emitter.data.max_speed);
 	double angle_radians = random_angle * DEG_TO_RAD;
 
-	Vector2 velocity = {random_speed * std::cos(angle_radians),
-						random_speed * std::sin(angle_radians)};
+	Vector2 velocity
+		= {random_speed * std::cos(angle_radians), random_speed * std::sin(angle_radians)};
 
 	for (Particle & particle : emitter.data.particles) {
 		if (!particle.active) {
-			particle.reset(emitter.data.end_lifespan, initial_position,
-						   velocity, random_angle);
+			particle.reset(emitter.data.end_lifespan, initial_position, velocity,
+						   random_angle);
 			break;
 		}
 	}
@@ -81,10 +77,8 @@ int ParticleSystem::calculate_update(int count, double emission) const {
 	return static_cast<int>(emission);
 }
 
-void ParticleSystem::check_bounds(ParticleEmitter & emitter,
-								  const Transform & transform) {
-	Vector2 offset = emitter.data.boundary.offset + transform.position
-					 + emitter.data.position;
+void ParticleSystem::check_bounds(ParticleEmitter & emitter, const Transform & transform) {
+	Vector2 offset = emitter.data.boundary.offset + transform.position + emitter.data.position;
 	double half_width = emitter.data.boundary.width / 2.0;
 	double half_height = emitter.data.boundary.height / 2.0;
 
@@ -95,8 +89,8 @@ void ParticleSystem::check_bounds(ParticleEmitter & emitter,
 
 	for (Particle & particle : emitter.data.particles) {
 		const Vector2 & position = particle.position;
-		bool within_bounds = (position.x >= LEFT && position.x <= RIGHT
-							  && position.y >= TOP && position.y <= BOTTOM);
+		bool within_bounds = (position.x >= LEFT && position.x <= RIGHT && position.y >= TOP
+							  && position.y <= BOTTOM);
 
 		if (!within_bounds) {
 			if (emitter.data.boundary.reset_on_exit) {
@@ -112,30 +106,25 @@ void ParticleSystem::check_bounds(ParticleEmitter & emitter,
 	}
 }
 
-double ParticleSystem::generate_random_angle(double min_angle,
-											 double max_angle) const {
+double ParticleSystem::generate_random_angle(double min_angle, double max_angle) const {
 	if (min_angle == max_angle) {
 		return min_angle;
 	} else if (min_angle < max_angle) {
 		return min_angle
-			   + static_cast<double>(std::rand()
-									 % static_cast<int>(max_angle - min_angle));
+			   + static_cast<double>(std::rand() % static_cast<int>(max_angle - min_angle));
 	} else {
 		double angle_offset = (360 - min_angle) + max_angle;
-		double random_angle = min_angle
-							  + static_cast<double>(
-								  std::rand() % static_cast<int>(angle_offset));
+		double random_angle
+			= min_angle + static_cast<double>(std::rand() % static_cast<int>(angle_offset));
 		return (random_angle >= 360) ? random_angle - 360 : random_angle;
 	}
 }
 
-double ParticleSystem::generate_random_speed(double min_speed,
-											 double max_speed) const {
+double ParticleSystem::generate_random_speed(double min_speed, double max_speed) const {
 	if (min_speed == max_speed) {
 		return min_speed;
 	} else {
 		return min_speed
-			   + static_cast<double>(std::rand()
-									 % static_cast<int>(max_speed - min_speed));
+			   + static_cast<double>(std::rand() % static_cast<int>(max_speed - min_speed));
 	}
 }
diff --git a/src/crepe/system/ParticleSystem.h b/src/crepe/system/ParticleSystem.h
index d7ca148..c647284 100644
--- a/src/crepe/system/ParticleSystem.h
+++ b/src/crepe/system/ParticleSystem.h
@@ -5,66 +5,75 @@
 #include "System.h"
 
