add pair/tuple conversions

src/json.hpp

@@ -465,6 +465,39 @@ using enable_if_t = typename std::enable_if<B, T>::type;
 template<typename T>
 using uncvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
 
+// implementation of C++14 index_sequence and affiliates
+// source: https://stackoverflow.com/a/32223343
+template <std::size_t... Ints>
+struct index_sequence
+{
+    using type = index_sequence;
+    using value_type = std::size_t;
+    static constexpr std::size_t size() noexcept
+    {
+        return sizeof...(Ints);
+    }
+};
+
+template <class Sequence1, class Sequence2>
+struct merge_and_renumber;
+
+template <std::size_t... I1, std::size_t... I2>
+struct merge_and_renumber<index_sequence<I1...>, index_sequence<I2...>>
+        : index_sequence < I1..., (sizeof...(I1) + I2)... >
+          { };
+
+template <std::size_t N>
+struct make_index_sequence
+    : merge_and_renumber < typename make_index_sequence < N / 2 >::type,
+      typename make_index_sequence < N - N / 2 >::type >
+{ };
+
+template<> struct make_index_sequence<0> : index_sequence<> { };
+template<> struct make_index_sequence<1> : index_sequence<0> { };
+
+template<typename... Ts>
+using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
+
 /*
 Implementation of two C++17 constructs: conjunction, negation. This is needed
 to avoid evaluating all the traits in a condition
@@ -866,6 +899,24 @@ void to_json(BasicJsonType& j, T (&arr)[N])
     external_constructor<value_t::array>::construct(j, arr);
 }
 
+template <typename BasicJsonType, typename... Args>
+void to_json(BasicJsonType& j, const std::pair<Args...>& p)
+{
+    j = {p.first, p.second};
+}
+
+template <typename BasicJsonType, typename Tuple, std::size_t... Idx>
+void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<Idx...>)
+{
+    j = {std::get<Idx>(t)...};
+}
+
+template <typename BasicJsonType, typename... Args>
+void to_json(BasicJsonType& j, const std::tuple<Args...>& t)
+{
+    to_json_tuple_impl(j, t, index_sequence_for<Args...> {});
+}
+
 ///////////////
 // from_json //
 ///////////////
@@ -1094,6 +1145,24 @@ void from_json(const BasicJsonType& j, ArithmeticType& val)
     }
 }
 
+template <typename BasicJsonType, typename... Args>
+void from_json(const BasicJsonType& j, std::pair<Args...>& p)
+{
+    p = {j.at(0), j.at(1)};
+}
+
+template <typename BasicJsonType, typename Tuple, std::size_t... Idx>
+void from_json_tuple_impl(const BasicJsonType& j, Tuple& t, index_sequence<Idx...>)
+{
+    t = std::make_tuple(j.at(Idx)...);
+}
+
+template <typename BasicJsonType, typename... Args>
+void from_json(const BasicJsonType& j, std::tuple<Args...>& t)
+{
+    from_json_tuple_impl(j, t, index_sequence_for<Args...> {});
+}
+
 struct to_json_fn
 {
   private:

test/src/unit-constructor1.cpp

@@ -239,6 +239,58 @@ TEST_CASE("constructors")
             CHECK(j == j_reference);
         }
 
+        SECTION("std::pair")
+        {
+            std::pair<float, std::string> p{1.0, "string"};
+            json j(p);
+
+            CHECK(j.type() == json::value_t::array);
+            REQUIRE(j.size() == 2);
+            CHECK(j[0] == std::get<0>(p));
+            CHECK(j[1] == std::get<1>(p));
+        }
+
+        SECTION("std::pair with discarded values")
+        {
+            json j{1, 2.0, "string"};
+
+            const auto p = j.get<std::pair<int, float>>();
+            CHECK(p.first == j[0]);
+            CHECK(p.second == j[1]);
+        }
+
+        SECTION("std::tuple")
+        {
+            const auto t = std::make_tuple(1.0, "string", 42, std::vector<int> {0, 1});
+            json j(t);
+
+            CHECK(j.type() == json::value_t::array);
+            REQUIRE(j.size() == 4);
+            CHECK(j[0] == std::get<0>(t));
+            CHECK(j[1] == std::get<1>(t));
+            CHECK(j[2] == std::get<2>(t));
+            CHECK(j[3][0] == 0);
+            CHECK(j[3][1] == 1);
+        }
+
+        SECTION("std::tuple with discarded values")
+        {
+            json j{1, 2.0, "string", 42};
+
+            const auto t = j.get<std::tuple<int, float, std::string>>();
+            CHECK(std::get<0>(t) == j[0]);
+            CHECK(std::get<1>(t) == j[1]);
+            CHECK(std::get<2>(t) == j[2]);
+        }
+
+        SECTION("std::pair/tuple failures")
+        {
+            json j{1};
+
+            CHECK_THROWS((j.get<std::pair<int, int>>()));
+            CHECK_THROWS((j.get<std::tuple<int, int>>()));
+        }
+
         SECTION("std::forward_list<json>")
         {
             std::forward_list<json> a {json(1), json(1u), json(2.2), json(false), json("string"), json()};