Fix iterators to meet (more) std::ranges requirements

Fixes #3130.
Related discussion: #3408

include/nlohmann/detail/iterators/iter_impl.hpp

@@ -51,9 +51,12 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     // make sure BasicJsonType is basic_json or const basic_json
     static_assert(is_basic_json<typename std::remove_const<BasicJsonType>::type>::value,
                   "iter_impl only accepts (const) basic_json");
+    // superficial check for the LegacyBidirectionalIterator named requirement
+    static_assert(std::is_base_of<std::bidirectional_iterator_tag, std::bidirectional_iterator_tag>::value
+                  &&  std::is_base_of<std::bidirectional_iterator_tag, typename array_t::iterator::iterator_category>::value,
+                  "basic_json iterator assumes array and object type iterators satisfy the LegacyBidirectionalIterator named requirement.");
 
   public:
-
     /// The std::iterator class template (used as a base class to provide typedefs) is deprecated in C++17.
     /// The C++ Standard has never required user-defined iterators to derive from std::iterator.
     /// A user-defined iterator should provide publicly accessible typedefs named

include/nlohmann/detail/iterators/iteration_proxy.hpp

@@ -6,6 +6,10 @@
 #include <tuple> // tuple_size, get, tuple_element
 #include <utility> // move
 
+#if JSON_HAS_RANGES
+    #include <ranges> // enable_borrowed_range
+#endif
+
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
@@ -25,14 +29,14 @@ template<typename IteratorType> class iteration_proxy_value
   public:
     using difference_type = std::ptrdiff_t;
     using value_type = iteration_proxy_value;
-    using pointer = value_type * ;
+    using pointer = value_type *;
-    using reference = value_type & ;
+    using reference = value_type &;
     using iterator_category = std::input_iterator_tag;
     using string_type = typename std::remove_cv< typename std::remove_reference<decltype( std::declval<IteratorType>().key() ) >::type >::type;
 
   private:
     /// the iterator
-    IteratorType anchor;
+    IteratorType anchor{};
     /// an index for arrays (used to create key names)
     std::size_t array_index = 0;
     /// last stringified array index
@@ -40,15 +44,30 @@ template<typename IteratorType> class iteration_proxy_value
     /// a string representation of the array index
     mutable string_type array_index_str = "0";
     /// an empty string (to return a reference for primitive values)
-    const string_type empty_str{};
+    string_type empty_str{};
 
   public:
-    explicit iteration_proxy_value(IteratorType it) noexcept
+    explicit iteration_proxy_value() = default;
+    explicit iteration_proxy_value(IteratorType it, std::size_t array_index_ = 0)
+    noexcept(std::is_nothrow_move_constructible<IteratorType>::value
+             && std::is_nothrow_default_constructible<string_type>::value)
         : anchor(std::move(it))
+        , array_index(array_index_)
     {}
 
+    iteration_proxy_value(iteration_proxy_value const&) = default;
+    iteration_proxy_value& operator=(iteration_proxy_value const&) = default;
+    // older GCCs are a bit fussy and require explicit noexcept specifiers on defaulted functions
+    iteration_proxy_value(iteration_proxy_value&&)
+    noexcept(std::is_nothrow_move_constructible<IteratorType>::value
+             && std::is_nothrow_move_constructible<string_type>::value) = default;
+    iteration_proxy_value& operator=(iteration_proxy_value&&)
+    noexcept(std::is_nothrow_move_assignable<IteratorType>::value
+             && std::is_nothrow_move_assignable<string_type>::value) = default;
+    ~iteration_proxy_value() = default;
+
     /// dereference operator (needed for range-based for)
-    iteration_proxy_value& operator*()
+    const iteration_proxy_value& operator*() const
     {
         return *this;
     }
@@ -62,6 +81,14 @@ template<typename IteratorType> class iteration_proxy_value
         return *this;
     }
 