 namespace crepe {
+
 class ParticleEmitter;
 class Transform;
+
 /**
- 	* \brief ParticleSystem class responsible for managing particle emission, updates, and bounds checking.
- 	*/
+ * \brief ParticleSystem class responsible for managing particle emission, updates, and bounds
+ * checking.
+ */
 class ParticleSystem : public System {
 public:
+	using System::System;
 	/**
-		* \brief Updates all particle emitters by emitting particles, updating particle states, and checking bounds.
-		*/
+	 * \brief Updates all particle emitters by emitting particles, updating particle states, and
+	 * checking bounds.
+	 */
 	void update() override;
 
 private:
 	/**
-		* \brief Emits a particle from the specified emitter based on its emission properties.
-		* 
-		* \param emitter Reference to the ParticleEmitter.
-		* \param transform Const reference to the Transform component associated with the emitter.
-		*/
+	 * \brief Emits a particle from the specified emitter based on its emission properties.
+	 * 
+	 * \param emitter Reference to the ParticleEmitter.
+	 * \param transform Const reference to the Transform component associated with the emitter.
+	 */
 	void emit_particle(ParticleEmitter & emitter, const Transform & transform);
 
 	/**
-		* \brief Calculates the number of times particles should be emitted based on emission rate and update count.
-		* 
-		* \param count Current update count.
-		* \param emission Emission rate.
-		* \return The number of particles to emit.
-		*/
+	 * \brief Calculates the number of times particles should be emitted based on emission rate
+	 * and update count.
+	 * 
+	 * \param count Current update count.
+	 * \param emission Emission rate.
+	 * \return The number of particles to emit.
+	 */
 	int calculate_update(int count, double emission) const;
 
 	/**
-		* \brief Checks whether particles are within the emitter’s boundary, resets or stops particles if they exit.
-		* 
-		* \param emitter Reference to the ParticleEmitter.
-		* \param transform Const reference to the Transform component associated with the emitter.
-		*/
+	 * \brief Checks whether particles are within the emitter’s boundary, resets or stops
+	 * particles if they exit.
+	 * 
+	 * \param emitter Reference to the ParticleEmitter.
+	 * \param transform Const reference to the Transform component associated with the emitter.
+	 */
 	void check_bounds(ParticleEmitter & emitter, const Transform & transform);
 
 	/**
-		* \brief Generates a random angle for particle emission within the specified range.
-		* 
-		* \param min_angle Minimum emission angle in degrees.
-		* \param max_angle Maximum emission angle in degrees.
-		* \return Random angle in degrees.
-		*/
+	 * \brief Generates a random angle for particle emission within the specified range.
+	 * 
+	 * \param min_angle Minimum emission angle in degrees.
+	 * \param max_angle Maximum emission angle in degrees.
+	 * \return Random angle in degrees.
+	 */
 	double generate_random_angle(double min_angle, double max_angle) const;
 
 	/**
-		* \brief Generates a random speed for particle emission within the specified range.
-		* 
-		* \param min_speed Minimum emission speed.
-		* \param max_speed Maximum emission speed.
-		* \return Random speed.
-		*/
+	 * \brief Generates a random speed for particle emission within the specified range.
+	 * 
+	 * \param min_speed Minimum emission speed.
+	 * \param max_speed Maximum emission speed.
+	 * \return Random speed.
+	 */
 	double generate_random_speed(double min_speed, double max_speed) const;
 
 private:
-	//! Counter to count updates to determine how many times emit_particle is called.
+	//! Counter to count updates to determine how many times emit_particle is
+	// called.
 	unsigned int update_count = 0;
-	//! Determines the lowest amount of emission rate (1000 = 0.001 = 1 particle per 1000 updates).
+	//! Determines the lowest amount of emission rate (1000 = 0.001 = 1 particle per 1000
+	// updates).
 	static constexpr unsigned int MAX_UPDATE_COUNT = 100;
 };
 
diff --git a/src/crepe/system/PhysicsSystem.cpp b/src/crepe/system/PhysicsSystem.cpp
index eb54ad3..4a7dbfb 100644
--- a/src/crepe/system/PhysicsSystem.cpp
+++ b/src/crepe/system/PhysicsSystem.cpp
@@ -11,7 +11,7 @@
 using namespace crepe;
 
