From 53a4d91a23af243397128775bb770fc9eef293c3 Mon Sep 17 00:00:00 2001
From: ADAM David Alan Martin <adam@recursive.engineer>
Date: Sun, 31 Mar 2024 10:33:03 -0400
Subject: [PATCH] Add a unique pointer dynamic cast utility.

---
 Utility/CMakeLists.txt                        |  1 +
 Utility/derived_pointer_cast.h                | 41 +++++++++++++
 Utility/derived_pointer_cast.test/0.cc        | 58 +++++++++++++++++++
 .../derived_pointer_cast.test/CMakeLists.txt  |  1 +
 4 files changed, 101 insertions(+)
 create mode 100644 Utility/derived_pointer_cast.h
 create mode 100644 Utility/derived_pointer_cast.test/0.cc
 create mode 100644 Utility/derived_pointer_cast.test/CMakeLists.txt

diff --git a/Utility/CMakeLists.txt b/Utility/CMakeLists.txt
index 68244fa..e59a3c6 100644
--- a/Utility/CMakeLists.txt
+++ b/Utility/CMakeLists.txt
@@ -2,3 +2,4 @@ target_sources( alepha PRIVATE
 	fancyTypeName.cc
 )
 
+add_subdirectory( derived_pointer_cast.test )
diff --git a/Utility/derived_pointer_cast.h b/Utility/derived_pointer_cast.h
new file mode 100644
index 0000000..f112d98
--- /dev/null
+++ b/Utility/derived_pointer_cast.h
@@ -0,0 +1,41 @@
+static_assert( __cplusplus > 2020'99 );
+
+#pragma once
+
+#include <Alepha/Alepha.h>
+
+#include <memory>
+
+namespace Alepha::Hydrogen::Utility  ::detail::  derived_pointer_cast_m
+{
+	inline namespace exports
+	{
+		template< typename Target, typename Source >
+		std::unique_ptr< Target > derived_pointer_cast( std::unique_ptr< Source > source );
+	}
+
+	template< typename Target, typename Source >
+	std::unique_ptr< Target >
+	exports::derived_pointer_cast( std::unique_ptr< Source > source )
+	{
+		if( Target *const conv= dynamic_cast< Target * >( source.get() ); not conv )
+		{
+			// TODO: More precise Alepha exception here.
+			throw std::bad_cast{};
+		}
+		else
+		{
+			// It's safe to release here, because we're taking ownership below.
+			source.release();
+
+			// We reuse the conversion pointer so we can avoid a double-trip thru the
+			// RTTI pathway.
+			return std::unique_ptr< Target >{ conv };
+		}
+	}
+}
+
+namespace Alepha::Hydrogen::Utility::inline exports::inline derived_pointer_cast_m
+{
+	using namespace detail::derived_pointer_cast_m::exports;
+}
diff --git a/Utility/derived_pointer_cast.test/0.cc b/Utility/derived_pointer_cast.test/0.cc
new file mode 100644
index 0000000..5cdfcca
--- /dev/null
+++ b/Utility/derived_pointer_cast.test/0.cc
@@ -0,0 +1,58 @@
+static_assert( __cplusplus > 2020'99 );
+
+#include "../derived_pointer_cast.h"
+
+#include <Alepha/Testing/TableTest.h>
+#include <Alepha/Testing/test.h>
+
+#include <Alepha/Utility/evaluation_helpers.h>
+
+static auto init= Alepha::Utility::enroll <=[]
+{
+	using namespace Alepha::Testing::exports;
+
+	struct Base
+	{
+		virtual ~Base()= default;
+	};
+
+	struct Derived : Base {};
+
+	struct Derived2 : Base {};
+
+	"Can `derived_pointer_cast` work on simple cases?"_test <=TableTest
+	<
+		[]( std::function< std::unique_ptr< Base >() > makeBase )
+		{
+			auto base= makeBase();
+			return static_cast< bool >( Alepha::Utility::derived_pointer_cast< Derived >( std::move( base ) ) );
+		}
+	>
+	::Cases
+	{
+		{ "Is derived", { [] { return std::make_unique< Derived >(); } }, true },
+		{ "Not derived", { [] { return std::make_unique< Base >(); } }, std::type_identity< std::bad_cast >{} },
+		{ "Other derived", { [] { return std::make_unique< Derived2 >(); } }, std::type_identity< std::bad_cast >{} },
+	};
+
+	struct Alien
+	{
+		virtual ~Alien()= default;
+	};
+
+	struct AlienDerived : Alien {};
+
+	"Alien Test"_test <=TableTest
+	<
+		[]( std::function< std::unique_ptr< Alien >() > makeBase )
+		{
+			auto base= makeBase();
+			return static_cast< bool >( Alepha::Utility::derived_pointer_cast< Derived >( std::move( base ) ) );
+		}
+	>
+	::Cases
+	{
+		{ "Alien", { [] { return std::make_unique< Alien >(); } }, std::type_identity< std::bad_cast >{} },
+		{ "Alien Derived", { [] { return std::make_unique< AlienDerived >(); } }, std::type_identity< std::bad_cast >{} },
+	};
+};
diff --git a/Utility/derived_pointer_cast.test/CMakeLists.txt b/Utility/derived_pointer_cast.test/CMakeLists.txt
new file mode 100644
index 0000000..b099603
--- /dev/null
+++ b/Utility/derived_pointer_cast.test/CMakeLists.txt
@@ -0,0 +1 @@
+unit_test( 0 )