+    iteration_proxy_value operator++(int)& // NOLINT(cert-dcl21-cpp)
+    {
+        auto tmp = iteration_proxy_value(anchor, array_index);
+        ++anchor;
+        ++array_index;
+        return tmp;
+    }
+
     /// equality operator (needed for InputIterator)
     bool operator==(const iteration_proxy_value& o) const
     {
@@ -122,25 +149,34 @@ template<typename IteratorType> class iteration_proxy
 {
   private:
     /// the container to iterate
-    typename IteratorType::reference container;
+    typename IteratorType::pointer container = nullptr;
 
   public:
+    explicit iteration_proxy() = default;
+
     /// construct iteration proxy from a container
     explicit iteration_proxy(typename IteratorType::reference cont) noexcept
-        : container(cont) {}
+        : container(&cont) {}
+
+    iteration_proxy(iteration_proxy const&) = default;
+    iteration_proxy& operator=(iteration_proxy const&) = default;
+    iteration_proxy(iteration_proxy&&) noexcept = default;
+    iteration_proxy& operator=(iteration_proxy&&) noexcept = default;
+    ~iteration_proxy() = default;
 
     /// return iterator begin (needed for range-based for)
-    iteration_proxy_value<IteratorType> begin() noexcept
+    iteration_proxy_value<IteratorType> begin() const noexcept
     {
-        return iteration_proxy_value<IteratorType>(container.begin());
+        return iteration_proxy_value<IteratorType>(container->begin());
     }
 
     /// return iterator end (needed for range-based for)
-    iteration_proxy_value<IteratorType> end() noexcept
+    iteration_proxy_value<IteratorType> end() const noexcept
     {
-        return iteration_proxy_value<IteratorType>(container.end());
+        return iteration_proxy_value<IteratorType>(container->end());
     }
 };
+
 // Structured Bindings Support
 // For further reference see https://blog.tartanllama.xyz/structured-bindings/
 // And see https://github.com/nlohmann/json/pull/1391
@@ -187,3 +223,8 @@ class tuple_element<N, ::nlohmann::detail::iteration_proxy_value<IteratorType >>
     #pragma clang diagnostic pop
 #endif
 } // namespace std
+
+#if JSON_HAS_RANGES
+    template <typename IteratorType>
+    inline constexpr bool ::std::ranges::enable_borrowed_range<::nlohmann::detail::iteration_proxy<IteratorType>> = true;
+#endif

include/nlohmann/detail/macro_scope.hpp

@@ -118,13 +118,11 @@
         #define JSON_HAS_RANGES 0
     #elif defined(__cpp_lib_ranges)
         #define JSON_HAS_RANGES 1
+    #else
+        #define JSON_HAS_RANGES 0
     #endif
 #endif
 
-#ifndef JSON_HAS_RANGES
-    #define JSON_HAS_RANGES 0
-#endif
-
 #if JSON_HEDLEY_HAS_ATTRIBUTE(no_unique_address)
     #define JSON_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #else

single_include/nlohmann/json.hpp

@@ -2345,13 +2345,11 @@ using is_detected_convertible =
         #define JSON_HAS_RANGES 0
     #elif defined(__cpp_lib_ranges)
         #define JSON_HAS_RANGES 1
+    #else
+        #define JSON_HAS_RANGES 0
     #endif
 #endif
 
-#ifndef JSON_HAS_RANGES
-    #define JSON_HAS_RANGES 0
-#endif
-
 #if JSON_HEDLEY_HAS_ATTRIBUTE(no_unique_address)
     #define JSON_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #else
@@ -4660,6 +4658,10 @@ constexpr const auto& from_json = detail::static_const<detail::from_json_fn>::va
 #include <tuple> // tuple_size, get, tuple_element
 #include <utility> // move
 
+#if JSON_HAS_RANGES
+    #include <ranges> // enable_borrowed_range
+#endif
+
 // #include <nlohmann/detail/meta/type_traits.hpp>
 