 void PhysicsSystem::update() {
-	ComponentManager & mgr = ComponentManager::get_instance();
+	ComponentManager & mgr = this->component_manager;
 	std::vector<std::reference_wrapper<Rigidbody>> rigidbodies
 		= mgr.get_components_by_type<Rigidbody>();
 	std::vector<std::reference_wrapper<Transform>> transforms
@@ -29,17 +29,15 @@ void PhysicsSystem::update() {
 						// Add gravity
 						if (rigidbody.data.use_gravity) {
 							rigidbody.data.linear_velocity.y
-								+= (rigidbody.data.mass
-									* rigidbody.data.gravity_scale * gravity);
+								+= (rigidbody.data.mass * rigidbody.data.gravity_scale
+									* gravity);
 						}
 						// Add damping
 						if (rigidbody.data.angular_damping != 0) {
-							rigidbody.data.angular_velocity
-								*= rigidbody.data.angular_damping;
+							rigidbody.data.angular_velocity *= rigidbody.data.angular_damping;
 						}
 						if (rigidbody.data.linear_damping != Vector2{0, 0}) {
-							rigidbody.data.linear_velocity
-								*= rigidbody.data.linear_damping;
+							rigidbody.data.linear_velocity *= rigidbody.data.linear_damping;
 						}
 
 						// Max velocity check
@@ -75,21 +73,17 @@ void PhysicsSystem::update() {
 
 						// Move object
 						if (!rigidbody.data.constraints.rotation) {
-							transform.rotation
-								+= rigidbody.data.angular_velocity;
-							transform.rotation
-								= std::fmod(transform.rotation, 360.0);
+							transform.rotation += rigidbody.data.angular_velocity;
+							transform.rotation = std::fmod(transform.rotation, 360.0);
 							if (transform.rotation < 0) {
 								transform.rotation += 360.0;
 							}
 						}
 						if (!rigidbody.data.constraints.x) {
-							transform.position.x
-								+= rigidbody.data.linear_velocity.x;
+							transform.position.x += rigidbody.data.linear_velocity.x;
 						}
 						if (!rigidbody.data.constraints.y) {
-							transform.position.y
-								+= rigidbody.data.linear_velocity.y;
+							transform.position.y += rigidbody.data.linear_velocity.y;
 						}
 					}
 				}
diff --git a/src/crepe/system/PhysicsSystem.h b/src/crepe/system/PhysicsSystem.h
index 038c120..227ab69 100644
--- a/src/crepe/system/PhysicsSystem.h
+++ b/src/crepe/system/PhysicsSystem.h
@@ -3,14 +3,16 @@
 #include "System.h"
 
 namespace crepe {
+
 /**
  * \brief System that controls all physics
  * 
- * This class is a physics system that uses a rigidbody and transform
- * to add physics to a game object.
+ * This class is a physics system that uses a rigidbody and transform to add physics to a game
+ * object.
  */
 class PhysicsSystem : public System {
 public:
+	using System::System;
 	/**
 	 * \brief updates the physics system.
 	 * 
diff --git a/src/crepe/system/RenderSystem.cpp b/src/crepe/system/RenderSystem.cpp
index 718036c..aa9910b 100644
--- a/src/crepe/system/RenderSystem.cpp
+++ b/src/crepe/system/RenderSystem.cpp
@@ -7,33 +7,19 @@
 #include "../api/Sprite.h"
 #include "../api/Transform.h"
 #include "../facade/SDLContext.h"
-#include "../util/log.h"
-#include "Asset.h"
+#include "../util/Log.h"
 
 #include "RenderSystem.h"
 
 using namespace crepe;
 
-RenderSystem::RenderSystem() { dbg_trace(); }
+void RenderSystem::clear_screen() const { SDLContext::get_instance().clear_screen(); }
 
