From: Mikko Rasa <tdb@tdb.fi>
Date: Fri, 23 Dec 2022 12:27:01 +0000 (+0200)
Subject: Rewrite the Variant class
X-Git-Url: http://git.tdb.fi/?a=commitdiff_plain;h=e42c69a8d3416be5071637cd4e241593458cb941;p=libs%2Fcore.git

Rewrite the Variant class

It now uses type erasure and small object optimization
---

diff --git a/source/core/variant.h b/source/core/variant.h
index eaf5a34..04c372b 100644
--- a/source/core/variant.h
+++ b/source/core/variant.h
@@ -17,109 +17,159 @@ public:
 
 class Variant
 {
-private:
-	struct StoreBase
-	{
-		virtual ~StoreBase() { }
+public:
+	static constexpr unsigned INTERNAL_SIZE = 2*sizeof(void *);
 
-		virtual const std::type_info &type_id() const = 0;
-		virtual StoreBase *clone() const = 0;
-		virtual bool type_equals(const StoreBase &) const = 0;
-		virtual bool value_equals(const StoreBase &) const = 0;
+	struct Functions
+	{
+		const std::type_info &(*get_type)();
+		bool (*compare)(const char *, const char *);
+		void (*clone)(char *, const char *);
+		void (*destroy)(char *);
 	};
 
+private:
+	const Functions *funcs = nullptr;
+	alignas(void *) char storage[INTERNAL_SIZE];
+
+public:
+	Variant() = default;
 	template<typename T>
-	struct Store: public StoreBase
-	{
-		T data;
+	Variant(const T &v) { assign(v); }
+	Variant(const Variant &v) { copy_from(v); }
+	~Variant() { if(funcs) funcs->destroy(storage); }
 
-		Store(const T &d): data(d) { }
+	template<typename T>
+	Variant &operator=(const T &v) { assign(v); return *this; }
 
-		const std::type_info &type_id() const override { return typeid(T); }
-		StoreBase *clone() const override { return new Store<T>(data); }
-		bool type_equals(const StoreBase &s) const override { return dynamic_cast<const Store<T> *>(&s); }
-		bool value_equals(const StoreBase &s) const override { return _value_equals<T>(s); }
+	Variant &operator=(const Variant &v) { if(&v!=this) copy_from(v); return *this; }
 
-		template<typename U>
-		typename std::enable_if<IsEqualityComparable<U>::value, bool>::type _value_equals(const StoreBase &s) const
-		{ const Store<T> *t = dynamic_cast<const Store<T> *>(&s); return (t && t->data==data); }
+private:
+	template<typename T>
+	void assign(const T &);
 
-		template<typename U>
-		typename std::enable_if<!IsEqualityComparable<U>::value, bool>::type _value_equals(const StoreBase &) const
-		{ return false; }
-	};
+	void copy_from(const Variant &);
 
-	StoreBase *store = nullptr;
+	template<typename T>
+	T &get();
 
 public:
-	Variant() = default;
 	template<typename T>
-	Variant(const T &v): store(new Store<typename std::remove_cv<T>::type>(v)) { }
-	Variant(const Variant &v): store(v.store ? v.store->clone() : nullptr) { }
-	~Variant() { delete store; }
+	T &value() { return get<T>(); }
 
 	template<typename T>
-	Variant &operator=(const T &v)
-	{
-		delete store;
-		store = new Store<typename std::remove_cv<T>::type>(v);
-		return *this;
-	}
+	const T &value() const { return const_cast<Variant *>(this)->get<T>(); }
 
-	Variant &operator=(const Variant &v)
-	{
-		if(&v==this)
-			return *this;
+	template<typename T>
+	bool check_type() const { return funcs==get_functions<typename std::remove_cv<T>::type>(); }
 
