make sure values are overwritten in from_json overloads

include/nlohmann/detail/conversions/from_json.hpp

@@ -136,6 +136,7 @@ void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
     {
         JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name())));
     }
+    l.clear();
     std::transform(j.rbegin(), j.rend(),
                    std::front_inserter(l), [](const BasicJsonType & i)
     {
@@ -182,14 +183,16 @@ auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, p
 {
     using std::end;
 
-    arr.reserve(j.size());
+    ConstructibleArrayType ret;
+    ret.reserve(j.size());
     std::transform(j.begin(), j.end(),
-                   std::inserter(arr, end(arr)), [](const BasicJsonType & i)
+                   std::inserter(ret, end(ret)), [](const BasicJsonType & i)
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
         return i.template get<typename ConstructibleArrayType::value_type>();
     });
+    arr = std::move(ret);
 }
 
 template <typename BasicJsonType, typename ConstructibleArrayType>
@@ -198,14 +201,16 @@ void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
 {
     using std::end;
 
+    ConstructibleArrayType ret;
     std::transform(
-        j.begin(), j.end(), std::inserter(arr, end(arr)),
+        j.begin(), j.end(), std::inserter(ret, end(ret)),
         [](const BasicJsonType & i)
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
         return i.template get<typename ConstructibleArrayType::value_type>();
     });
+    arr = std::move(ret);
 }
 
 template <typename BasicJsonType, typename ConstructibleArrayType,
@@ -239,15 +244,17 @@ void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, "type must be object, but is " + std::string(j.type_name())));
     }
 
+    ConstructibleObjectType ret;
     auto inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
     using value_type = typename ConstructibleObjectType::value_type;
     std::transform(
         inner_object->begin(), inner_object->end(),
-        std::inserter(obj, obj.begin()),
+        std::inserter(ret, ret.begin()),
         [](typename BasicJsonType::object_t::value_type const & p)
     {
         return value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
     });
+    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments
@@ -319,6 +326,7 @@ void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>&
     {
         JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name())));
     }
+    m.clear();
     for (const auto& p : j)
     {
         if (JSON_UNLIKELY(not p.is_array()))
@@ -338,6 +346,7 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
     {
         JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name())));
     }
+    m.clear();
     for (const auto& p : j)
     {
         if (JSON_UNLIKELY(not p.is_array()))

include/nlohmann/detail/meta/type_traits.hpp

@@ -192,10 +192,19 @@ struct is_constructible_object_type_impl <
     using object_t = typename BasicJsonType::object_t;
 
     static constexpr bool value =
-        (std::is_constructible<typename ConstructibleObjectType::key_type, typename object_t::key_type>::value and
-         std::is_same<typename object_t::mapped_type, typename ConstructibleObjectType::mapped_type>::value) or
-        (has_from_json<BasicJsonType, typename ConstructibleObjectType::mapped_type>::value or
-         has_non_default_from_json<BasicJsonType, typename ConstructibleObjectType::mapped_type >::value);
+        (std::is_default_constructible<ConstructibleObjectType>::value and
+         (std::is_move_assignable<ConstructibleObjectType>::value or
+          std::is_copy_assignable<ConstructibleObjectType>::value) and
+         (std::is_constructible<typename ConstructibleObjectType::key_type,
+          typename object_t::key_type>::value and
+          std::is_same <
+          typename object_t::mapped_type,
+          typename ConstructibleObjectType::mapped_type >::value)) or
+        (has_from_json<BasicJsonType,
+         typename ConstructibleObjectType::mapped_type>::value or
+         has_non_default_from_json <
+         BasicJsonType,
+         typename ConstructibleObjectType::mapped_type >::value);
 };
 
