BLI: improve various C++ data structures

The changes come from the `functions` branch, where I'm using
these structures a lot.

This also includes a new `BLI::Optional<T>` type, which is similar
to `std::Optional<T>` which can be used when Blender starts using
C++17.

1 files changed, 3 insertions, 3 deletions

diff --git a/source/blender/blenlib/BLI_vector_set.h b/source/blender/blenlib/BLI_vector_set.h
index fb21f7ed987..99d955c60d8 100644
--- a/source/blender/blenlib/BLI_vector_set.h
+++ b/source/blender/blenlib/BLI_vector_set.h
@@ -292,12 +292,12 @@ template<typename T, typename Allocator = GuardedAllocator> class VectorSet {
     return m_elements[index];
   }
 
-  operator ArrayRef<T>() const
+  ArrayRef<T> as_ref() const
   {
-    return m_elements;
+    return *this;
   }
 
-  operator MutableArrayRef<T>()
+  operator ArrayRef<T>() const
   {
     return m_elements;
   }