anonymous namespace renamed to detail

src/json.hpp

@@ -106,6 +106,14 @@ SOFTWARE.
 */
 namespace nlohmann
 {
+// TODO add real documentation before PR
+
+// Traits structure declaration, users can specialize it for their own types
+// 
+// constructing a json object from a user-defined type will call the
+// 'json to_json(T)' function
+//
+// whereas calling json::get<T> will call 'T from_json(json const&)'
 template <typename T, typename = void>
 struct json_traits;
 
@@ -113,8 +121,8 @@ struct json_traits;
 @brief unnamed namespace with internal helper functions
 @since version 1.0.0
 */
-// TODO transform this anon ns to detail?
+
-namespace
+namespace detail
 {
 /*!
 @brief Helper to determine whether there's a key_type for T.
@@ -140,6 +148,7 @@ struct has_mapped_type
 };
 
 // taken from http://stackoverflow.com/questions/10711952/how-to-detect-existence-of-a-class-using-sfinae
+// used to determine if json_traits is defined for a given type T
 template <typename T>
 struct has_destructor
 {
@@ -158,7 +167,23 @@ struct has_json_traits
   static constexpr bool value = has_destructor<json_traits<T>>::value;
 };
 
-template <> struct has_json_traits<void> : std::false_type {};
+struct to_json_fn
+{
+  template <typename T>
+  constexpr auto operator()(T&& val) const -> decltype(to_json(std::forward<T>(val)))
+  {
+    return to_json(std::forward<T>(val));
+  }
+};
+
+struct from_json_fn
+{
+  template <typename Json, typename T>
+  constexpr auto operator()(Json const& from, T& to) const -> decltype(from_json(from, to))
+  {
+    return from_json(from, to);
+  }
+};
 
 /*!
 @brief helper class to create locales with decimal point
@@ -181,6 +206,23 @@ struct DecimalSeparator : std::numpunct<char>
 };
 
 
+// taken from ranges-v3
+// TODO add doc
+template <typename T>
+struct __static_const
+{
+  static constexpr T value{};
+};
+
+template <typename T>
+constexpr T __static_const<T>::value;
+
+inline namespace
+{
+  constexpr auto const& to_json = __static_const<detail::to_json_fn>::value;
+  constexpr auto const& from_json = __static_const<detail::from_json_fn>::value;
+}
+
 /*!
 @brief a class to store JSON values
 
@@ -1337,10 +1379,24 @@ class basic_json
         assert_invariant();
     }
 
+    // constructor chosen if json_traits is specialized for type T
+    // note: constructor is marked explicit to avoid the following issue:
+    //
+    // struct not_equality_comparable{};
+    // 
+    // not_equality_comparable{} == not_equality_comparable{};
+    // 
+    // this will construct implicitely 2 json objects and call operator== on them
+    // which can cause nasty bugs on the user's in json-unrelated code
+    // 
+    // the trade-off is expressivety in initializer-lists
+    // auto j = json{{"a", json(not_equality_comparable{})}};
+    // 
+    // we can remove this constraint though, since lots of ctor are not explicit already
     template <
         typename T,
         typename =
-            typename std::enable_if<has_json_traits<typename std::remove_cv<
+            typename std::enable_if<detail::has_json_traits<typename std::remove_cv<
                 typename std::remove_reference<T>::type>::type>::value>::type>
     explicit basic_json(T &&val)
         : basic_json(json_traits<typename std::remove_cv<
@@ -2705,10 +2761,13 @@ class basic_json
     // value access //
     //////////////////
 
+    // get_impl overload chosen if json_traits struct is specialized for type T
+    // simply returns json_traits<T>::from_json(*this);
+    // TODO add alias templates (enable_if_t etc)
     template <
         typename T,
-        typename =
+        typename = typename std::enable_if<
-            typename std::enable_if<has_json_traits<typename std::remove_cv<
+            detail::has_json_traits<typename std::remove_cv<
                 typename std::remove_reference<T>::type>::type>::value>::type>
     auto get_impl(T *) const -> decltype(
         json_traits<typename std::remove_cv<typename std::remove_reference<
@@ -2717,6 +2776,24 @@ class basic_json
           typename std::remove_reference<T>::type>::type>::from_json(*this);
     }
 