 template <typename BasicJsonType, typename ConstructibleObjectType>
@@ -278,20 +287,24 @@ struct is_constructible_array_type_impl <
     BasicJsonType, ConstructibleArrayType,
     enable_if_t<not std::is_same<ConstructibleArrayType,
     typename BasicJsonType::value_type>::value and
-    is_detected<value_type_t, ConstructibleArrayType>::value and
-    is_detected<iterator_t, ConstructibleArrayType>::value and
-    is_complete_type<
-    detected_t<value_type_t, ConstructibleArrayType>>::value >>
+    std::is_default_constructible<ConstructibleArrayType>::value and
+(std::is_move_assignable<ConstructibleArrayType>::value or
+ std::is_copy_assignable<ConstructibleArrayType>::value) and
+is_detected<value_type_t, ConstructibleArrayType>::value and
+is_detected<iterator_t, ConstructibleArrayType>::value and
+is_complete_type<
+detected_t<value_type_t, ConstructibleArrayType>>::value >>
 {
     static constexpr bool value =
         // This is needed because json_reverse_iterator has a ::iterator type,
-        // furthermore, std::back_insert_iterator (and other iterators) have a base class `iterator`...
-        // Therefore it is detected as a ConstructibleArrayType.
-        // The real fix would be to have an Iterable concept.
-        not is_iterator_traits <
-        iterator_traits<ConstructibleArrayType >>::value and
-
-        (std::is_same<typename ConstructibleArrayType::value_type, typename BasicJsonType::array_t::value_type>::value or
+        // furthermore, std::back_insert_iterator (and other iterators) have a
+        // base class `iterator`... Therefore it is detected as a
+        // ConstructibleArrayType. The real fix would be to have an Iterable
+        // concept.
+        not is_iterator_traits<iterator_traits<ConstructibleArrayType>>::value and
+
+        (std::is_same<typename ConstructibleArrayType::value_type,
+         typename BasicJsonType::array_t::value_type>::value or
          has_from_json<BasicJsonType,
          typename ConstructibleArrayType::value_type>::value or
          has_non_default_from_json <

single_include/nlohmann/json.hpp

@@ -1059,10 +1059,19 @@ struct is_constructible_object_type_impl <
     using object_t = typename BasicJsonType::object_t;
 
     static constexpr bool value =
-        (std::is_constructible<typename ConstructibleObjectType::key_type, typename object_t::key_type>::value and
-         std::is_same<typename object_t::mapped_type, typename ConstructibleObjectType::mapped_type>::value) or
-        (has_from_json<BasicJsonType, typename ConstructibleObjectType::mapped_type>::value or
-         has_non_default_from_json<BasicJsonType, typename ConstructibleObjectType::mapped_type >::value);
+        (std::is_default_constructible<ConstructibleObjectType>::value and
+         (std::is_move_assignable<ConstructibleObjectType>::value or
+          std::is_copy_assignable<ConstructibleObjectType>::value) and
+         (std::is_constructible<typename ConstructibleObjectType::key_type,
+          typename object_t::key_type>::value and
+          std::is_same <
+          typename object_t::mapped_type,
+          typename ConstructibleObjectType::mapped_type >::value)) or
+        (has_from_json<BasicJsonType,
+         typename ConstructibleObjectType::mapped_type>::value or
+         has_non_default_from_json <
+         BasicJsonType,
+         typename ConstructibleObjectType::mapped_type >::value);
 };
 
