tombstone: Extract out relational operators

This patch extracts out the relational operators in struct tombstone
to a class capable of generating them from a tri-compare function.

Signed-off-by: Duarte Nunes <duarte@scylladb.com>

tombstone.hh

@@ -26,12 +26,13 @@
 #include "timestamp.hh"
 #include "gc_clock.hh"
 #include "hashing.hh"
+#include "utils/with_relational_operators.hh"
 
 /**
  * Represents deletion operation. Can be commuted with other tombstones via apply() method.
  * Can be empty.
  */
-struct tombstone final {
+struct tombstone final : public with_relational_operators<tombstone> {
     api::timestamp_type timestamp;
     gc_clock::time_point deletion_time;
 
@@ -58,30 +59,6 @@ struct tombstone final {
         }
     }
 
-    bool operator<(const tombstone& t) const {
-        return compare(t) < 0;
-    }
-
-    bool operator<=(const tombstone& t) const {
-        return compare(t) <= 0;
-    }
-
-    bool operator>(const tombstone& t) const {
-        return compare(t) > 0;
-    }
-
-    bool operator>=(const tombstone& t) const {
-        return compare(t) >= 0;
-    }
-
-    bool operator==(const tombstone& t) const {
-        return compare(t) == 0;
-    }
-
-    bool operator!=(const tombstone& t) const {
-        return compare(t) != 0;
-    }
-
     explicit operator bool() const {
         return timestamp != api::missing_timestamp;
     }

utils/with_relational_operators.hh

@@ -0,0 +1,67 @@
+/*
+ * Copyright (C) 2017 ScyllaDB
+ */
+
+/*
+ * This file is part of Scylla.
+ *
+ * Scylla is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Scylla is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Scylla.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <seastar/util/gcc6-concepts.hh>
+#include <type_traits>
+
+GCC6_CONCEPT(
+template<typename T>
+concept bool HasTriCompare =
+    requires(const T& t) {
+        { t.compare(t) } -> int;
+    } && std::is_same<std::result_of_t<decltype(&T::compare)(T, T)>, int>::value; //FIXME: #1449
+)
+
+template<typename T>
+class with_relational_operators {
+private:
+    template<typename U>
+    GCC6_CONCEPT( requires HasTriCompare<U> )
+    int do_compare(const U& t) const {
+        return static_cast<const U*>(this)->compare(t);
+    }
+public:
+    bool operator<(const T& t) const {
+        return do_compare(t) < 0;
+    }
+
+    bool operator<=(const T& t) const {
+        return do_compare(t) <= 0;
+    }
+
+    bool operator>(const T& t) const {
+        return do_compare(t) > 0;
+    }
+
+    bool operator>=(const T& t) const {
+        return do_compare(t) >= 0;
+    }
+
+    bool operator==(const T& t) const {
+        return do_compare(t) == 0;
+    }
+
+    bool operator!=(const T& t) const {
+        return do_compare(t) != 0;
+    }
+};