Avoid argument copying in WTF hash-based containers

Source/wtf/HashMapTest.cpp

Index: Source/wtf/HashMapTest.cpp
diff --git a/Source/wtf/HashMapTest.cpp b/Source/wtf/HashMapTest.cpp
index cbdbca6eb6c1c11b65ad3537b508e6267e0067ea..fff8121e7c1eb2ed525425fd0a8fcc03c2225aa3 100644
--- a/Source/wtf/HashMapTest.cpp
+++ b/Source/wtf/HashMapTest.cpp
@@ -303,6 +303,56 @@ TEST(HashMapTest, ValueTypeDestructed)
     EXPECT_EQ(0, InstanceCounter::counter);
 }
 
+class CopyCounter {
+public:
+    CopyCounter() = default;
+    explicit CopyCounter(int data) : m_data(data) { }
+
+    CopyCounter(const CopyCounter& copy) : m_data(copy.m_data) { ++counter; }
+
+    int data() const { return m_data; }
+
+    bool isHashTableDeletedValue() const { return m_data == HashTableDeletedValue; }
+
+    static int counter;

[tkent, 2015/06/18 23:41:50]

counter -> s_counter

+private:
+    int m_data;
+};
+
+int CopyCounter::counter = 0;
+
+struct CopyCounterHash {
+    static unsigned hash(const CopyCounter& key)
+    {
+        return IntHash<unsigned>::hash(key.data());
+    }
+
+    static bool equal(const CopyCounter& a, const CopyCounter& b)
+    {
+        return a.data() == b.data();
+    }
+
+    static const bool safeToCompareToEmptyOrDeleted = true;
+};
+
+
+struct CopyCounterHashTraits: SimpleClassHashTraits<CopyCounter> {
+    static const bool hasIsEmptyValueFunction = true;
+    static bool isEmptyValue(const CopyCounter& counter) { return !counter.data(); }
+};
+
+
+TEST(HashMapTest, LookupNoKeyCopies)
+{
+    HashMap<CopyCounter, int, CopyCounterHash, CopyCounterHashTraits> map;
+
+    map.contains(CopyCounter(1));
+    auto it = map.find(CopyCounter(1));
+    ALLOW_UNUSED_LOCAL(it);
+
+    EXPECT_EQ(0, CopyCounter::counter);
+}
+
 } // anonymous namespace
 
 } // namespace WTF