 // #include <nlohmann/detail/value_t.hpp>
@@ -4681,14 +4683,14 @@ template<typename IteratorType> class iteration_proxy_value
   public:
     using difference_type = std::ptrdiff_t;
     using value_type = iteration_proxy_value;
-    using pointer = value_type * ;
+    using pointer = value_type *;
-    using reference = value_type & ;
+    using reference = value_type &;
     using iterator_category = std::input_iterator_tag;
     using string_type = typename std::remove_cv< typename std::remove_reference<decltype( std::declval<IteratorType>().key() ) >::type >::type;
 
   private:
     /// the iterator
-    IteratorType anchor;
+    IteratorType anchor{};
     /// an index for arrays (used to create key names)
     std::size_t array_index = 0;
     /// last stringified array index
@@ -4696,15 +4698,30 @@ template<typename IteratorType> class iteration_proxy_value
     /// a string representation of the array index
     mutable string_type array_index_str = "0";
     /// an empty string (to return a reference for primitive values)
-    const string_type empty_str{};
+    string_type empty_str{};
 
   public:
-    explicit iteration_proxy_value(IteratorType it) noexcept
+    explicit iteration_proxy_value() = default;
+    explicit iteration_proxy_value(IteratorType it, std::size_t array_index_ = 0)
+    noexcept(std::is_nothrow_move_constructible<IteratorType>::value
+             && std::is_nothrow_default_constructible<string_type>::value)
         : anchor(std::move(it))
+        , array_index(array_index_)
     {}
 
+    iteration_proxy_value(iteration_proxy_value const&) = default;
+    iteration_proxy_value& operator=(iteration_proxy_value const&) = default;
+    // older GCCs are a bit fussy and require explicit noexcept specifiers on defaulted functions
+    iteration_proxy_value(iteration_proxy_value&&)
+    noexcept(std::is_nothrow_move_constructible<IteratorType>::value
+             && std::is_nothrow_move_constructible<string_type>::value) = default;
+    iteration_proxy_value& operator=(iteration_proxy_value&&)
+    noexcept(std::is_nothrow_move_assignable<IteratorType>::value
+             && std::is_nothrow_move_assignable<string_type>::value) = default;
+    ~iteration_proxy_value() = default;
+
     /// dereference operator (needed for range-based for)
-    iteration_proxy_value& operator*()
+    const iteration_proxy_value& operator*() const
     {
         return *this;
     }
@@ -4718,6 +4735,14 @@ template<typename IteratorType> class iteration_proxy_value
         return *this;
     }
 
+    iteration_proxy_value operator++(int)& // NOLINT(cert-dcl21-cpp)
+    {
+        auto tmp = iteration_proxy_value(anchor, array_index);
+        ++anchor;
+        ++array_index;
+        return tmp;
+    }
+
     /// equality operator (needed for InputIterator)
     bool operator==(const iteration_proxy_value& o) const
     {
@@ -4778,25 +4803,34 @@ template<typename IteratorType> class iteration_proxy
 {
   private:
     /// the container to iterate
-    typename IteratorType::reference container;
+    typename IteratorType::pointer container = nullptr;
 
   public:
+    explicit iteration_proxy() = default;
+
     /// construct iteration proxy from a container
     explicit iteration_proxy(typename IteratorType::reference cont) noexcept
-        : container(cont) {}
+        : container(&cont) {}
+
+    iteration_proxy(iteration_proxy const&) = default;
+    iteration_proxy& operator=(iteration_proxy const&) = default;
+    iteration_proxy(iteration_proxy&&) noexcept = default;
+    iteration_proxy& operator=(iteration_proxy&&) noexcept = default;
+    ~iteration_proxy() = default;
 
     /// return iterator begin (needed for range-based for)
-    iteration_proxy_value<IteratorType> begin() noexcept
+    iteration_proxy_value<IteratorType> begin() const noexcept
     {
-        return iteration_proxy_value<IteratorType>(container.begin());
+        return iteration_proxy_value<IteratorType>(container->begin());
     }
 
     /// return iterator end (needed for range-based for)
-    iteration_proxy_value<IteratorType> end() noexcept
+    iteration_proxy_value<IteratorType> end() const noexcept
     {
-        return iteration_proxy_value<IteratorType>(container.end());
+        return iteration_proxy_value<IteratorType>(container->end());
     }
 };
+
 // Structured Bindings Support
 // For further reference see https://blog.tartanllama.xyz/structured-bindings/
 // And see https://github.com/nlohmann/json/pull/1391
@@ -4844,6 +4878,11 @@ class tuple_element<N, ::nlohmann::detail::iteration_proxy_value<IteratorType >>
 #endif
 } // namespace std
 
