Merge pull request #2258 from nlohmann/issue2179

Add ordered_json specialization with ordered object keys

single_include/nlohmann/json.hpp

@@ -2807,6 +2807,19 @@ uses the standard template types.
 @since version 1.0.0
 */
 using json = basic_json<>;
+
+template<class Key, class T, class IgnoredLess, class Allocator>
+struct ordered_map;
+
+/*!
+@brief ordered JSON class
+
+This type preserves the insertion order of object keys.
+
+@since version 3.9.0
+*/
+using ordered_json = basic_json<nlohmann::ordered_map>;
+
 }  // namespace nlohmann
 
 #endif  // INCLUDE_NLOHMANN_JSON_FWD_HPP_
@@ -15973,6 +15986,79 @@ class serializer
 
 // #include <nlohmann/json_fwd.hpp>
 
+// #include <nlohmann/ordered_map.hpp>
+
+
+#include <functional> // less
+#include <memory> // allocator
+#include <utility> // pair
+#include <vector> // vector
+
+namespace nlohmann
+{
+
+/// ordered_map: a minimal map-like container that preserves insertion order
+/// for use within nlohmann::basic_json<ordered_map>
+template <class Key, class T, class IgnoredLess = std::less<Key>,
+          class Allocator = std::allocator<std::pair<const Key, T>>>
+                  struct ordered_map : std::vector<std::pair<const Key, T>, Allocator>
+{
+    using key_type = Key;
+    using mapped_type = T;
+    using Container = std::vector<std::pair<const Key, T>, Allocator>;
+    using typename Container::iterator;
+    using typename Container::size_type;
+    using typename Container::value_type;
+
+    // Explicit constructors instead of `using Container::Container`
+    // otherwise older compilers choke on it (GCC <= 5.5, xcode <= 9.4)
+    ordered_map(const Allocator& alloc = Allocator()) : Container{alloc} {}
+    template <class It>
+    ordered_map(It first, It last, const Allocator& alloc = Allocator())
+        : Container{first, last, alloc} {}
+    ordered_map(std::initializer_list<T> init, const Allocator& alloc = Allocator() )
+        : Container{init, alloc} {}
+
+    std::pair<iterator, bool> emplace(key_type&& key, T&& t)
+    {
+        for (auto it = this->begin(); it != this->end(); ++it)
+        {
+            if (it->first == key)
+            {
+                return {it, false};
+            }
+        }
+        Container::emplace_back(key, t);
+        return {--this->end(), true};
+    }
+
+    T& operator[](Key&& key)
+    {
+        return emplace(std::move(key), T{}).first->second;
+    }
+
+    size_type erase(const Key& key)
+    {
+        for (auto it = this->begin(); it != this->end(); ++it)
+        {
+            if (it->first == key)
+            {
+                // Since we cannot move const Keys, re-construct them in place
+                for (auto next = it; ++next != this->end(); ++it)
+                {
+                    it->~value_type(); // Destroy but keep allocation
+                    new (&*it) value_type{std::move(*next)};
+                }
+                Container::pop_back();
+                return 1;
+            }
+        }
+        return 0;
+    }
+};
+
+}  // namespace nlohmann
+
 
 /*!
 @brief namespace for Niels Lohmann
@@ -16099,7 +16185,7 @@ class basic_json
         detail::parser_callback_t<basic_json>cb = nullptr,
         const bool allow_exceptions = true,
         const bool ignore_comments = false
-    )
+                                 )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
                 std::move(cb), allow_exceptions, ignore_comments);
@@ -24635,7 +24721,7 @@ template<>
 inline void swap<nlohmann::json>(nlohmann::json& j1, nlohmann::json& j2) noexcept(
     is_nothrow_move_constructible<nlohmann::json>::value&&
     is_nothrow_move_assignable<nlohmann::json>::value
-)
+                              )
 {
     j1.swap(j2);
 }