diff --git a/typeidx.cpp b/typeidx.cpp
index cbd99c2b..5cc349ac 100644
--- a/typeidx.cpp
+++ b/typeidx.cpp
@@ -11,9 +11,17 @@
 #include <atomic>
 
 ///////////////////////////////////////////////////////////////////////////////
+static std::atomic<int> counter;
+
+
+//-----------------------------------------------------------------------------
 int
 cruft::detail::typeidx_next ()
 {
-    static std::atomic<int> counter = 0;
-    return counter++;
+    // We can't return zero, but we want to optimise access to the static
+    // (which means we can't have it locally due to initialisation locking).
+    //
+    // Ensure we use pre-increment so that we can utilise zero static
+    // initialisation instead.
+    return ++counter;
 }
diff --git a/typeidx.hpp b/typeidx.hpp
index 0782b73b..eacc7136 100644
--- a/typeidx.hpp
+++ b/typeidx.hpp
@@ -3,29 +3,31 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  *
- * Copyright 2017 Danny Robson <danny@nerdcruft.net>
+ * Copyright 2017-2019 Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef CRUFT_UTIL_TYPEIDX_HPP
-#define CRUFT_UTIL_TYPEIDX_HPP
+#pragma once
 
 #include <cstdint>
 
 namespace cruft {
     namespace detail { int typeidx_next (void); }
 
-    /// return a globally unique runtime ID for a given type.
+    /// Return a globally unique runtime ID for a given type.
     ///
-    /// this is intended to be used as a lightweight type check for variants
+    /// This is intended to be used as a lightweight type check for variants
     /// and such without requiring RTTI.
     ///
-    /// the identifier is constructed at runtime and is not guaranteed to be
+    /// The identifier is constructed at runtime and is not guaranteed to be
     /// stable across executions (particularly in the case of threads and
-    /// other non-determinism). however it _is_ threadsafe to call this.
+    /// other non-determinism). However it _is_ threadsafe to call this.
     ///
-    /// the range of identifiers is _probably_ contiguous starting from zero.
-    /// this should not be relied upon for correctness, but may be used for
-    /// performance concerns.
+    /// The value 0 will never be returned from this function and can be used
+    /// to indicate an 'unknown' state.
+    ///
+    /// The range of identifiers is _probably_ contiguous. This should not
+    /// be relied upon for correctness, but may be used for performance
+    /// concerns.
     template <typename T>
     int
     typeidx (void)
@@ -34,5 +36,3 @@ namespace cruft {
         return id;
     }
 }
-
-#endif