-RenderSystem::~RenderSystem() { dbg_trace(); }
-
-RenderSystem & RenderSystem::get_instance() {
-	static RenderSystem instance;
-	return instance;
-}
-
-void RenderSystem::clear_screen() const {
-	SDLContext::get_instance().clear_screen();
-}
-
-void RenderSystem::present_screen() const {
-	SDLContext::get_instance().present_screen();
-}
+void RenderSystem::present_screen() const { SDLContext::get_instance().present_screen(); }
 void RenderSystem::update_camera() {
-	ComponentManager & mgr = ComponentManager::get_instance();
-	std::vector<std::reference_wrapper<Camera>> cameras
-		= mgr.get_components_by_type<Camera>();
+	ComponentManager & mgr = this->component_manager;
+
+	std::vector<std::reference_wrapper<Camera>> cameras = mgr.get_components_by_type<Camera>();
 
 	for (Camera & cam : cameras) {
 		SDLContext::get_instance().camera(cam);
@@ -63,6 +49,7 @@ RenderSystem::sort(std::vector<std::reference_wrapper<Sprite>> & objs) {
 }
 
 void RenderSystem::render_sprites() {
+	ComponentManager & mgr = this->component_manager;
 
 	ComponentManager & mgr = ComponentManager::get_instance();
 
@@ -72,8 +59,7 @@ void RenderSystem::render_sprites() {
 	
 	SDLContext & render = SDLContext::get_instance();
 	for (const Sprite & sprite : sorted_sprites) {
-		auto transforms
-			= mgr.get_components_by_id<Transform>(sprite.game_object_id);
+		auto transforms = mgr.get_components_by_id<Transform>(sprite.game_object_id);
 		render.draw(sprite, transforms[0], *curr_cam);
 	}
 }
diff --git a/src/crepe/system/RenderSystem.h b/src/crepe/system/RenderSystem.h
index a83633a..7661cbd 100644
--- a/src/crepe/system/RenderSystem.h
+++ b/src/crepe/system/RenderSystem.h
@@ -13,19 +13,13 @@ namespace crepe {
  * \class RenderSystem
  * \brief Manages rendering operations for all game objects.
  *
- * RenderSystem is responsible for rendering sprites, clearing and presenting the screen, 
- * and managing the active camera. It functions as a singleton, providing centralized 
- * rendering services for the application.
+ * RenderSystem is responsible for rendering sprites, clearing and presenting the screen, and
+ * managing the active camera. It functions as a singleton, providing centralized rendering
+ * services for the application.
  */
 class RenderSystem : public System {
-
 public:
-	/**
-	 * \brief Gets the singleton instance of RenderSystem.
-	 * \return Reference to the RenderSystem instance.
-	 */
-	static RenderSystem & get_instance();
-
+	using System::System;
 	/**
 	 * \brief Updates the RenderSystem for the current frame.
 	 * This method is called to perform all rendering operations for the current game frame.
@@ -33,10 +27,6 @@ public:
 	void update() override;
 
 private:
-	// Private constructor to enforce singleton pattern.
-	RenderSystem();
-	~RenderSystem();
-
 	//! Clears the screen in preparation for rendering.
 	void clear_screen() const;
 
@@ -67,4 +57,5 @@ private:
 	Camera * curr_cam = nullptr;
 	// TODO: needs a better solution
 };
+
 } // namespace crepe
diff --git a/src/crepe/system/ScriptSystem.cpp b/src/crepe/system/ScriptSystem.cpp
index f2673e7..c4d724c 100644
--- a/src/crepe/system/ScriptSystem.cpp
+++ b/src/crepe/system/ScriptSystem.cpp
@@ -5,7 +5,6 @@
 #include "../ComponentManager.h"
 #include "../api/BehaviorScript.h"
 #include "../api/Script.h"
-#include "../util/log.h"
 
 #include "ScriptSystem.h"
 
@@ -13,16 +12,23 @@ using namespace std;
 using namespace crepe;
 
 void ScriptSystem::update() {
-	using namespace std;
 	dbg_trace();
 
