💩 first try on #1045

include/nlohmann/detail/iterators/iteration_proxy.hpp

@@ -2,6 +2,7 @@
 
 #include <cstddef> // size_t
 #include <string> // string, to_string
+#include <iterator> // input_iterator_tag
 
 #include <nlohmann/detail/value_t.hpp>
 
@@ -16,6 +17,13 @@ template<typename IteratorType> class iteration_proxy
     /// helper class for iteration
     class iteration_proxy_internal
     {
+      public:
+        using difference_type = std::ptrdiff_t;
+        using value_type = iteration_proxy_internal;
+        using pointer = iteration_proxy_internal*;
+        using reference = iteration_proxy_internal&;
+        using iterator_category = std::input_iterator_tag;
+
       private:
         /// the iterator
         IteratorType anchor;
@@ -25,6 +33,9 @@ template<typename IteratorType> class iteration_proxy
       public:
         explicit iteration_proxy_internal(IteratorType it) noexcept : anchor(it) {}
 
+        iteration_proxy_internal(const iteration_proxy_internal&) = default;
+        iteration_proxy_internal& operator=(const iteration_proxy_internal&) = default;
+
         /// dereference operator (needed for range-based for)
         iteration_proxy_internal& operator*()
         {
@@ -40,6 +51,12 @@ template<typename IteratorType> class iteration_proxy
             return *this;
         }
 
+        /// equality operator (needed for InputIterator)
+        bool operator==(const iteration_proxy_internal& o) const noexcept
+        {
+            return anchor == o.anchor;
+        }
+
         /// inequality operator (needed for range-based for)
         bool operator!=(const iteration_proxy_internal& o) const noexcept
         {

single_include/nlohmann/json.hpp

@@ -4491,6 +4491,7 @@ class iter_impl
 
 #include <cstddef> // size_t
 #include <string> // string, to_string
+#include <iterator> // input_iterator_tag
 
 // #include <nlohmann/detail/value_t.hpp>
 
@@ -4506,6 +4507,13 @@ template<typename IteratorType> class iteration_proxy
     /// helper class for iteration
     class iteration_proxy_internal
     {
+      public:
+        using difference_type = std::ptrdiff_t;
+        using value_type = iteration_proxy_internal;
+        using pointer = iteration_proxy_internal*;
+        using reference = iteration_proxy_internal&;
+        using iterator_category = std::input_iterator_tag;
+
       private:
         /// the iterator
         IteratorType anchor;
@@ -4515,6 +4523,9 @@ template<typename IteratorType> class iteration_proxy
       public:
         explicit iteration_proxy_internal(IteratorType it) noexcept : anchor(it) {}
 
+        iteration_proxy_internal(const iteration_proxy_internal&) = default;
+        iteration_proxy_internal& operator=(const iteration_proxy_internal&) = default;
+
         /// dereference operator (needed for range-based for)
         iteration_proxy_internal& operator*()
         {
@@ -4530,6 +4541,12 @@ template<typename IteratorType> class iteration_proxy
             return *this;
         }
 
+        /// equality operator (needed for InputIterator)
+        bool operator==(const iteration_proxy_internal& o) const noexcept
+        {
+            return anchor == o.anchor;
+        }
+
         /// inequality operator (needed for range-based for)
         bool operator!=(const iteration_proxy_internal& o) const noexcept
         {

test/src/unit-regression.cpp

@@ -1615,4 +1615,25 @@ TEST_CASE("regression tests")
         float_json j2 = {1000.0, 2000.0, 3000.0};
         CHECK(float_json::from_ubjson(float_json::to_ubjson(j2, true, true)) == j2);
     }
+
+    SECTION("issue #1045 - Using STL algorithms with JSON containers with expected results?")
+    {
+        json diffs = nlohmann::json::array();
+        json m1{{"key1", 42}};
+        json m2{{"key2", 42}};
+        auto p1 = m1.items();
+        auto p2 = m2.items();
+
+        using it_type = decltype(p1.begin());
+
+        std::set_difference(
+            p1.begin(), p1.end(),
+            p2.begin(), p2.end(),
+            std::inserter(diffs, diffs.end()), [&](const it_type & e1, const it_type & e2) -> bool
+        {
+            return (e1.key() < e2.key()) and (e1.value() < e2.value());
+        });
+
+        CHECK(diffs.size() == 2);
+    }
 }