From 90fc7638aee23270fe005f86dd1b492d67015c13 Mon Sep 17 00:00:00 2001
From: Mikko Rasa <tdb@tdb.fi>
Date: Tue, 20 Apr 2021 18:11:45 +0300
Subject: [PATCH] Add equality comparison for Variant

Besides value comparison, it's also possible to check if two Variant
objects have the same type.
---
 source/core/meta.h    | 12 ++++++++++++
 source/core/variant.h | 21 +++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/source/core/meta.h b/source/core/meta.h
index 8015f42..6998c5d 100644
--- a/source/core/meta.h
+++ b/source/core/meta.h
@@ -58,6 +58,18 @@ struct Sfinae
 	};
 };
 
+
+struct CheckEqualityComparable: Sfinae
+{
+	static int &v;
+	template<typename T>
+	static Yes f(int (*)[sizeof(reinterpret_cast<const T &>(v)==reinterpret_cast<const T &>(v))]);
+	using Sfinae::f;
+};
+
+template<typename T>
+struct IsEqualityComparable: Sfinae::Evaluate<CheckEqualityComparable, T> { };
+
 } // namespace Msp
 
 #endif
diff --git a/source/core/variant.h b/source/core/variant.h
index 515350d..2dd9732 100644
--- a/source/core/variant.h
+++ b/source/core/variant.h
@@ -24,6 +24,8 @@ private:
 
 		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;
 	};
 
 	template<typename T>
@@ -35,6 +37,16 @@ private:
 
 		virtual const std::type_info &type_id() const { return typeid(T); }
 		virtual StoreBase *clone() const { return new Store<T>(data); }
+		virtual bool type_equals(const StoreBase &s) const { return dynamic_cast<const Store<T> *>(&s); }
+		virtual bool value_equals(const StoreBase &s) const { return value_equals_<T>(s); }
+
+		template<typename U>
+		typename EnableIf<IsEqualityComparable<U>::value, bool>::Yes value_equals_(const StoreBase &s) const
+		{ const Store<T> *t = dynamic_cast<const Store<T> *>(&s); return (t && t->data==data); }
+
+		template<typename U>
+		typename EnableIf<IsEqualityComparable<U>::value, bool>::No value_equals_(const StoreBase &) const
+		{ return false; }
 	};
 
 	StoreBase *store;
@@ -91,6 +103,15 @@ public:
 		return dynamic_cast<Store<typename RemoveConst<T>::Type> *>(store)!=0;
 	}
 
+	bool check_same_type(const Variant &v) const
+	{ return store && v.store && store->type_equals(*v.store); }
+
+	bool operator==(const Variant &v) const
+	{ return store && v.store && store->value_equals(*v.store); }
+
+	bool operator!=(const Variant &v) const
+	{ return !(operator==(v)); }
+
 	template<typename T>
 	operator T() const
 	{ return value<T>(); }
-- 
2.43.0