From 1b2ea5c2a6effb2948fc54a59c37dc9a12bcf1cf Mon Sep 17 00:00:00 2001
From: _Redstone_c_ <myredstone1024@gmail.com>
Date: Fri, 20 Jan 2023 21:21:26 +0800
Subject: [PATCH] feat(templates): add TRetainedRef and the corresponding
 testing

---
 .../Private/Testing/TemplatesTesting.cpp      | 16 ++--
 .../Public/Templates/RetainedReference.h      | 73 +++++++++++++++++++
 .../Source/Public/Templates/Templates.h       |  1 +
 3 files changed, 85 insertions(+), 5 deletions(-)
 create mode 100644 Redcraft.Utility/Source/Public/Templates/RetainedReference.h

diff --git a/Redcraft.Utility/Source/Private/Testing/TemplatesTesting.cpp b/Redcraft.Utility/Source/Private/Testing/TemplatesTesting.cpp
index 2c3d8e8..0d68e86 100644
--- a/Redcraft.Utility/Source/Private/Testing/TemplatesTesting.cpp
+++ b/Redcraft.Utility/Source/Private/Testing/TemplatesTesting.cpp
@@ -1475,8 +1475,8 @@ void TestPropagateConst()
 	{
 		struct FTestA
 		{
-			void Check(bool bFlag) { check(!bFlag); }
-			void Check(bool bFlag) const { check(bFlag); }
+			void Check(bool bFlag)       { always_check(!bFlag); }
+			void Check(bool bFlag) const { always_check( bFlag); }
 		};
 
 		struct FTestB
@@ -1504,9 +1504,9 @@ void TestPropagateConst()
 		TempA = TempB;
 		TempB = TempC;
 
-		check(TempA.IsValid());
-		check(TempA == &IntA);
-		check(TempB == TempC);
+		always_check(TempA.IsValid());
+		always_check(TempA == &IntA);
+		always_check(TempB == TempC);
 	}
 }
 
@@ -1544,6 +1544,12 @@ void TestMiscTemplates()
 	always_check(TestFunctionB(AddressOf(ObjectA)) == 0);
 	always_check(AddressOf(TestMiscTemplates) == &TestMiscTemplates);
 
+	struct FTestRetainedRef { explicit FTestRetainedRef(TRetainedRef<const int64> InRef) { } };
+	
+	int64 IntA;
+	FTestRetainedRef TempA(IntA);
+//	FTestRetainedRef TempB(114514);
+
 }
 
 NAMESPACE_END(Testing)
diff --git a/Redcraft.Utility/Source/Public/Templates/RetainedReference.h b/Redcraft.Utility/Source/Public/Templates/RetainedReference.h
new file mode 100644
index 0000000..b633f86
--- /dev/null
+++ b/Redcraft.Utility/Source/Public/Templates/RetainedReference.h
@@ -0,0 +1,73 @@
+#pragma once
+
+#include "CoreTypes.h"
+#include "Templates/Utility.h"
+#include "TypeTraits/TypeTraits.h"
+
+NAMESPACE_REDCRAFT_BEGIN
+NAMESPACE_MODULE_BEGIN(Redcraft)
+NAMESPACE_MODULE_BEGIN(Utility)
+
+/**
+ * A helper class which replaces T& as a function parameter when the reference is
+ * intended to be retained by the function (e.g. as a class member).  The benefit
+ * of this class is that it is a compile error to pass an rvalue which might otherwise
+ * bind to a const reference, which is dangerous when the reference is retained.
+ *
+ *	struct FRAIIType
+ *	{
+ *		explicit FRAIIType(TRetainedRef<const FThing> InThing) : Ref(InThing) { }
+ *
+ *		void DoSomething() { Ref.Something(); }
+ *
+ *		FThing& Ref;
+ *	};
+ *
+ */
+template <typename T>
+struct TRetainedRef
+{
+	/** Retain a non-const lvalue reference. */
+	FORCEINLINE constexpr TRetainedRef(T& InRef) : Ref(InRef) { }
+
+	/** Can't retain a rvalue reference. */
+	TRetainedRef(const T& ) = delete;
+	TRetainedRef(      T&&) = delete;
+	TRetainedRef(const T&&) = delete;
+
+	/** @return The managed reference. */
+	FORCEINLINE constexpr operator T&() const { return Ref; }
+	FORCEINLINE constexpr      T& Get() const { return Ref; }
+
+private:
+
+	T& Ref;
+
+};
+
+template <typename T>
+struct TRetainedRef<const T>
+{
+	/** Retain a non-const lvalue reference. */
+	FORCEINLINE constexpr TRetainedRef(T& InRef) : Ref(InRef) { }
+
+	/** Retain a const lvalue reference. */
+	FORCEINLINE constexpr TRetainedRef(const T& InRef) : Ref(InRef) { }
+
+	/** Can't retain a rvalue reference. */
+	TRetainedRef(      T&&) = delete;
+	TRetainedRef(const T&&) = delete;
+
+	/** @return The managed reference. */
+	FORCEINLINE constexpr operator const T&() const { return Ref; }
+	FORCEINLINE constexpr      const T& Get() const { return Ref; }
+
+private:
+
+	const T& Ref;
+
+};
+
+NAMESPACE_MODULE_END(Utility)
+NAMESPACE_MODULE_END(Redcraft)
+NAMESPACE_REDCRAFT_END
diff --git a/Redcraft.Utility/Source/Public/Templates/Templates.h b/Redcraft.Utility/Source/Public/Templates/Templates.h
index 5039e9f..7318a61 100644
--- a/Redcraft.Utility/Source/Public/Templates/Templates.h
+++ b/Redcraft.Utility/Source/Public/Templates/Templates.h
@@ -16,3 +16,4 @@
 #include "Templates/Atomic.h"
 #include "Templates/ScopeHelper.h"
 #include "Templates/PropagateConst.h"
+#include "Templates/RetainedReference.h"