From 7dda773426881fa54354c23677aaa911608b3eb9 Mon Sep 17 00:00:00 2001
From: Andreas Kling <kling@serenityos.org>
Date: Sat, 4 Sep 2021 00:44:10 +0200
Subject: [PATCH] AK: Add rvalue-ref qualifiers for Optional's value() and
 value_or()

This avoids a value copy when calling value() or value_or() on a
temporary Optional. This is very common when using the HashMap::get()
API like this:

    auto value = hash_map.get(key).value_or(fallback_value);
---
 AK/Optional.h             | 18 +++++++++++++++---
 Tests/AK/TestOptional.cpp | 21 +++++++++++++++++++++
 2 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/AK/Optional.h b/AK/Optional.h
index ff07456591..bbe0d38ba6 100644
--- a/AK/Optional.h
+++ b/AK/Optional.h
@@ -127,18 +127,23 @@ public:
 
     [[nodiscard]] ALWAYS_INLINE bool has_value() const { return m_has_value; }
 
-    [[nodiscard]] ALWAYS_INLINE T& value()
+    [[nodiscard]] ALWAYS_INLINE T& value() &
     {
         VERIFY(m_has_value);
         return *__builtin_launder(reinterpret_cast<T*>(&m_storage));
     }
 
-    [[nodiscard]] ALWAYS_INLINE T const& value() const
+    [[nodiscard]] ALWAYS_INLINE T const& value() const&
     {
         VERIFY(m_has_value);
         return *__builtin_launder(reinterpret_cast<T const*>(&m_storage));
     }
 
+    [[nodiscard]] ALWAYS_INLINE T value() &&
+    {
+        return release_value();
+    }
+
     [[nodiscard]] T release_value()
     {
         VERIFY(m_has_value);
@@ -148,13 +153,20 @@ public:
         return released_value;
     }
 
-    [[nodiscard]] ALWAYS_INLINE T value_or(T const& fallback) const
+    [[nodiscard]] ALWAYS_INLINE T value_or(T const& fallback) const&
     {
         if (m_has_value)
             return value();
         return fallback;
     }
 
+    [[nodiscard]] ALWAYS_INLINE T value_or(T&& fallback) &&
+    {
+        if (m_has_value)
+            return move(value());
+        return move(fallback);
+    }
+
     ALWAYS_INLINE T const& operator*() const { return value(); }
     ALWAYS_INLINE T& operator*() { return value(); }
 
diff --git a/Tests/AK/TestOptional.cpp b/Tests/AK/TestOptional.cpp
index 485a37f248..006fe20716 100644
--- a/Tests/AK/TestOptional.cpp
+++ b/Tests/AK/TestOptional.cpp
@@ -34,6 +34,27 @@ TEST_CASE(move_optional)
     EXPECT_EQ(x.has_value(), false);
 }
 
+TEST_CASE(optional_rvalue_ref_qualified_getters)
+{
+    struct DontCopyMe {
+        DontCopyMe() { }
+        ~DontCopyMe() = default;
+        DontCopyMe(DontCopyMe&&) = default;
+        DontCopyMe& operator=(DontCopyMe&&) = default;
+        DontCopyMe(DontCopyMe const&) = delete;
+        DontCopyMe& operator=(DontCopyMe const&) = delete;
+
+        int x { 13 };
+    };
+
+    auto make_an_optional = []() -> Optional<DontCopyMe> {
+        return DontCopyMe {};
+    };
+
+    EXPECT_EQ(make_an_optional().value().x, 13);
+    EXPECT_EQ(make_an_optional().value_or(DontCopyMe {}).x, 13);
+}
+
 TEST_CASE(optional_leak_1)
 {
     struct Structure {