+    // this one is quite atrocious
+    // this overload is chosen ONLY if json_traits struct is not specialized, and if the expression nlohmann::from_json(*this, T&) is valid
+    // I chose to prefer the json_traits specialization if it exists, since it's a more advanced use.
+    // But we can of course change this behaviour
+    template <typename T>
+    auto get_impl(T *) const -> typename std::enable_if<
+        not detail::has_json_traits<typename std::remove_cv<T>::type>::value,
+        typename std::remove_cv<typename std::remove_reference<
+            decltype(::nlohmann::from_json(std::declval<basic_json>(),
+                                           std::declval<T &>()),
+                     std::declval<T>())>::type>::type>::type
+    {
+      typename std::remove_cv<typename std::remove_reference<T>::type>::type
+          ret;
+      ::nlohmann::from_json(*this, ret);
+      return ret;
+    }
+
     /// get an object (explicit)
     template <class T,
               typename std::enable_if<
@@ -2750,7 +2827,7 @@ class basic_json
                  not std::is_same<basic_json_t, typename T::value_type>::value and
                  not std::is_arithmetic<T>::value and
                  not std::is_convertible<std::string, T>::value and
-                 not has_mapped_type<T>::value, int>::type = 0>
+                 not detail::has_mapped_type<T>::value, int>::type = 0>
     T get_impl(T* /*unused*/) const
     {
         if (is_array())
@@ -2791,7 +2868,7 @@ class basic_json
     /// get an array (explicit)
     template<class T, typename std::enable_if<
                  std::is_same<basic_json, typename T::value_type>::value and
-                 not has_mapped_type<T>::value, int>::type = 0>
+                 not detail::has_mapped_type<T>::value, int>::type = 0>
     T get_impl(T* /*unused*/) const
     {
         if (is_array())

test/src/unit-constructor3.cpp

@@ -42,22 +42,12 @@ struct pod_type {
   short c;
 };
 
-inline bool operator==(pod_type const& lhs, pod_type const& rhs) noexcept
-{
-  return std::tie(lhs.a, lhs.b, lhs.c) == std::tie(rhs.a, rhs.b, rhs.c);
-}
-
 struct bit_more_complex_type {
   pod_type a;
   pod_type b;
   std::string c;
 };
 
-inline bool operator==(bit_more_complex_type const &lhs,
-                       bit_more_complex_type const &rhs) noexcept {
-  return std::tie(lhs.a, lhs.b, lhs.c) == std::tie(rhs.a, rhs.b, rhs.c);
-}
-
 // best optional implementation ever
 template <typename T>
 class optional_type
@@ -68,11 +58,97 @@ public:
   explicit operator bool() const noexcept { return _val != nullptr; }
 
   T const &operator*() const { return *_val; }
+  optional_type& operator=(T const& t)
+  {
+    _val = std::make_shared<T>(t);
+    return *this;
+  }
 
 private:
   std::shared_ptr<T> _val;
 };
 
+struct no_json_traits_type
+{
+  int a;
+};
+
+// free to/from_json functions
+
+json to_json(empty_type)
+{
+  return json::object();
+}
+
+json to_json(pod_type const& p)
+{
+  return {{"a", p.a}, {"b", p.b}, {"c", p.c}};
+}
+
+json to_json(bit_more_complex_type const& p)
+{
+  using nlohmann::to_json;
+  return json{{"a", to_json(p.a)}, {"b", to_json(p.b)}, {"c", p.c}};
+}
+
+template <typename T>
+json to_json(optional_type<T> const& opt)
+{
+  using nlohmann::to_json;
+  if (!opt)
+    return nullptr;
+  return to_json(*opt);
+}
+
+json to_json(no_json_traits_type const& p)
+{
+  json ret;
+  ret["a"] = p.a;
+  return ret;
+}
+
+void from_json(json const&j, empty_type& t)
+{
+  assert(j.empty());
+  t = empty_type{};
+}
+
+void from_json(json const&j, pod_type& t)
+{
+  t = {j["a"].get<int>(), j["b"].get<char>(), j["c"].get<short>()};
+}
+
+void from_json(json const&j, bit_more_complex_type& t)
+{
+   // relying on json_traits struct here..
+   t = {j["a"].get<udt::pod_type>(), j["b"].get<udt::pod_type>(),
+        j["c"].get<std::string>()};
+}
+
+void from_json(json const& j, no_json_traits_type& t)
+{
+  t.a = j["a"].get<int>();
+}
+
+template <typename T>
+void from_json(json const& j, optional_type<T>& t)
+{
+  if (j.is_null())
+    t = optional_type<T>{};
+  else
+    t = j.get<T>();
+}
+
+inline bool operator==(pod_type const& lhs, pod_type const& rhs) noexcept
+{
+  return std::tie(lhs.a, lhs.b, lhs.c) == std::tie(rhs.a, rhs.b, rhs.c);
+}
+
+inline bool operator==(bit_more_complex_type const &lhs,
+                       bit_more_complex_type const &rhs) noexcept {
+  return std::tie(lhs.a, lhs.b, lhs.c) == std::tie(rhs.a, rhs.b, rhs.c);
+}
+
 template <typename T>
 inline bool operator==(optional_type<T> const& lhs, optional_type<T> const& rhs)
 {
@@ -82,6 +158,11 @@ inline bool operator==(optional_type<T> const& lhs, optional_type<T> const& rhs)
     return false;
   return *lhs == *rhs;
 }
+
+inline bool operator==(no_json_traits_type const& lhs, no_json_traits_type const& rhs)
+{
+  return lhs.a == rhs.a;
+}
 }
 