+#if JSON_HAS_RANGES
+    template <typename IteratorType>
+    inline constexpr bool ::std::ranges::enable_borrowed_range<::nlohmann::detail::iteration_proxy<IteratorType>> = true;
+#endif
+
 // #include <nlohmann/detail/meta/cpp_future.hpp>
 
 // #include <nlohmann/detail/meta/type_traits.hpp>
@@ -12197,9 +12236,12 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
     // make sure BasicJsonType is basic_json or const basic_json
     static_assert(is_basic_json<typename std::remove_const<BasicJsonType>::type>::value,
                   "iter_impl only accepts (const) basic_json");
+    // superficial check for the LegacyBidirectionalIterator named requirement
+    static_assert(std::is_base_of<std::bidirectional_iterator_tag, std::bidirectional_iterator_tag>::value
+                  &&  std::is_base_of<std::bidirectional_iterator_tag, typename array_t::iterator::iterator_category>::value,
+                  "basic_json iterator assumes array and object type iterators satisfy the LegacyBidirectionalIterator named requirement.");
 
   public:
-
     /// The std::iterator class template (used as a base class to provide typedefs) is deprecated in C++17.
     /// The C++ Standard has never required user-defined iterators to derive from std::iterator.
     /// A user-defined iterator should provide publicly accessible typedefs named

tests/src/unit-items.cpp

@@ -80,7 +80,7 @@ TEST_CASE("iterator_wrapper")
             json j = { {"A", 1}, {"B", 2} };
             int counter = 1;
 
-            for (auto& i : json::iterator_wrapper(j))
+            for (auto& i : json::iterator_wrapper(j)) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -226,7 +226,7 @@ TEST_CASE("iterator_wrapper")
             const json j = { {"A", 1}, {"B", 2} };
             int counter = 1;
 
-            for (auto& i : json::iterator_wrapper(j))
+            for (auto& i : json::iterator_wrapper(j)) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -361,7 +361,7 @@ TEST_CASE("iterator_wrapper")
             json j = { "A", "B" };
             int counter = 1;
 
-            for (auto& i : json::iterator_wrapper(j))
+            for (auto& i : json::iterator_wrapper(j)) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -507,7 +507,7 @@ TEST_CASE("iterator_wrapper")
             const json j = { "A", "B" };
             int counter = 1;
 
-            for (auto& i : json::iterator_wrapper(j))
+            for (auto& i : json::iterator_wrapper(j)) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -624,7 +624,7 @@ TEST_CASE("iterator_wrapper")
             json j = 1;
             int counter = 1;
 
-            for (auto& i : json::iterator_wrapper(j))
+            for (auto& i : json::iterator_wrapper(j)) // NOLINT(readability-qualified-auto)
             {
                 ++counter;
                 CHECK(i.key() == "");
@@ -693,7 +693,7 @@ TEST_CASE("iterator_wrapper")
             const json j = 1;
             int counter = 1;
 
-            for (auto& i : json::iterator_wrapper(j))
+            for (auto& i : json::iterator_wrapper(j)) // NOLINT(readability-qualified-auto)
             {
                 ++counter;
                 CHECK(i.key() == "");
@@ -777,7 +777,7 @@ TEST_CASE("items()")
             json j = { {"A", 1}, {"B", 2} };
             int counter = 1;
 
-            for (auto& i : j.items())
+            for (auto& i : j.items()) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -939,7 +939,7 @@ TEST_CASE("items()")
             const json j = { {"A", 1}, {"B", 2} };
             int counter = 1;
 
-            for (auto& i : j.items())
+            for (auto& i : j.items()) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -1074,7 +1074,7 @@ TEST_CASE("items()")
             json j = { "A", "B" };
             int counter = 1;
 
-            for (auto& i : j.items())
+            for (auto& i : j.items()) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -1220,7 +1220,7 @@ TEST_CASE("items()")
             const json j = { "A", "B" };
             int counter = 1;
 
-            for (auto& i : j.items())
+            for (auto& i : j.items()) // NOLINT(readability-qualified-auto)
             {
                 switch (counter++)
                 {
@@ -1337,7 +1337,7 @@ TEST_CASE("items()")
             json j = 1;
             int counter = 1;
 
