diff --git a/dlib/general_hash/count_bits.h b/dlib/general_hash/count_bits.h
index 9b0529502..01acad2ab 100644
--- a/dlib/general_hash/count_bits.h
+++ b/dlib/general_hash/count_bits.h
@@ -54,6 +54,26 @@ namespace dlib
         return count_bits(a^b);
     }
 
+// ----------------------------------------------------------------------------------------
+
+    template <
+        typename T
+        >
+    T hamming_distance (
+        const std::pair<T,T>& a,
+        const std::pair<T,T>& b
+    )
+    /*!
+        requires
+            - T is an unsigned integral type or a std::pair that, recursively, eventually
+              contains unsigned integral types.
+        ensures
+            - returns the number of bits which differ between a and b.
+    !*/
+    {
+        return hamming_distance(a.first,b.first) + hamming_distance(a.second, b.second);
+    }
+
 // ----------------------------------------------------------------------------------------
 
 }