 namespace nlohmann
@@ -163,7 +244,7 @@ TEST_CASE("constructors for user-defined types", "[udt]")
 {
   SECTION("empty type")
   {
-    udt::empty_type const e;
+    udt::empty_type const e{};
     auto const j = json{e};
     auto k = json::object();
     CHECK(j == k);
@@ -300,3 +381,118 @@ TEST_CASE("get<> for user-defined types", "[udt]")
     }
   }
 }
+
+TEST_CASE("to_json free function", "[udt]")
+{
+  SECTION("pod_type")
+  {
+    auto const e = udt::pod_type{42, 42, 42};
+    auto const expected = json{{"a", 42}, {"b", 42}, {"c", 42}};
+
+    auto const j = nlohmann::to_json(e);
+    CHECK(j == expected);
+  }
+
+  SECTION("bit_more_complex_type")
+  {
+    auto const e =
+        udt::bit_more_complex_type{{42, 42, 42}, {41, 41, 41}, "forty"};
+    auto const expected = json{{"a", {{"a", 42}, {"b", 42}, {"c", 42}}},
+                        {"b", {{"a", 41}, {"b", 41}, {"c", 41}}},
+                        {"c", "forty"}};
+    auto const j = nlohmann::to_json(e);
+    CHECK(j == expected);
+  }
+
+  SECTION("optional_type")
+  {
+    SECTION("from null")
+    {
+      udt::optional_type<udt::pod_type> o;
+
+      json expected;
+      auto const j = nlohmann::to_json(o);
+      CHECK(expected == j);
+    }
+
+    SECTION("from value")
+    {
+      udt::optional_type<udt::pod_type> o{{42, 42, 42}};
+
+      auto const expected = json{{"a", 42}, {"b", 42}, {"c", 42}};
+      auto const j = nlohmann::to_json(o);
+      CHECK(expected == j);
+    }
+  }
+
+  SECTION("no json_traits specialization")
+  {
+    udt::no_json_traits_type t{42};
+
+    json expected;
+    expected["a"] = 42;
+    auto const j = nlohmann::to_json(t);
+    CHECK(j == expected);
+  }
+}
+
+TEST_CASE("from_json free function", "[udt]")
+{
+  SECTION("pod_type")
+  {
+    auto const expected = udt::pod_type{42, 42, 42};
+    auto const j = json{{"a", 42}, {"b", 42}, {"c", 42}};
+
+    udt::pod_type p;
+    nlohmann::from_json(j, p);
+    CHECK(p == expected);
+  }
+
+  SECTION("bit_more_complex_type")
+  {
+    auto const expected =
+        udt::bit_more_complex_type{{42, 42, 42}, {41, 41, 41}, "forty"};
+    auto const j = json{{"a", {{"a", 42}, {"b", 42}, {"c", 42}}},
+                        {"b", {{"a", 41}, {"b", 41}, {"c", 41}}},
+                        {"c", "forty"}};
+    udt::bit_more_complex_type p;
+    nlohmann::from_json(j, p);
+    CHECK(p == expected);
+  }
+
+  SECTION("optional_type")
+  {
+    SECTION("from null")
+    {
+      udt::optional_type<udt::pod_type> expected;
+      json j;
+      udt::optional_type<udt::pod_type> o;
+
+      nlohmann::from_json(j, o);
+      CHECK(expected == o);
+    }
+
+    SECTION("from value")
+    {
+      udt::optional_type<udt::pod_type> expected{{42, 42, 42}};
+      auto const j = json{{"a", 42}, {"b", 42}, {"c", 42}};
+      udt::optional_type<udt::pod_type> o;
+
+      nlohmann::from_json(j, o);
+      CHECK(expected == o);
+    }
+  }
+
+  SECTION("no json_traits specialization")
+  {
+    udt::no_json_traits_type expected{42};
+    udt::no_json_traits_type res;
+    json j;
+    j["a"] = 42;
+    nlohmann::from_json(j, res);
+    CHECK(res == expected);
+
+    res = j.get<udt::no_json_traits_type>();
+    CHECK(res == expected);
+  }
+}