-		delete store;
-		store = (v.store ? v.store->clone() : nullptr);
-		return *this;
-	}
+	bool check_same_type(const Variant &v) const { return (funcs && funcs==v.funcs); }
+
+	bool operator==(const Variant &v) const { return (has_same_type(v) && funcs->compare(storage, v.storage)); }
+	bool operator!=(const Variant &v) const { return !(operator==(v)); }
+
+	template<typename T>
+	operator T() const { return value<T>(); }
 
 private:
 	template<typename T>
-	Store<typename std::remove_cv<T>::type> *get_typed_store() const
-	{
-		typedef typename std::remove_cv<T>::type NCT;
-		Store<NCT> *s = dynamic_cast<Store<NCT> *>(store);
-		if(!s)
-			throw type_mismatch(typeid(T), (store ? store->type_id() : typeid(void)));
-		return s;
-	}
+	static constexpr bool is_small() { return (sizeof(T)<=INTERNAL_SIZE && alignof(T)<=alignof(void *)); }
+
+	template<typename T, typename U>
+	using EnableSmall = typename std::enable_if<is_small<T>(), U>::type;
+
+	template<typename T, typename U>
+	using EnableLarge = typename std::enable_if<!is_small<T>(), U>::type;
 
-public:
 	template<typename T>
-	T &value()
-	{
-		return get_typed_store<T>()->data;
-	}
+	static const Functions *get_functions();
 
 	template<typename T>
-	const T &value() const
-	{
-		return get_typed_store<T>()->data;
-	}
+	static const std::type_info &get_type() { return typeid(T); }
 
 	template<typename T>
-	bool check_type() const
-	{
-		return dynamic_cast<Store<typename std::remove_cv<T>::type> *>(store);
-	}
+	static EnableSmall<T, void> create(char *s, const T &v)
+	{ new(s) T(v); }
 
-	bool check_same_type(const Variant &v) const
-	{ return store && v.store && store->type_equals(*v.store); }
+	template<typename T>
+	static EnableLarge<T, void> create(char *s, const T &v)
+	{ *reinterpret_cast<T **>(s) = new T(v); }
 
-	bool operator==(const Variant &v) const
-	{ return store && v.store && store->value_equals(*v.store); }
+	template<typename T>
+	static typename std::enable_if<!IsEqualityComparable<T>::value, bool>::type compare(const char *, const char *)
+	{ return false; }
 
-	bool operator!=(const Variant &v) const
-	{ return !(operator==(v)); }
+	template<typename T>
+	static typename std::enable_if<IsEqualityComparable<T>::value, EnableSmall<T, bool>>::type compare(const char *s1, const char *s2)
+	{ return *reinterpret_cast<const T *>(s1)==*reinterpret_cast<const T *>(s2); }
 
 	template<typename T>
-	operator T() const
-	{ return value<T>(); }
+	static typename std::enable_if<IsEqualityComparable<T>::value, EnableLarge<T, bool>>::type compare(const char *s1, const char *s2)
+	{ return **reinterpret_cast<const T *const *>(s1)==**reinterpret_cast<const T *const *>(s2); }
+
+	template<typename T>
+	static EnableSmall<T, void> clone(char *s, const char *v)
+	{ new(s) T(*reinterpret_cast<const T *>(v)); }
+
+	template<typename T>
+	static EnableLarge<T, void> clone(char *s, const char *v)
+	{ *reinterpret_cast<T **>(s) = new T(**reinterpret_cast<const T *const *>(v)); }
+
+	template<typename T>
+	static EnableSmall<T, void> destroy(char *s)
+	{ reinterpret_cast<T *>(s)->~T(); }
+
+	template<typename T>
+	static EnableLarge<T, void> destroy(char *s)
+	{ delete *reinterpret_cast<T **>(s); }
 };
 
+
+template<typename T>
+inline void Variant::assign(const T &value)
+{
+	if(funcs)
+		funcs->destroy(storage);
+
+	funcs = get_functions<typename std::remove_cv<T>::type>();
+	create(storage, value);
+}
+
+inline void Variant::copy_from(const Variant &v)
+{
+	if(funcs)
+		funcs->destroy(storage);
+
+	funcs = v.funcs;
+	if(funcs)
+		funcs->clone(storage, v.storage);
+}
+
+template<typename T>
+inline T &Variant::get()
+{
+	if(!has_type<T>())
+		throw type_mismatch(typeid(T), (funcs ? funcs->get_type() : typeid(void)));
+
+	if(sizeof(T)<=INTERNAL_SIZE)
+		return *reinterpret_cast<T *>(storage);
+	else
+		return **reinterpret_cast<T **>(storage);
+}
+
+template<typename T>
+inline const Variant::Functions *Variant::get_functions()
+{
+	static Functions funcs =
+	{
+		&get_type<T>,
+		&compare<T>,
+		&clone<T>,
+		&destroy<T>
+	};
+	return &funcs;
+}
+
 } // namespace Msp
 
 #endif