Make RelationRange a proper view (#727)

Author: m-fila

include/podio/RelationRange.h

@@ -1,7 +1,7 @@
 #ifndef PODIO_RELATIONRANGE_H
 #define PODIO_RELATIONRANGE_H
 
-#include <iterator>
+#include <ranges>
 #include <vector>
 
 namespace podio {
@@ -57,4 +57,11 @@ class RelationRange {
 };
 } // namespace podio
 
+// Opt-in to view concept
+template <typename ReferenceType>
+inline constexpr bool std::ranges::enable_view<podio::RelationRange<ReferenceType>> = true;
+// Opt-in to borrowed_range concept
+template <typename ReferenceType>
+inline constexpr bool std::ranges::enable_borrowed_range<podio::RelationRange<ReferenceType>> = true;
+
 #endif // PODIO_RELATIONRANGE_H

tests/unittests/std_interoperability.cpp

@@ -1,7 +1,9 @@
 #include "datamodel/ExampleHit.h"
 #include "datamodel/ExampleHitCollection.h"
 #include "datamodel/MutableExampleHit.h"
+
 #include "podio/LinkCollection.h"
+#include "podio/RelationRange.h"
 
 #include <catch2/catch_test_macros.hpp>
 
@@ -1195,5 +1197,16 @@ TEST_CASE("LinkCollection and range concepts", "[links][iterator][std]") {
   STATIC_REQUIRE(std::ranges::viewable_range<link_collection>);
 }
 
+TEST_CASE("RelationRange as range", "[relations][ranges][std]") {
+  using relation_range = podio::RelationRange<ExampleHit>;
+
+  STATIC_REQUIRE(std::ranges::contiguous_range<relation_range>);
+  STATIC_REQUIRE(std::ranges::sized_range<relation_range>);
+  STATIC_REQUIRE(std::ranges::common_range<relation_range>);
+  STATIC_REQUIRE(std::ranges::viewable_range<relation_range>);
+  STATIC_REQUIRE(std::ranges::view<relation_range>);
+  STATIC_REQUIRE(std::ranges::borrowed_range<relation_range>);
+}
+
 #undef DOCUMENTED_STATIC_FAILURE
 #undef DOCUMENTED_FAILURE