FixedSizeQueue: Bugfixes and improvements

- Fixed a bug where pushing items over queue's size left it in a corrupted state
- For non-trivial types, have clear() and pop() run destructors
- Added emplace(args...)
- Added empty()

FixedSizeQueue has semantics of a circular buffer,
so pushing items continuously is expected to keep overwriting oldest elements gracefully.

Tests have been updated to verify correctness of a previously bugged behaviour
and to verify correctness of destructing non-trivial types

Source/UnitTests/Common/FixedSizeQueueTest.cpp

@@ -31,3 +31,85 @@ TEST(FixedSizeQueue, Simple)
 
   EXPECT_EQ(0u, q.size());
 }
+
+TEST(FixedSizeQueue, RingBuffer)
+{
+  // Testing if queue works when used as a ring buffer
+  FixedSizeQueue<int, 5> q;
+
+  EXPECT_EQ(0u, q.size());
+
+  q.push(0);
+  q.push(1);
+  q.push(2);
+  q.push(3);
+  q.push(4);
+  q.push(5);
+
+  EXPECT_EQ(5u, q.size());
+  EXPECT_EQ(1, q.pop_front());
+
+  EXPECT_EQ(4u, q.size());
+}
+
+// Local classes cannot have static fields,
+// therefore this has to be declared in global scope.
+class NonTrivialTypeTestData
+{
+public:
+  static inline int num_objects = 0;
+  static inline int total_constructed = 0;
+  static inline int default_constructed = 0;
+  static inline int total_destructed = 0;
+
+  NonTrivialTypeTestData()
+  {
+    num_objects++;
+    total_constructed++;
+    default_constructed++;
+  }
+
+  NonTrivialTypeTestData(int /*val*/)
+  {
+    num_objects++;
+    total_constructed++;
+  }
+
+  ~NonTrivialTypeTestData()
+  {
+    num_objects--;
+    total_destructed++;
+  }
+};
+
+TEST(FixedSizeQueue, NonTrivialTypes)
+{
+  // Testing if construction/destruction of non-trivial types happens as expected
+  FixedSizeQueue<NonTrivialTypeTestData, 2> q;
+
+  EXPECT_EQ(0u, q.size());
+
+  EXPECT_EQ(2, NonTrivialTypeTestData::num_objects);
+  EXPECT_EQ(2, NonTrivialTypeTestData::total_constructed);
+  EXPECT_EQ(2, NonTrivialTypeTestData::default_constructed);
+  EXPECT_EQ(0, NonTrivialTypeTestData::total_destructed);
+
+  q.emplace(4);
+  q.emplace(6);
+  q.emplace(8);
+
+  EXPECT_EQ(2, NonTrivialTypeTestData::num_objects);
+  EXPECT_EQ(2 + 3, NonTrivialTypeTestData::total_constructed);
+  EXPECT_EQ(2, NonTrivialTypeTestData::default_constructed);
+  EXPECT_EQ(3, NonTrivialTypeTestData::total_destructed);
+  EXPECT_EQ(2u, q.size());
+
+  q.pop();
+  q.pop();
+
+  EXPECT_EQ(2, NonTrivialTypeTestData::num_objects);
+  EXPECT_EQ(2 + 3 + 2, NonTrivialTypeTestData::total_constructed);
+  EXPECT_EQ(2 + 2, NonTrivialTypeTestData::default_constructed);
+  EXPECT_EQ(3 + 2, NonTrivialTypeTestData::total_destructed);
+  EXPECT_EQ(0u, q.size());
+}