merge `master` into `loek/scripts`

26 files changed, 292 insertions, 107 deletions

diff --git a/src/crepe/Asset.cpp b/src/crepe/Asset.cpp
deleted file mode 100644
index 9c41ecb..0000000
--- a/src/crepe/Asset.cpp
+++ /dev/null
@@ -1,16 +0,0 @@
-#include <filesystem>
-
-#include "Asset.h"
-
-using namespace crepe;
-using namespace std;
-
-// FIXME: restore this
-// src(std::filesystem::canonical(src))
-Asset::Asset(const std::string & src) : src(src) {
-	this->file = std::ifstream(this->src, std::ios::in | std::ios::binary);
-}
-
-istream & Asset::get_stream() { return this->file; }
-
-const string & Asset::get_canonical() const { return this->src; }
diff --git a/src/crepe/Asset.h b/src/crepe/Asset.h
deleted file mode 100644
index 9051c5e..0000000
--- a/src/crepe/Asset.h
+++ /dev/null
@@ -1,41 +0,0 @@
-#pragma once
-
-#include <fstream>
-#include <iostream>
-#include <string>
-
-namespace crepe {
-
-/**
- * \brief Asset location helper
- *
- * This class is used to locate and canonicalize paths to game asset files, and should *always*
- * be used when retrieving files from disk.
- */
-class Asset {
-public:
-	/**
-	 * \param src  Unique identifier to asset
-	 */
-	Asset(const std::string & src);
-
-public:
-	/**
-	 * \brief Get an input stream to the contents of this asset
-	 * \return Input stream with file contents
-	 */
-	std::istream & get_stream();
-	/**
-	 * \brief Get the canonical path to this asset
-	 * \return Canonical path to this asset
-	 */
-	const std::string & get_canonical() const;
-
-private:
-	//! Canonical path to asset
-	const std::string src;
-	//! File handle (stream)
-	std::ifstream file;
-};
-
-} // namespace crepe
diff --git a/src/crepe/CMakeLists.txt b/src/crepe/CMakeLists.txt
index 3b05742..7e176e7 100644
--- a/src/crepe/CMakeLists.txt
+++ b/src/crepe/CMakeLists.txt
@@ -1,5 +1,4 @@
 target_sources(crepe PUBLIC
-	Asset.cpp
 	Particle.cpp
 	ComponentManager.cpp
 	Component.cpp
@@ -7,7 +6,6 @@ target_sources(crepe PUBLIC
 )
 
 target_sources(crepe PUBLIC FILE_SET HEADERS FILES
-	Asset.h
 	ComponentManager.h
 	ComponentManager.hpp
 	Component.h
diff --git a/src/crepe/ComponentManager.h b/src/crepe/ComponentManager.h
index 2107453..0956d1e 100644
--- a/src/crepe/ComponentManager.h
+++ b/src/crepe/ComponentManager.h
@@ -8,6 +8,7 @@
 #include "api/Vector2.h"
 
 #include "Component.h"
+#include "types.h"
 
 namespace crepe {
 
@@ -112,7 +113,7 @@ public:
 	 * \return A vector of all components of the specific type and id
 	 */
 	template <typename T>
-	std::vector<std::reference_wrapper<T>> get_components_by_id(game_object_id_t id) const;
+	RefVector<T> get_components_by_id(game_object_id_t id) const;
 	/**
 	 * \brief Get all components of a specific type
 	 * 
@@ -122,7 +123,7 @@ public:
 	 * \return A vector of all components of the specific type
 	 */
 	template <typename T>
-	std::vector<std::reference_wrapper<T>> get_components_by_type() const;
+	RefVector<T> get_components_by_type() const;
 
 private:
 	/**
diff --git a/src/crepe/ComponentManager.hpp b/src/crepe/ComponentManager.hpp
index be99cac..4d5eaf4 100644
--- a/src/crepe/ComponentManager.hpp
+++ b/src/crepe/ComponentManager.hpp
@@ -81,15 +81,14 @@ void ComponentManager::delete_components() {
 }
 
 template <typename T>
-std::vector<std::reference_wrapper<T>>
-ComponentManager::get_components_by_id(game_object_id_t id) const {
+RefVector<T> ComponentManager::get_components_by_id(game_object_id_t id) const {
 	using namespace std;
 
