Make object holds a constant reference to its material.

1 files changed, 1 insertions, 1 deletions

diff --git a/object.h b/object.h
index 7f35237..ab79423 100644
--- a/object.h
+++ b/object.h
@@ -41,7 +41,7 @@ public:
     virtual ~object() = default;
 
     // Get this object's material.
-    virtual material &get_material() const = 0;
+    virtual const material & get_material() const = 0;
 };