json_formatter: Add support for standard range containers

This commit extends the JSON formatting capabilities to support all
standard range containers through the C++20 ranges concept.

Key improvements:
- Add generic range support using std::ranges::input_range concept
- Enable JSON formatting for any range type without materialization
- Maintain existing support for vector, map, and unordered_map
- Provide a flexible, type-agnostic serialization approach

Example use cases:
- Formatting std::span
- Serializing range views
- Converting filter and transform views directly to JSON

Signed-off-by: Kefu Chai <[email protected]>

include/seastar/json/formatter.hh

@@ -22,6 +22,7 @@
 #pragma once
 
 #ifndef SEASTAR_MODULE
+#include <ranges>
 #include <string>
 #include <vector>
 #include <unordered_map>
@@ -37,6 +38,15 @@
 
 namespace seastar {
 
+namespace internal {
+
+template<typename T>
+concept is_map = requires {
+    typename T::mapped_type;
+};
+
+}
+
 namespace json {
 
 SEASTAR_MODULE_EXPORT
@@ -64,8 +74,18 @@ class formatter {
                         to_json(p.first) + ":" + to_json(p.second);
     }
 
-    template<typename Iter>
-    static sstring to_json(state s, Iter i, Iter e) {
+    template<typename K, typename V>
+    static sstring to_json(state s, const std::tuple<K, V>& p) {
+        if (s == state::array) {
+            return "{" + to_json(state::none, p) + "}";
+        } else {
+            auto& [key, value] = p;
+            return to_json(key) + ":" + to_json(value);
+        }
+    }
+
+    template<typename Iterator, typename Sentinel>
+    static sstring to_json(state s, Iterator i, Sentinel e) {
         std::stringstream res;
         res << begin(s);
         size_t n = 0;
@@ -100,8 +120,8 @@ class formatter {
         }
     }
 
-    template<typename Iter>
-    static future<> write(output_stream<char>& stream, state s, Iter i, Iter e) {
+    template<typename Iterator, typename Sentinel>
+    static future<> write(output_stream<char>& stream, state s, Iterator i, Sentinel e) {
         return do_with(true, [&stream, s, i, e] (bool& first) {
             return stream.write(begin(s)).then([&first, &stream, s, i, e] {
                 return do_for_each(i, e, [&first, &stream, s] (auto& m) {
@@ -191,23 +211,18 @@ public:
     static sstring to_json(bool d);
 
     /**
-     * return a json formatted list of a given vector of params
-     * @param vec the vector to format
-     * @return the given vector in a json format
+     * converts a given range to a JSON-formatted string
+     *
+     * @param range A standard range type
+     * @return A string containing the JSON representation of the input range
      */
-    template<typename... Args>
-    static sstring to_json(const std::vector<Args...>& vec) {
-        return to_json(state::array, vec.begin(), vec.end());
-    }
-
-    template<typename... Args>
-    static sstring to_json(const std::map<Args...>& map) {
-        return to_json(state::map, map.begin(), map.end());
-    }
-
-    template<typename... Args>
-    static sstring to_json(const std::unordered_map<Args...>& map) {
-        return to_json(state::map, map.begin(), map.end());
+    template<std::ranges::input_range Range>
+    static sstring to_json(const Range& range) {
+        if constexpr (internal::is_map<Range>) {
+            return to_json(state::map, std::ranges::begin(range), std::ranges::end(range));
+        } else {
+            return to_json(state::array, std::ranges::begin(range), std::ranges::end(range));
+        }
     }
 
     /**

tests/unit/json_formatter_test.cc

@@ -62,6 +62,14 @@ SEASTAR_TEST_CASE(test_collections) {
     return make_ready_future();
 }
 
+SEASTAR_TEST_CASE(test_ranges) {
+    BOOST_CHECK_EQUAL("[1,2,3,4]", formatter::to_json(std::views::iota(1, 5)));
+#ifdef __cpp_lib_ranges_enumerate
+    BOOST_CHECK_EQUAL("[{0:5},{1:6},{2:7},{3:8}]", formatter::to_json(std::views::iota(5, 9) | std::views::enumerate));
+#endif
+    return make_ready_future();
+}
+
 struct object_json : public json_base {
     json_element<sstring> subject;
     json_list<long> values;