 	// Determine the type of T (this is used as the key of the unordered_map<>)
 	type_index type = typeid(T);
 
 	// Create an empty vector<>
-	vector<reference_wrapper<T>> component_vector;
+	RefVector<T> component_vector;
 
 	if (this->components.find(type) == this->components.end()) return component_vector;
 
@@ -114,14 +113,14 @@ ComponentManager::get_components_by_id(game_object_id_t id) const {
 }
 
 template <typename T>
-std::vector<std::reference_wrapper<T>> ComponentManager::get_components_by_type() const {
+RefVector<T> ComponentManager::get_components_by_type() const {
 	using namespace std;
 
 	// Determine the type of T (this is used as the key of the unordered_map<>)
 	type_index type = typeid(T);
 
 	// Create an empty vector<>
-	vector<reference_wrapper<T>> component_vector;
+	RefVector<T> component_vector;
 
 	// Find the type (in the unordered_map<>)
 	if (this->components.find(type) == this->components.end()) return component_vector;
diff --git a/src/crepe/api/Asset.cpp b/src/crepe/api/Asset.cpp
new file mode 100644
index 0000000..e148367
--- /dev/null
+++ b/src/crepe/api/Asset.cpp
@@ -0,0 +1,54 @@
+#include <filesystem>
+#include <stdexcept>
+#include <whereami.h>
+
+#include "api/Config.h"
+
+#include "Asset.h"
+
+using namespace crepe;
+using namespace std;
+
+Asset::Asset(const string & src) : src(find_asset(src)) {}
+Asset::Asset(const char * src) : src(find_asset(src)) {}
+
+const string & Asset::get_path() const noexcept { return this->src; }
+
+string Asset::find_asset(const string & src) const {
+	auto & cfg = Config::get_instance();
+	string & root_pattern = cfg.asset.root_pattern;
+
+	// if root_pattern is empty, find_asset must return all paths as-is
+	if (root_pattern.empty()) return src;
+
+	// absolute paths do not need to be resolved, only canonicalized
+	filesystem::path path = src;
+	if (path.is_absolute()) return filesystem::canonical(path);
+
+	// find directory matching root_pattern
+	filesystem::path root = this->whereami();
+	while (1) {
+		if (filesystem::exists(root / root_pattern)) break;
+		if (!root.has_parent_path())
+			throw runtime_error(format("Asset: Cannot find root pattern ({})", root_pattern));
+		root = root.parent_path();
+	}
+
+	// join path to root (base directory) and canonicalize
+	return filesystem::canonical(root / path);
+}
+
+string Asset::whereami() const noexcept {
+	string path;
+	size_t path_length = wai_getExecutablePath(NULL, 0, NULL);
+	path.resize(path_length + 1); // wai writes null byte
+	wai_getExecutablePath(path.data(), path_length, NULL);
+	path.resize(path_length);
+	return path;
+}
+
+bool Asset::operator==(const Asset & other) const noexcept { return this->src == other.src; }
+
+size_t std::hash<const Asset>::operator()(const Asset & asset) const noexcept {
+	return std::hash<string>{}(asset.get_path());
+};
diff --git a/src/crepe/api/Asset.h b/src/crepe/api/Asset.h
new file mode 100644
index 0000000..bfd0ac7
--- /dev/null
+++ b/src/crepe/api/Asset.h
@@ -0,0 +1,84 @@
+#pragma once
+
+#include <string>
+
+namespace crepe {
+
+/**
+ * \brief Asset location helper
+ *
+ * This class is used to locate game asset files, and should *always* be used
+ * instead of reading file paths directly.
+ */
+class Asset {
+public:
+	/**
+	 * \param src  Unique identifier to asset
+	 */
+	Asset(const std::string & src);
