add std::pair<CompatibleStringType, T> support

src/json.hpp

@@ -285,7 +285,7 @@ class invalid_iterator : public exception
 
 Exceptions have ids 3xx.
 
-name / id                     | example massage | description
+name / id                     | example message | description
 ----------------------------- | --------------- | -------------------------
 json.exception.type_error.301 | cannot create object from initializer list | To create an object from an initializer list, the initializer list must consist only of a list of pairs whose first element is a string. When this constraint is violated, an array is created instead.
 json.exception.type_error.302 | type must be object, but is array | During implicit or explicit value conversion, the JSON type must be compatible to the target type. For instance, a JSON string can only be converted into string types, but not into numbers or boolean types.
@@ -324,7 +324,7 @@ class type_error : public exception
 
 Exceptions have ids 4xx.
 
-name / id                       | example massage | description
+name / id                       | example message | description
 ------------------------------- | --------------- | -------------------------
 json.exception.out_of_range.401 | array index 3 is out of range | The provided array index @a i is larger than @a size-1.
 json.exception.out_of_range.402 | array index '-' (3) is out of range | The special array index `-` in a JSON Pointer never describes a valid element of the array, but the index past the end. That is, it can only be used to add elements at this position, but not to read it.
@@ -355,9 +355,10 @@ class out_of_range : public exception
 
 Exceptions have ids 5xx.
 
-name / id                      | example massage | description
+name / id                      | example message | description
 ------------------------------ | --------------- | -------------------------
 json.exception.other_error.501 | unsuccessful: {"op":"test","path":"/baz", "value":"bar"} | A JSON Patch operation 'test' failed. The unsuccessful operation is also printed.
+json.exception.other_error.502 | invalid object size for conversion | Some conversions to user-defined types impose constraints on the object size (e.g. std::pair)
 
 @since version 3.0.0
 */
@@ -865,6 +866,14 @@ void to_json(BasicJsonType& j, T (&arr)[N])
     external_constructor<value_t::array>::construct(j, arr);
 }
 
+template <typename BasicJsonType, typename CompatibleString, typename T,
+         enable_if_t<std::is_constructible<typename BasicJsonType::string_t,
+                     CompatibleString>::value, int> = 0>
+void to_json(BasicJsonType& j, std::pair<CompatibleString, T> const& p)
+{
+  j[p.first] = p.second;
+}
+
 ///////////////
 // from_json //
 ///////////////
@@ -1086,6 +1095,27 @@ void from_json(const BasicJsonType& j, ArithmeticType& val)
     }
 }
 
+template <typename BasicJsonType, typename CompatibleString, typename T,
+         enable_if_t<std::is_constructible<typename BasicJsonType::string_t,
+                     CompatibleString>::value, int> = 0>
+void from_json(const BasicJsonType& j, std::pair<CompatibleString, T>& p)
+{
+    if (not j.is_object())
+    {
+        JSON_THROW(type_error::create(302, "type must be object, but is " + j.type_name()));
+    }
+
+    auto const inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
+    auto const size = inner_object->size();
+    if (size != 1)
+    {
+        JSON_THROW(other_error::create(502, "conversion to std::pair requires the object to have exactly one field, but it has " + std::to_string(size)));
+    }
+    auto const& obj = *inner_object->begin();
+    // cannot use *inner_object, need to convert both members 
+    p = std::make_pair(obj.first, obj.second.template get<T>());
+}
+
 struct to_json_fn
 {
   private:

test/src/unit-constructor1.cpp

@@ -156,6 +156,20 @@ TEST_CASE("constructors")
             CHECK(j == j_reference);
         }
 
+        SECTION("std::pair<CompatibleString, T>")
+        {
+          std::pair<std::string, std::string> p{"first", "second"};
+          json j(p);
+
+          CHECK((j.get<decltype(p)>() == p));
+
+          std::pair<std::string, int> p2{"first", 1};
+          // use char const*
+          json j2(std::make_pair("first", 1));
+
+          CHECK((j2.get<decltype(p2)>() == p2));
+        }
+
         SECTION("std::map<const char*, json>")
         {
             std::map<const char*, json> o {{"a", json(1)}, {"b", json(1u)}, {"c", json(2.2)}, {"d", json(false)}, {"e", json("string")}, {"f", json()}};
@@ -164,6 +178,7 @@ TEST_CASE("constructors")
             CHECK(j == j_reference);
         }
 
+
         SECTION("std::multimap<json::string_t, json>")
         {
             std::multimap<json::string_t, json> o {{"a", json(1)}, {"b", json(1u)}, {"c", json(2.2)}, {"d", json(false)}, {"e", json("string")}, {"f", json()}};
@@ -954,6 +969,17 @@ TEST_CASE("constructors")
                 "[json.exception.type_error.301] cannot create object from initializer list");
             }
 
+            SECTION("std::pair<CompatibleString, T> with error")
+            {
+              json j{{"too", "much"}, {"string", "fields"}};
+              CHECK_THROWS_AS((j.get<std::pair<std::string, std::string>>()), json::other_error);
+              CHECK_THROWS_WITH((j.get<std::pair<std::string, std::string>>()),
+                                "[json.exception.other_error.502] conversion "
+                                "to std::pair requires the object to have "
+                                "exactly one field, but it has 2");
+            }
+
+
             SECTION("empty array")
             {
                 json j = json::array();