enum_set: Add iterator

Sometimes it is useful to be able to query for all the members of an
`enum_set`, rather than just add, remove, and query for membership. (The
patch following this one makes use of this in the auth. sub-system).

We use the bitset iterator in Seastar to help with the implementation.

enum_set.hh

@@ -21,6 +21,7 @@
 
 #pragma once
 
+#include <boost/iterator/transform_iterator.hpp>
 #include <seastar/core/bitset-iter.hh>
 
 #include <algorithm>
@@ -146,7 +147,16 @@ private:
     static constexpr unsigned shift_for() {
         return Enum::template sequence_for<Elem>();
     }
+
+    static auto make_iterator(mask_iterator iter) {
+        return boost::make_transform_iterator(std::move(iter), [](typename Enum::sequence_type s) {
+            return enum_type(s);
+        });
+    }
+
 public:
+    using iterator = std::invoke_result_t<decltype(&enum_set::make_iterator), mask_iterator>;
+
     constexpr enum_set() : _mask(0) {}
 
     /**
@@ -229,6 +239,14 @@ public:
         return _mask;
     }
 
+    iterator begin() const {
+        return make_iterator(mask_iterator(_mask));
+    }
+
+    iterator end() const {
+        return make_iterator(mask_iterator(0));
+    }
+
     template<enum_type... items>
     struct frozen {
         template<enum_type first>

tests/enum_set_test.cc

@@ -23,10 +23,26 @@
 
 #include "enum_set.hh"
 
+#include <iostream>
+#include <iterator>
+#include <unordered_set>
+
 #include <boost/test/unit_test.hpp>
 
+#include "to_string.hh"
+
 enum class fruit { apple = 3, pear = 7, banana = 8 };
 
+static std::ostream& operator<<(std::ostream& os, fruit f) {
+    switch (f) {
+        case fruit::apple: os << "apple"; break;
+        case fruit::pear: os << "pear"; break;
+        case fruit::banana: os << "banana"; break;
+    }
+
+    return os;
+}
+
 using fruit_enum = super_enum<fruit, fruit::apple, fruit::pear, fruit::banana>;
 
 //
@@ -91,3 +107,22 @@ BOOST_AUTO_TEST_CASE(set_remove) {
     fs.remove(fruit::pear);
     BOOST_REQUIRE(!fs.contains(fruit::pear));
 }
+
+BOOST_AUTO_TEST_CASE(set_iterator) {
+    const auto fs1 = fruit_set::of<fruit::pear, fruit::banana>();
+
+    std::unordered_set<fruit> us1;
+    std::copy(fs1.begin(), fs1.end(), std::inserter(us1, us1.begin()));
+
+    BOOST_REQUIRE_EQUAL(us1, (std::unordered_set<fruit>{fruit::pear, fruit::banana}));
+
+    //
+    // Empty set.
+    //
+
+    const auto fs2 = fruit_set();
+    std::unordered_set<fruit> us2;
+    std::copy(fs2.begin(), fs2.end(), std::inserter(us2, us2.begin()));
+
+    BOOST_REQUIRE(us2.empty());
+}