+	/**
+	 * \param src  Unique identifier to asset
+	 */
+	Asset(const char * src);
+
+public:
+	/**
+	 * \brief Get the path to this asset
+	 * \return path to this asset
+	 */
+	const std::string & get_path() const noexcept;
+
+	/**
+	 * \brief Comparison operator
+	 * \param other Possibly different instance of \c Asset to test equality against
+	 * \return True if \c this and \c other are equal
+	 */
+	bool operator==(const Asset & other) const noexcept;
+
+private:
+	//! path to asset
+	const std::string src;
+
+private:
+	/**
+	 * \brief Locate asset path, or throw exception if it cannot be found
+	 *
+	 * This function resolves asset locations relative to crepe::Config::root_pattern if it is
+	 * set and \p src is a relative path. If \p src is an absolute path, it is canonicalized.
+	 * This function only returns if the file can be found.
+	 *
+	 * \param src Arbitrary path to resource file
+	 *
+	 * \returns \p src if crepe::Config::root_pattern is empty
+	 * \returns Canonical path to \p src
+	 *
+	 * \throws std::runtime_error if root_pattern cannot be found
+	 * \throws std::filesystem::filesystem_error if the resolved path does not exist
+	 * \throws std::filesystem::filesystem_error if the path cannot be canonicalized
+	 */
+	std::string find_asset(const std::string & src) const;
+	/**
+	 * \returns The path to the current executable
+	 */
+	std::string whereami() const noexcept;
+};
+
+} // namespace crepe
+
+namespace std {
+
+//! Hash helper struct
+template <>
+struct hash<const crepe::Asset> {
+	/**
+	 * \brief Hash operator for crepe::Asset
+	 *
+	 * This function hashes a crepe::Asset instance, allowing it to be used as a key in an \c
+	 * std::unordered_map.
+	 *
+	 * \returns Hash value
+	 */
+	size_t operator()(const crepe::Asset & asset) const noexcept;
+};
+
+} // namespace std
diff --git a/src/crepe/api/CMakeLists.txt b/src/crepe/api/CMakeLists.txt
index a63c5ff..602ab52 100644
--- a/src/crepe/api/CMakeLists.txt
+++ b/src/crepe/api/CMakeLists.txt
@@ -22,6 +22,7 @@ target_sources(crepe PUBLIC
 	IMouseListener.cpp
 	LoopManager.cpp
 	LoopTimer.cpp
+	Asset.cpp
 	EventHandler.cpp
 	Script.cpp
 )
@@ -57,4 +58,5 @@ target_sources(crepe PUBLIC FILE_SET HEADERS FILES
 	IMouseListener.h
 	LoopManager.h
 	LoopTimer.h
+	Asset.h
 )
diff --git a/src/crepe/api/Config.h b/src/crepe/api/Config.h
index 3ab877a..13eabd1 100644
--- a/src/crepe/api/Config.h
+++ b/src/crepe/api/Config.h
@@ -62,6 +62,20 @@ public:
 		 */
 		double gravity = 1;
 	} physics;
+
+	//! Asset loading options
+	struct {
+		/**
+		 * \brief Pattern to match for Asset base directory
+		 *
+		 * All non-absolute paths resolved using \c Asset will be made relative to
+		 * the first parent directory relative to the calling executable where
+		 * appending this pattern results in a path that exists. If this string is
+		 * empty, path resolution is disabled, and Asset will return all paths
+		 * as-is.
+		 */
+		std::string root_pattern = ".crepe-root";
+	} asset;
 };
 
 } // namespace crepe
diff --git a/src/crepe/api/GameObject.cpp b/src/crepe/api/GameObject.cpp
index 287e81d..4874426 100644
--- a/src/crepe/api/GameObject.cpp
+++ b/src/crepe/api/GameObject.cpp
@@ -23,12 +23,10 @@ void GameObject::set_parent(const GameObject & parent) {
 	ComponentManager & mgr = this->component_manager;
 