 template <typename BasicJsonType, typename ConstructibleObjectType>
@@ -1145,20 +1154,24 @@ struct is_constructible_array_type_impl <
     BasicJsonType, ConstructibleArrayType,
     enable_if_t<not std::is_same<ConstructibleArrayType,
     typename BasicJsonType::value_type>::value and
-    is_detected<value_type_t, ConstructibleArrayType>::value and
-    is_detected<iterator_t, ConstructibleArrayType>::value and
-    is_complete_type<
-    detected_t<value_type_t, ConstructibleArrayType>>::value >>
+    std::is_default_constructible<ConstructibleArrayType>::value and
+(std::is_move_assignable<ConstructibleArrayType>::value or
+ std::is_copy_assignable<ConstructibleArrayType>::value) and
+is_detected<value_type_t, ConstructibleArrayType>::value and
+is_detected<iterator_t, ConstructibleArrayType>::value and
+is_complete_type<
+detected_t<value_type_t, ConstructibleArrayType>>::value >>
 {
     static constexpr bool value =
         // This is needed because json_reverse_iterator has a ::iterator type,
-        // furthermore, std::back_insert_iterator (and other iterators) have a base class `iterator`...
-        // Therefore it is detected as a ConstructibleArrayType.
-        // The real fix would be to have an Iterable concept.
-        not is_iterator_traits <
-        iterator_traits<ConstructibleArrayType >>::value and
-
-        (std::is_same<typename ConstructibleArrayType::value_type, typename BasicJsonType::array_t::value_type>::value or
+        // furthermore, std::back_insert_iterator (and other iterators) have a
+        // base class `iterator`... Therefore it is detected as a
+        // ConstructibleArrayType. The real fix would be to have an Iterable
+        // concept.
+        not is_iterator_traits<iterator_traits<ConstructibleArrayType>>::value and
+
+        (std::is_same<typename ConstructibleArrayType::value_type,
+         typename BasicJsonType::array_t::value_type>::value or
          has_from_json<BasicJsonType,
          typename ConstructibleArrayType::value_type>::value or
          has_non_default_from_json <
@@ -1411,6 +1424,7 @@ void from_json(const BasicJsonType& j, std::forward_list<T, Allocator>& l)
     {
         JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name())));
     }
+    l.clear();
     std::transform(j.rbegin(), j.rend(),
                    std::front_inserter(l), [](const BasicJsonType & i)
     {
@@ -1457,14 +1471,16 @@ auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, p
 {
     using std::end;
 
-    arr.reserve(j.size());
+    ConstructibleArrayType ret;
+    ret.reserve(j.size());
     std::transform(j.begin(), j.end(),
-                   std::inserter(arr, end(arr)), [](const BasicJsonType & i)
+                   std::inserter(ret, end(ret)), [](const BasicJsonType & i)
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
         return i.template get<typename ConstructibleArrayType::value_type>();
     });
+    arr = std::move(ret);
 }
 
 template <typename BasicJsonType, typename ConstructibleArrayType>
@@ -1473,14 +1489,16 @@ void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
 {
     using std::end;
 
+    ConstructibleArrayType ret;
     std::transform(
-        j.begin(), j.end(), std::inserter(arr, end(arr)),
+        j.begin(), j.end(), std::inserter(ret, end(ret)),
         [](const BasicJsonType & i)
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
         return i.template get<typename ConstructibleArrayType::value_type>();
     });
+    arr = std::move(ret);
 }
 
 template <typename BasicJsonType, typename ConstructibleArrayType,
@@ -1514,15 +1532,17 @@ void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
         JSON_THROW(type_error::create(302, "type must be object, but is " + std::string(j.type_name())));
     }
 
+    ConstructibleObjectType ret;
     auto inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
     using value_type = typename ConstructibleObjectType::value_type;
     std::transform(
         inner_object->begin(), inner_object->end(),
-        std::inserter(obj, obj.begin()),
+        std::inserter(ret, ret.begin()),
         [](typename BasicJsonType::object_t::value_type const & p)
     {
         return value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
     });
+    obj = std::move(ret);
 }
 
 // overload for arithmetic types, not chosen for basic_json template arguments
@@ -1594,6 +1614,7 @@ void from_json(const BasicJsonType& j, std::map<Key, Value, Compare, Allocator>&
     {
         JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name())));
     }
+    m.clear();
     for (const auto& p : j)
     {
         if (JSON_UNLIKELY(not p.is_array()))
@@ -1613,6 +1634,7 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
     {
         JSON_THROW(type_error::create(302, "type must be array, but is " + std::string(j.type_name())));
     }
+    m.clear();
     for (const auto& p : j)
     {
         if (JSON_UNLIKELY(not p.is_array()))