-	forward_list<Script *> scripts = this->get_scripts();
-	for (Script * script : scripts) script->update();
+	forward_list<reference_wrapper<Script>> scripts = this->get_scripts();
+
+	for (auto & script_ref : scripts) {
+		Script & script = script_ref.get();
+		if (!script.initialized) {
+			script.init();
+			script.initialized = true;
+		}
+		script.update();
+	}
 }
 
-forward_list<Script *> ScriptSystem::get_scripts() {
-	forward_list<Script *> scripts = {};
-	ComponentManager & mgr = ComponentManager::get_instance();
+forward_list<reference_wrapper<Script>> ScriptSystem::get_scripts() const {
+	forward_list<reference_wrapper<Script>> scripts = {};
+	ComponentManager & mgr = this->component_manager;
 	vector<reference_wrapper<BehaviorScript>> behavior_scripts
 		= mgr.get_components_by_type<BehaviorScript>();
 
@@ -31,7 +37,7 @@ forward_list<Script *> ScriptSystem::get_scripts() {
 		if (!behavior_script.active) continue;
 		Script * script = behavior_script.script.get();
 		if (script == nullptr) continue;
-		scripts.push_front(script);
+		scripts.push_front(*script);
 	}
 
 	return scripts;
diff --git a/src/crepe/system/ScriptSystem.h b/src/crepe/system/ScriptSystem.h
index 4fa6141..deb89cb 100644
--- a/src/crepe/system/ScriptSystem.h
+++ b/src/crepe/system/ScriptSystem.h
@@ -8,13 +8,32 @@ namespace crepe {
 
 class Script;
 
+/**
+ * \brief Script system
+ * 
+ * The script system is responsible for all \c BehaviorScript components, and
+ * calls the methods on classes derived from \c Script.
+ */
 class ScriptSystem : public System {
 public:
-	void update();
+	using System::System;
+	/**
+	 * \brief Call Script::update() on all active \c BehaviorScript instances
+	 *
+	 * This routine updates all scripts sequentially using the Script::update()
+	 * method. It also calls Script::init() if this has not been done before on
+	 * the \c BehaviorScript instance.
+	 */
+	void update() override;
 
 private:
-	// TODO: to forward_list<reference_wrapper>
-	std::forward_list<Script *> get_scripts();
+	/**
+	 * \brief Aggregate all active \c BehaviorScript components and return a list
+	 * of references to their \c Script instances (utility)
+	 *
+	 * \returns List of active \c Script instances
+	 */
+	std::forward_list<std::reference_wrapper<Script>> get_scripts() const;
 };
 
 } // namespace crepe
diff --git a/src/crepe/system/System.cpp b/src/crepe/system/System.cpp
new file mode 100644
index 0000000..937a423
--- /dev/null
+++ b/src/crepe/system/System.cpp
@@ -0,0 +1,7 @@
+#include "../util/Log.h"
+
+#include "System.h"
+
+using namespace crepe;
+
+System::System(ComponentManager & mgr) : component_manager(mgr) { dbg_trace(); }
diff --git a/src/crepe/system/System.h b/src/crepe/system/System.h
index 3b81bef..28ea20e 100644
--- a/src/crepe/system/System.h
+++ b/src/crepe/system/System.h
@@ -2,13 +2,28 @@
 
 namespace crepe {
 
+class ComponentManager;
+
+/**
+ * \brief Base ECS system class
+ *
+ * This class is used as the base for all system classes. Classes derived from
+ * System must implement the System::update() method and copy Script::Script
+ * with the `using`-syntax.
+ */
 class System {
 public:
+	/**
+	 * \brief Process all components this system is responsible for.
+	 */
 	virtual void update() = 0;
 
 public:
-	System() = default;
+	System(ComponentManager &);
 	virtual ~System() = default;
+
+protected:
+	ComponentManager & component_manager;
 };
 
 } // namespace crepe