 	// Set parent on own Metadata component
-	vector<reference_wrapper<Metadata>> this_metadata
-		= mgr.get_components_by_id<Metadata>(this->id);
+	RefVector<Metadata> this_metadata = mgr.get_components_by_id<Metadata>(this->id);
 	this_metadata.at(0).get().parent = parent.id;
 
 	// Add own id to children list of parent's Metadata component
-	vector<reference_wrapper<Metadata>> parent_metadata
-		= mgr.get_components_by_id<Metadata>(parent.id);
+	RefVector<Metadata> parent_metadata = mgr.get_components_by_id<Metadata>(parent.id);
 	parent_metadata.at(0).get().children.push_back(this->id);
 }
diff --git a/src/crepe/api/Script.h b/src/crepe/api/Script.h
index 8356fd2..43efd15 100644
--- a/src/crepe/api/Script.h
+++ b/src/crepe/api/Script.h
@@ -78,7 +78,7 @@ protected:
 	 * \returns List of component references
 	 */
 	template <typename T>
-	std::vector<std::reference_wrapper<T>> get_components() const;
+	RefVector<T> get_components() const;
 
 	/**
 	 * \brief Log a message using Log::logf
diff --git a/src/crepe/api/Script.hpp b/src/crepe/api/Script.hpp
index e60be07..e94278d 100644
--- a/src/crepe/api/Script.hpp
+++ b/src/crepe/api/Script.hpp
@@ -10,7 +10,7 @@ namespace crepe {
 template <typename T>
 T & Script::get_component() const {
 	using namespace std;
-	vector<reference_wrapper<T>> all_components = this->get_components<T>();
+	RefVector<T> all_components = this->get_components<T>();
 	if (all_components.size() < 1)
 		throw runtime_error(
 			format("Script: no component found with type = {}", typeid(T).name()));
@@ -19,7 +19,7 @@ T & Script::get_component() const {
 }
 
 template <typename T>
-std::vector<std::reference_wrapper<T>> Script::get_components() const {
+RefVector<T> Script::get_components() const {
 	ComponentManager & mgr = *this->component_manager_ref;
 
 	return mgr.get_components_by_id<T>(*this->game_object_id_ref);
diff --git a/src/crepe/api/Texture.cpp b/src/crepe/api/Texture.cpp
index 734a5bb..9be9421 100644
--- a/src/crepe/api/Texture.cpp
+++ b/src/crepe/api/Texture.cpp
@@ -26,7 +26,7 @@ Texture::~Texture() {
 
 void Texture::load(unique_ptr<Asset> res) {
 	SDLContext & ctx = SDLContext::get_instance();
-	this->texture = std::move(ctx.texture_from_path(res->get_canonical()));
+	this->texture = std::move(ctx.texture_from_path(res->get_path()));
 }
 
 int Texture::get_width() const {
diff --git a/src/crepe/facade/Sound.cpp b/src/crepe/facade/Sound.cpp
index 7aa89a9..4d3abf5 100644
--- a/src/crepe/facade/Sound.cpp
+++ b/src/crepe/facade/Sound.cpp
@@ -16,7 +16,7 @@ Sound::Sound(const char * src) {
 	this->load(make_unique<Asset>(src));
 }
 
-void Sound::load(unique_ptr<Asset> res) { this->sample.load(res->get_canonical().c_str()); }
+void Sound::load(unique_ptr<Asset> res) { this->sample.load(res->get_path().c_str()); }
 
 void Sound::play() {
 	SoundContext & ctx = SoundContext::get_instance();
diff --git a/src/crepe/facade/Sound.h b/src/crepe/facade/Sound.h
index 32b6478..4c68f32 100644
--- a/src/crepe/facade/Sound.h
+++ b/src/crepe/facade/Sound.h
@@ -4,7 +4,7 @@
 #include <soloud/soloud.h>
 #include <soloud/soloud_wav.h>
 