-            for (auto& i : j.items())
+            for (auto& i : j.items()) // NOLINT(readability-qualified-auto)
             {
                 ++counter;
                 CHECK(i.key() == "");
@@ -1406,7 +1406,7 @@ TEST_CASE("items()")
             const json j = 1;
             int counter = 1;
 
-            for (auto& i : j.items())
+            for (auto& i : j.items()) // NOLINT(readability-qualified-auto)
             {
                 ++counter;
                 CHECK(i.key() == "");

tests/src/unit-iterators2.cpp

@@ -32,6 +32,12 @@ SOFTWARE.
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
+#if JSON_HAS_RANGES
+    // JSON_HAS_CPP_20 (magic keyword; do not remove)
+    #include <algorithm>
+    #include <ranges>
+#endif
+
 TEST_CASE("iterators 2")
 {
     SECTION("iterator comparisons")
@@ -881,4 +887,100 @@ TEST_CASE("iterators 2")
             }
         }
     }
+
+
+#if JSON_HAS_RANGES
+    SECTION("ranges")
+    {
+        SECTION("concepts")
+        {
+            using nlohmann::detail::iteration_proxy_value;
+            CHECK(std::bidirectional_iterator<json::iterator>);
+            CHECK(std::input_iterator<iteration_proxy_value<json::iterator>>);
+
+            CHECK(std::is_same<json::iterator, std::ranges::iterator_t<json>>::value);
+            CHECK(std::ranges::bidirectional_range<json>);
+
+            using nlohmann::detail::iteration_proxy;
+            using items_type = decltype(std::declval<json&>().items());
+            CHECK(std::is_same<items_type, iteration_proxy<json::iterator>>::value);
+            CHECK(std::is_same<iteration_proxy_value<json::iterator>, std::ranges::iterator_t<items_type>>::value);
+            CHECK(std::ranges::input_range<items_type>);
+        }
+
+        // libstdc++ algorithms don't work with Clang 15 (04/2022)
+#if !defined(__clang__) || (defined(__clang__) && defined(__GLIBCXX__))
+        SECTION("algorithms")
+        {
+            SECTION("copy")
+            {
+                json j{"foo", "bar"};
+                auto j_copied = json::array();
+
+                std::ranges::copy(j, std::back_inserter(j_copied));
+
+                CHECK(j == j_copied);
+            }
+
+            SECTION("find_if")
+            {
+                json j{1, 3, 2, 4};
+                auto j_even = json::array();
+
+#if JSON_USE_IMPLICIT_CONVERSIONS
+                auto it = std::ranges::find_if(j, [](int v) noexcept
+                {
+                    return (v % 2) == 0;
+                });
+#else
+                auto it = std::ranges::find_if(j, [](const json & j) noexcept
+                {
+                    int v;
+                    j.get_to(v);
+                    return (v % 2) == 0;
+                });
+#endif
+
+                CHECK(*it == 2);
+            }
+        }
+#endif
+
+        // libstdc++ views don't work with Clang 15 (04/2022)
+        // libc++ hides limited ranges implementation behind guard macro
+#if !(defined(__clang__) && (defined(__GLIBCXX__) || defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)))
+        SECTION("views")
+        {
+            SECTION("reverse")
+            {
+                json j{1, 2, 3, 4, 5};
+                json j_expected{5, 4, 3, 2, 1};
+
+                auto reversed = j | std::views::reverse;
+                CHECK(reversed == j_expected);
+            }
+
+            SECTION("transform")
+            {
+                json j
+                {
+                    { "a_key", "a_value"},
+                    { "b_key", "b_value"},
+                    { "c_key", "c_value"},
+                };
+                json j_expected{"a_key", "b_key", "c_key"};
+
+                auto transformed = j.items() | std::views::transform([](const auto & item)
+                {
+                    return item.key();
+                });
+                auto j_transformed = json::array();
+                std::ranges::copy(transformed, std::back_inserter(j_transformed));
+
+                CHECK(j_transformed == j_expected);
+            }
+        }
+#endif
+    }
+#endif
 }