test/src/unit-conversions.cpp

@@ -140,6 +140,54 @@ TEST_CASE("value conversion")
         }
     }
 
+    SECTION("get an object (explicit, get_to)")
+    {
+        json::object_t o_reference = {{"object", json::object()},
+            {"array", {1, 2, 3, 4}},
+            {"number", 42},
+            {"boolean", false},
+            {"null", nullptr},
+            {"string", "Hello world"}
+        };
+        json j(o_reference);
+
+        SECTION("json::object_t")
+        {
+            json::object_t o = {{"previous", "value"}};
+            j.get_to(o);
+            CHECK(json(o) == j);
+        }
+
+        SECTION("std::map<json::string_t, json>")
+        {
+            std::map<json::string_t, json> o{{"previous", "value"}};
+            j.get_to(o);
+            CHECK(json(o) == j);
+        }
+
+        SECTION("std::multimap<json::string_t, json>")
+        {
+            std::multimap<json::string_t, json> o{{"previous", "value"}};
+            j.get_to(o);
+            CHECK(json(o) == j);
+        }
+
+        SECTION("std::unordered_map<json::string_t, json>")
+        {
+            std::unordered_map<json::string_t, json> o{{"previous", "value"}};
+            j.get_to(o);
+            CHECK(json(o) == j);
+        }
+
+        SECTION("std::unordered_multimap<json::string_t, json>")
+        {
+            std::unordered_multimap<json::string_t, json> o{{"previous", "value"}};
+            j.get_to(o);
+            CHECK(json(o) == j);
+        }
+    }
+
+
     SECTION("get an object (implicit)")
     {
         json::object_t o_reference = {{"object", json::object()},
@@ -226,11 +274,6 @@ TEST_CASE("value conversion")
 #if not defined(JSON_NOEXCEPTION)
             SECTION("reserve is called on containers that supports it")
             {
-                // making the call to from_json throw in order to check capacity
-                std::vector<float> v;
-                CHECK_THROWS_AS(nlohmann::from_json(j, v), json::type_error&);
-                CHECK(v.capacity() == j.size());
-
                 // make sure all values are properly copied
                 std::vector<int> v2 = json({1, 2, 3, 4, 5, 6, 7, 8, 9, 10});
                 CHECK(v2.size() == 10);
@@ -302,6 +345,55 @@ TEST_CASE("value conversion")
         }
     }
 
+    SECTION("get an array (explicit, get_to)")
+    {
+        json::array_t a_reference{json(1),     json(1u),       json(2.2),
+                                  json(false), json("string"), json()};
+        json j(a_reference);
+
+        SECTION("json::array_t")
+        {
+            json::array_t a{"previous", "value"};
+            j.get_to(a);
+            CHECK(json(a) == j);
+        }
+
+        SECTION("std::valarray<json>")
+        {
+            std::valarray<json> a{"previous", "value"};
+            j.get_to(a);
+            CHECK(json(a) == j);
+        }
+
+        SECTION("std::list<json>")
+        {
+            std::list<json> a{"previous", "value"};
+            j.get_to(a);
+            CHECK(json(a) == j);
+        }
+
+        SECTION("std::forward_list<json>")
+        {
+            std::forward_list<json> a{"previous", "value"};
+            j.get_to(a);
+            CHECK(json(a) == j);
+        }
+
+        SECTION("std::vector<json>")
+        {
+            std::vector<json> a{"previous", "value"};
+            j.get_to(a);
+            CHECK(json(a) == j);
+        }
+
+        SECTION("std::deque<json>")
+        {
+            std::deque<json> a{"previous", "value"};
+            j.get_to(a);
+            CHECK(json(a) == j);
+        }
+    }
+
     SECTION("get an array (implicit)")
     {
         json::array_t a_reference{json(1),     json(1u),       json(2.2),
@@ -433,6 +525,35 @@ TEST_CASE("value conversion")
 #endif
     }
 
+    SECTION("get a string (explicit, get_to)")
+    {
+        json::string_t s_reference{"Hello world"};
+        json j(s_reference);
+
+        SECTION("string_t")
+        {
+            json::string_t s = "previous value";
+            j.get_to(s);
+            CHECK(json(s) == j);
+        }
+
+        SECTION("std::string")
+        {
+            std::string s = "previous value";
+            j.get_to(s);
+            CHECK(json(s) == j);
+        }
+#if defined(JSON_HAS_CPP_17)
+        SECTION("std::string_view")
+        {
+            std::string s = "previous value";
+            std::string_view sv = s;
+            j.get_to(sv);
+            CHECK(json(sv) == j);
+        }
+#endif
+    }
+
     SECTION("get null (explicit)")
     {
         std::nullptr_t n = nullptr;