-#include "../Asset.h"
+#include "../api/Asset.h"
 
 namespace crepe {
 
diff --git a/src/crepe/system/AnimatorSystem.cpp b/src/crepe/system/AnimatorSystem.cpp
index 9d18873..676e485 100644
--- a/src/crepe/system/AnimatorSystem.cpp
+++ b/src/crepe/system/AnimatorSystem.cpp
@@ -1,6 +1,4 @@
 #include <cstdint>
-#include <functional>
-#include <vector>
 
 #include "api/Animator.h"
 #include "facade/SDLContext.h"
@@ -13,8 +11,7 @@ using namespace crepe;
 void AnimatorSystem::update() {
 	ComponentManager & mgr = this->component_manager;
 
-	std::vector<std::reference_wrapper<Animator>> animations
-		= mgr.get_components_by_type<Animator>();
+	RefVector<Animator> animations = mgr.get_components_by_type<Animator>();
 
 	uint64_t tick = SDLContext::get_instance().get_ticks();
 	for (Animator & a : animations) {
diff --git a/src/crepe/system/ParticleSystem.cpp b/src/crepe/system/ParticleSystem.cpp
index 7316309..fcf7522 100644
--- a/src/crepe/system/ParticleSystem.cpp
+++ b/src/crepe/system/ParticleSystem.cpp
@@ -14,8 +14,7 @@ using namespace crepe;
 void ParticleSystem::update() {
 	// Get all emitters
 	ComponentManager & mgr = this->component_manager;
-	std::vector<std::reference_wrapper<ParticleEmitter>> emitters
-		= mgr.get_components_by_type<ParticleEmitter>();
+	RefVector<ParticleEmitter> emitters = mgr.get_components_by_type<ParticleEmitter>();
 
 	for (ParticleEmitter & emitter : emitters) {
 		// Get transform linked to emitter
diff --git a/src/crepe/system/PhysicsSystem.cpp b/src/crepe/system/PhysicsSystem.cpp
index 4a7dbfb..bcde431 100644
--- a/src/crepe/system/PhysicsSystem.cpp
+++ b/src/crepe/system/PhysicsSystem.cpp
@@ -12,10 +12,8 @@ using namespace crepe;
 
 void PhysicsSystem::update() {
 	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
-		= mgr.get_components_by_type<Transform>();
+	RefVector<Rigidbody> rigidbodies = mgr.get_components_by_type<Rigidbody>();
+	RefVector<Transform> transforms = mgr.get_components_by_type<Transform>();
 
 	double gravity = Config::get_instance().physics.gravity;
 	for (Rigidbody & rigidbody : rigidbodies) {
diff --git a/src/crepe/system/RenderSystem.cpp b/src/crepe/system/RenderSystem.cpp
index e379771..ad510f5 100644
--- a/src/crepe/system/RenderSystem.cpp
+++ b/src/crepe/system/RenderSystem.cpp
@@ -24,7 +24,7 @@ void RenderSystem::present_screen() { this->context.present_screen(); }
 void RenderSystem::update_camera() {
 	ComponentManager & mgr = this->component_manager;
 
-	std::vector<std::reference_wrapper<Camera>> cameras = mgr.get_components_by_type<Camera>();
+	RefVector<Camera> cameras = mgr.get_components_by_type<Camera>();
 
 	if (cameras.size() == 0) throw std::runtime_error("No cameras in current scene");
 
@@ -42,10 +42,8 @@ bool sorting_comparison(const Sprite & a, const Sprite & b) {
 	return false;
 }
 
-std::vector<std::reference_wrapper<Sprite>>
-RenderSystem::sort(std::vector<std::reference_wrapper<Sprite>> & objs) const {
-
-	std::vector<std::reference_wrapper<Sprite>> sorted_objs(objs);
+RefVector<Sprite> RenderSystem::sort(RefVector<Sprite> & objs) const {
+	RefVector<Sprite> sorted_objs(objs);
 	std::sort(sorted_objs.begin(), sorted_objs.end(), sorting_comparison);
 
 	return sorted_objs;
@@ -87,8 +85,8 @@ void RenderSystem::render_normal(const Sprite & sprite, const Transform & tm) {
 void RenderSystem::render() {
 
