SERVER-54358 enable for (auto [name, elem] : someBsonObj) {...}

3 files changed, 44 insertions, 0 deletions

diff --git a/src/mongo/bson/bson_obj_test.cpp b/src/mongo/bson/bson_obj_test.cpp
index 049fab19d52..3745f77f135 100644
--- a/src/mongo/bson/bson_obj_test.cpp
+++ b/src/mongo/bson/bson_obj_test.cpp
@@ -621,6 +621,16 @@ TEST(Looping, Cpp11Auto) {
     ASSERT_EQUALS(count, 1 + 2 + 3);
 }
 
+TEST(Looping, Cpp17StructuredBindings) {
+    int count = 0;
+    for (auto [name, e] : BSON("a" << 1 << "a" << 2 << "a" << 3)) {
+        ASSERT_EQUALS(name, "a");
+        count += e.Int();
+    }
+
+    ASSERT_EQUALS(count, 1 + 2 + 3);
+}
+
 TEST(BSONObj, getFields) {
     auto e = BSON("a" << 1 << "b" << 2 << "c" << 3 << "d" << 4 << "e" << 5 << "f" << 6);
     std::array<StringData, 3> fieldNames{"c", "d", "f"};
diff --git a/src/mongo/bson/bsonelement.h b/src/mongo/bson/bsonelement.h
index 722ea0937b3..a0cf82dcf02 100644
--- a/src/mongo/bson/bsonelement.h
+++ b/src/mongo/bson/bsonelement.h
@@ -901,6 +901,24 @@ public:
 
 private:
     /**
+     * This is to enable structured bindings for BSONElement, it should not be used explicitly.
+     * When used in a structed binding, BSONElement behaves as-if it is a
+     * std::pair<StringData, BSONElement>.
+     *
+     * Example:
+     *   for (auto [name, elem] : someBsonObj) {...}
+     */
+    template <size_t I>
+    friend auto get(const BSONElement& elem) {
+        static_assert(I <= 1);
+        if constexpr (I == 0) {
+            return elem.fieldNameStringData();
+        } else if constexpr (I == 1) {
+            return elem;
+        }
+    }
+
+    /**
      * Get a string's value. Also gives you start of the real data for an embedded object.
      * You must assure data is of an appropriate type first, like the type check in
      * valueStringDataSafe(). You should use the string's size when performing any operations
@@ -1227,3 +1245,13 @@ inline BSONElement::BSONElement() {
     totalSize = 1;
 }
 }  // namespace mongo
+
+// These template specializations in namespace std are required in order to support structured
+// bindings using the "tuple protocol".
+namespace std {
+template <>
+struct tuple_size<mongo::BSONElement> : std::integral_constant<size_t, 2> {};
+template <size_t I>
+struct tuple_element<I, mongo::BSONElement>
+    : std::tuple_element<I, std::pair<mongo::StringData, mongo::BSONElement>> {};
+}  // namespace std
diff --git a/src/mongo/bson/bsonobj.h b/src/mongo/bson/bsonobj.h
index 10c6483e70d..2f1e511704d 100644
--- a/src/mongo/bson/bsonobj.h
+++ b/src/mongo/bson/bsonobj.h
@@ -662,6 +662,12 @@ public:
      *      for (BSONElement elem : BSON("a" << 1 << "b" << 2)) {
      *          ... // Do something with elem
      *      }
+     *
+     * You can also loop over a bson object as-if it were a map<StringData, BSONElement>:
+     *
+     *      for (auto [fieldName, elem] : BSON("a" << 1 << "b" << 2)) {
+     *          ... // Do something with fieldName and elem
+     *      }
      */
     iterator begin() const;
     iterator end() const;