- Document limitation of TArray and silence warnings about non-trivial types being trivially moved

src/utility/tarray.h

@@ -31,6 +31,20 @@
 ** THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 **---------------------------------------------------------------------------
 **
+** NOTE: TArray takes advantage of the assumption that the contained type is
+** able to be trivially moved. The definition of trivially movable by the C++
+** standard is more strict than the actual set of types that can be moved with
+** memmove. For example, FString uses non-trivial constructors/destructor in
+** order to maintain the reference count, but can be "safely" by passed if the
+** opaque destructor call is avoided. Similarly types like TArray itself which
+** only null the owning pointers when moving which can be skipped if the
+** destructor is not called.
+**
+** It is possible that with LTO TArray could be made safe for non-trivial types,
+** but we don't wish to rely on LTO to reach expected performance. The set of
+** types which can not be contained by TArray as a result of this choice is
+** actually extremely small.
+**
 */
 
 
@@ -352,7 +366,8 @@ public:
 			Array[index].~T();
 			if (index < --Count)
 			{
-				memmove (&Array[index], &Array[index+1], sizeof(T)*(Count - index));
+				// Cast to void to assume trivial move
+				memmove ((void*)&Array[index], (const void*)&Array[index+1], sizeof(T)*(Count - index));
 			}
 		}
 	}
@@ -372,7 +387,8 @@ public:
 			Count -= deletecount;
 			if (index < Count)
 			{
-				memmove (&Array[index], &Array[index+deletecount], sizeof(T)*(Count - index));
+				// Cast to void to assume trivial move
+				memmove ((void*)&Array[index], (const void*)&Array[index+deletecount], sizeof(T)*(Count - index));
 			}
 		}
 	}
@@ -394,7 +410,8 @@ public:
 			Resize (Count + 1);
 
 			// Now move items from the index and onward out of the way
-			memmove (&Array[index+1], &Array[index], sizeof(T)*(Count - index - 1));
+			// Cast to void to assume trivial move
+			memmove ((void*)&Array[index+1], (const void*)&Array[index], sizeof(T)*(Count - index - 1));
 
 			// And put the new element in
 			::new ((void *)&Array[index]) T(item);