 	ComponentManager & mgr = this->component_manager;
-	vector<reference_wrapper<Sprite>> sprites = mgr.get_components_by_type<Sprite>();
-	vector<reference_wrapper<Sprite>> sorted_sprites = this->sort(sprites);
+	RefVector<Sprite> sprites = mgr.get_components_by_type<Sprite>();
+	RefVector<Sprite> sorted_sprites = this->sort(sprites);
 
 	for (const Sprite & sprite : sorted_sprites) {
 		if (!sprite.active) continue;
diff --git a/src/crepe/system/RenderSystem.h b/src/crepe/system/RenderSystem.h
index d25a6e3..30b41cf 100644
--- a/src/crepe/system/RenderSystem.h
+++ b/src/crepe/system/RenderSystem.h
@@ -65,8 +65,7 @@ private:
 	 * \param objs the vector that will do a sorting algorithm on 
 	 * \return returns a sorted reference vector
 	 */
-	std::vector<std::reference_wrapper<Sprite>>
-	sort(std::vector<std::reference_wrapper<Sprite>> & objs) const;
+	RefVector<Sprite> sort(RefVector<Sprite> & objs) const;
 
 	/**
 	 * \todo Include color handling for sprites.
diff --git a/src/crepe/system/ScriptSystem.cpp b/src/crepe/system/ScriptSystem.cpp
index c4d724c..c33309c 100644
--- a/src/crepe/system/ScriptSystem.cpp
+++ b/src/crepe/system/ScriptSystem.cpp
@@ -1,6 +1,4 @@
-#include <forward_list>
 #include <functional>
-#include <vector>
 
 #include "../ComponentManager.h"
 #include "../api/BehaviorScript.h"
@@ -14,7 +12,7 @@ using namespace crepe;
 void ScriptSystem::update() {
 	dbg_trace();
 
-	forward_list<reference_wrapper<Script>> scripts = this->get_scripts();
+	RefVector<Script> scripts = this->get_scripts();
 
 	for (auto & script_ref : scripts) {
 		Script & script = script_ref.get();
@@ -26,18 +24,17 @@ void ScriptSystem::update() {
 	}
 }
 
-forward_list<reference_wrapper<Script>> ScriptSystem::get_scripts() const {
-	forward_list<reference_wrapper<Script>> scripts = {};
+RefVector<Script> ScriptSystem::get_scripts() const {
+	RefVector<Script> scripts = {};
 	ComponentManager & mgr = this->component_manager;
-	vector<reference_wrapper<BehaviorScript>> behavior_scripts
-		= mgr.get_components_by_type<BehaviorScript>();
+	RefVector<BehaviorScript> behavior_scripts = mgr.get_components_by_type<BehaviorScript>();
 
 	for (auto behavior_script_ref : behavior_scripts) {
 		BehaviorScript & behavior_script = behavior_script_ref.get();
 		if (!behavior_script.active) continue;
 		Script * script = behavior_script.script.get();
 		if (script == nullptr) continue;
-		scripts.push_front(*script);
+		scripts.push_back(*script);
 	}
 
 	return scripts;
diff --git a/src/crepe/system/ScriptSystem.h b/src/crepe/system/ScriptSystem.h
index deb89cb..32e1fcd 100644
--- a/src/crepe/system/ScriptSystem.h
+++ b/src/crepe/system/ScriptSystem.h
@@ -1,9 +1,9 @@
 #pragma once
 
-#include <forward_list>
-
 #include "System.h"
 
+#include "../types.h"
+
 namespace crepe {
 
 class Script;
@@ -33,7 +33,7 @@ private:
 	 *
 	 * \returns List of active \c Script instances
 	 */
-	std::forward_list<std::reference_wrapper<Script>> get_scripts() const;
+	RefVector<Script> get_scripts() const;
 };
 
