From 347b13ee60ea35b11aff0efa8814f74a7c2e244d Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Sun, 12 May 2019 07:51:42 +1000
Subject: [PATCH] coord/comparator: allow delayed and defaulted deduction of
 arguments

---
 coord/comparator.hpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/coord/comparator.hpp b/coord/comparator.hpp
index 22f3b082..1efd620a 100644
--- a/coord/comparator.hpp
+++ b/coord/comparator.hpp
@@ -15,6 +15,11 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 namespace cruft::coord {
+    /// A StrictWeakOrder over coordinate types.
+    template <typename CoordT = void>
+    struct ordering;
+
+
     /// A StrictWeakOrder over coordinate types.
     ///
     /// The comparison operates across successive elements of the two
@@ -35,4 +40,18 @@ namespace cruft::coord {
             return false;
         }
     };
+
+
+    /// A StrictWeakOrder over coordinate types.
+    ///
+    /// Deduction is deferred until the function is called, in the same way
+    /// that std::less<> operates.
+    template <>
+    struct ordering<void> {
+        template <typename CoordT>
+        bool operator() (CoordT const &a, CoordT const &b) const noexcept
+        {
+            return ordering<CoordT> {} (a, b);
+        }
+    };
 }