diff --git a/oop/oop.lua b/oop/oop.lua
index f65faf4..0a723bc 100644
--- a/oop/oop.lua
+++ b/oop/oop.lua
@@ -1,5 +1,11 @@
 local oop = {}
 
+oop.Visibility = {
+    PUBLIC = "public",
+    PROTECTED = "protected",
+    PRIVATE = "private"
+}
+
 local tpack = table.pack or function(...)
     return { n = select("#", ...), ... }
 end
@@ -30,12 +36,12 @@ oop._call_stack = {}
 
 local function normalize_visibility(visibility)
     if visibility == nil then
-        return "public"
+        return oop.Visibility.PUBLIC
     end
-    if visibility == "public" or visibility == "protected" or visibility == "private" then
+    if visibility == oop.Visibility.PUBLIC or visibility == oop.Visibility.PROTECTED or visibility == oop.Visibility.PRIVATE then
         return visibility
     end
-    error("visibility must be 'public', 'protected', or 'private'")
+    error("visibility must be Visibility.PUBLIC, Visibility.PROTECTED, or Visibility.PRIVATE")
 end
 
 local function is_class_table(target)
@@ -58,21 +64,36 @@ local function current_caller()
 end
 
 local function can_access(owner, visibility, caller)
-    if visibility == "public" then
+    if visibility == oop.Visibility.PUBLIC then
         return true
     end
     if not caller or not owner then
         return false
     end
-    if visibility == "private" then
+    if visibility == oop.Visibility.PRIVATE then
         return caller == owner
     end
-    if visibility == "protected" then
+    if visibility == oop.Visibility.PROTECTED then
         return oop.issubclass(caller, owner)
     end
     return false
 end
 
+local function class_name(cls)
+    if type(cls) ~= "table" then
+        return tostring(cls)
+    end
+    return rawget(cls, "__name") or "Anonymous"
+end
+
+local function visibility_error(kind, member, visibility, owner, caller)
+    local from = caller and class_name(caller) or "outside"
+    local where = owner and class_name(owner) or "unknown"
+    local msg = "access violation: " .. kind .. " '" .. tostring(member) .. "' is " .. visibility
+        .. " in " .. where .. " (from " .. from .. ")"
+    error(msg, 3)
+end
+
 function oop.setattr(target, key, value, visibility)
     target[key] = value
     if is_class_table(target) then
@@ -135,7 +156,7 @@ function lookup_visibility(cls, key)
         end
         cur = rawget(cur, "__base")
     end
-    return "public", nil
+    return oop.Visibility.PUBLIC, nil
 end
 
 function oop.class(def, base)
@@ -143,6 +164,7 @@ function oop.class(def, base)
     cls.__name = "Anonymous"
     cls.__base = base
     cls.__visibility = {}
+    cls.visibility = oop.Visibility
     cls.__index = function(self, key)
         local fields = rawget(self, "__fields")
         if fields then
@@ -150,7 +172,7 @@ function oop.class(def, base)
             if field_val ~= nil then
                 local vis, owner = lookup_visibility(cls, key)
                 if not can_access(owner, vis, current_caller()) then
-                    error("attempt to access " .. vis .. " member '" .. tostring(key) .. "'")
+                    visibility_error("field", key, vis, owner, current_caller())
                 end
                 return field_val
             end
@@ -163,7 +185,7 @@ function oop.class(def, base)
         local vis, vis_owner = lookup_visibility(cls, key)
         local access_owner = vis_owner or owner
         if not can_access(access_owner, vis, current_caller()) then
-            error("attempt to access " .. vis .. " member '" .. tostring(key) .. "'")
+            visibility_error("member", key, vis, access_owner, current_caller())
         end
         if type(val) == "function" then
             return function(...)
@@ -179,7 +201,7 @@ function oop.class(def, base)
         end
         local vis, owner = lookup_visibility(cls, key)
         if owner and not can_access(owner, vis, current_caller()) then
-            error("attempt to set " .. vis .. " member '" .. tostring(key) .. "'")
+            visibility_error("field", key, vis, owner, current_caller())
         end
         local fields = rawget(self, "__fields")
         if not fields then
@@ -284,6 +306,7 @@ function oop.install(env)
     target.setattr = oop.setattr
     target.isinstance = oop.isinstance
     target.issubclass = oop.issubclass
+    target.Visibility = oop.Visibility
 end
 
 oop.install()