 } // namespace crepe
diff --git a/src/crepe/types.h b/src/crepe/types.h
index 0d459e8..914c76c 100644
--- a/src/crepe/types.h
+++ b/src/crepe/types.h
@@ -1,9 +1,16 @@
 #pragma once
 
 #include <cstdint>
+#include <functional>
+#include <vector>
 
 namespace crepe {
 
+//! GameObject ID
 typedef uint32_t game_object_id_t;
 
-}
+//! vector of reference_wrapper
+template <typename T>
+using RefVector = std::vector<std::reference_wrapper<T>>;
+
+} // namespace crepe
diff --git a/src/crepe/util/CMakeLists.txt b/src/crepe/util/CMakeLists.txt
index 4be738a..94ed906 100644
--- a/src/crepe/util/CMakeLists.txt
+++ b/src/crepe/util/CMakeLists.txt
@@ -9,5 +9,7 @@ target_sources(crepe PUBLIC FILE_SET HEADERS FILES
 	Log.hpp
 	Proxy.h
 	Proxy.hpp
+	OptionalRef.h
+	OptionalRef.hpp
 )
 
diff --git a/src/crepe/util/OptionalRef.h b/src/crepe/util/OptionalRef.h
new file mode 100644
index 0000000..3201667
--- /dev/null
+++ b/src/crepe/util/OptionalRef.h
@@ -0,0 +1,58 @@
+#pragma once
+
+namespace crepe {
+
+/**
+ * \brief Optional reference utility
+ *
+ * This class doesn't need to know the full definition of \c T to be used.
+ *
+ * \tparam T Value type
+ */
+template <typename T>
+class OptionalRef {
+public:
+	//! Initialize empty (nonexistant) reference
+	OptionalRef() = default;
+	//! Initialize reference with value
+	OptionalRef(T & ref);
+	/**
+	 * \brief Assign new reference
+	 *
+	 * \param ref Reference to assign
+	 *
+	 * \return Reference to this (required for operator)
+	 */
+	OptionalRef<T> & operator=(T & ref);
+	/**
+	 * \brief Retrieve this reference
+	 *
+	 * \returns Internal reference if it is set
+	 *
+	 * \throws std::runtime_error if this function is called while the reference it not set
+	 */
+	operator T &() const;
+	/**
+	 * \brief Check if this reference is not empty
+	 *
+	 * \returns `true` if reference is set, or `false` if it is not
+	 */
+	explicit operator bool() const noexcept;
+
+	/**
+	 * \brief Make this reference empty
+	 */
+	void clear() noexcept;
+
+private:
+	/**
+	 * \brief Reference to the value of type \c T
+	 *
+	 * \note This raw pointer is *not* managed, and only used as a reference!
+	 */
+	T * ref = nullptr;
+};
+
+} // namespace crepe
+
+#include "OptionalRef.hpp"
diff --git a/src/crepe/util/OptionalRef.hpp b/src/crepe/util/OptionalRef.hpp
new file mode 100644
index 0000000..4608c9e
--- /dev/null
+++ b/src/crepe/util/OptionalRef.hpp
@@ -0,0 +1,37 @@
+#pragma once
+
+#include <stdexcept>
+
+#include "OptionalRef.h"
+
+namespace crepe {
+
+template <typename T>
+OptionalRef<T>::OptionalRef(T & ref) {
+	this->ref = &ref;
+}
+
+template <typename T>
+OptionalRef<T>::operator T &() const {
+	if (this->ref == nullptr)
+		throw std::runtime_error("OptionalRef: attempt to dereference nullptr");
+	return *this->ref;
+}
+
+template <typename T>
+OptionalRef<T> & OptionalRef<T>::operator=(T & ref) {
+	this->ref = &ref;
+	return *this;
+}
+
+template <typename T>
+OptionalRef<T>::operator bool() const noexcept {
+	return this->ref != nullptr;
+}
+
+template <typename T>
+void OptionalRef<T>::clear() noexcept {
+	this->ref = nullptr;
+}
+
+} // namespace crepe