🐛 fix assertion failure #2838

include/nlohmann/json.hpp

@@ -5371,8 +5371,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add element to array (move semantics)
+        const auto capacity = m_value.array->capacity();
         m_value.array->push_back(std::move(val));
-        set_parent(m_value.array->back());
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of last element is sufficient
+            set_parent(m_value.array->back());
+        }
+        else
+        {
+            // capacity has changed: update all elements' parents
+            set_parents();
+        }
         // if val is moved from, basic_json move constructor marks it null so we do not call the destructor
     }
 
@@ -5407,7 +5417,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add element to array
+        const auto capacity = m_value.array->capacity();
         m_value.array->push_back(val);
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of last element is sufficient
+            set_parent(m_value.array->back());
+        }
+        else
+        {
+            // capacity has changed: update all elements' parents
+            set_parents();
+        }
         set_parent(m_value.array->back());
     }
 
@@ -5562,12 +5583,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add element to array (perfect forwarding)
-#ifdef JSON_HAS_CPP_17
+        const auto capacity = m_value.array->capacity();
-        return set_parent(m_value.array->emplace_back(std::forward<Args>(args)...));
-#else
         m_value.array->emplace_back(std::forward<Args>(args)...);
-        return set_parent(m_value.array->back());
+
-#endif
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of last element is sufficient
+            return set_parent(m_value.array->back());
+        }
+
+        // capacity has changed: update all elements' parents
+        set_parents();
+        return m_value.array->back();
     }
 
     /*!
@@ -5630,12 +5657,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @note: This uses std::distance to support GCC 4.8,
     ///        see https://github.com/nlohmann/json/pull/1257
     template<typename... Args>
-    iterator insert_iterator(const_iterator pos, Args&& ... args)
+    iterator insert_iterator(const_iterator pos, typename iterator::difference_type cnt, Args&& ... args)
     {
         iterator result(this);
         JSON_ASSERT(m_value.array != nullptr);
 
         auto insert_pos = std::distance(m_value.array->begin(), pos.m_it.array_iterator);
+        const auto capacity = m_value.array->capacity();
         m_value.array->insert(pos.m_it.array_iterator, std::forward<Args>(args)...);
         result.m_it.array_iterator = m_value.array->begin() + insert_pos;
 
@@ -5643,6 +5671,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, cnt, val);
         // but the return value of insert is missing in GCC 4.8, so it is written this way instead.
 
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of inserted elements is sufficient
+            set_parents(result, cnt);
+        }
+        else
+        {
+            // capacity has changed: update all elements' parents
+            set_parents();
+        }
+
         return result;
     }
 
@@ -5680,7 +5719,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             }
 
             // insert to array and return iterator
-            return set_parents(insert_iterator(pos, val), static_cast<typename iterator::difference_type>(1));
+            return insert_iterator(pos, static_cast<typename iterator::difference_type>(1), val);
         }
 
         JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name()), *this));
@@ -5731,7 +5770,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             }
 
             // insert to array and return iterator
-            return set_parents(insert_iterator(pos, cnt, val), static_cast<typename iterator::difference_type>(cnt));
+            return insert_iterator(pos, static_cast<typename iterator::difference_type>(cnt), cnt, val);
         }
 
         JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name()), *this));
@@ -5793,7 +5832,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // insert to array and return iterator
-        return set_parents(insert_iterator(pos, first.m_it.array_iterator, last.m_it.array_iterator), std::distance(first, last));
+        return insert_iterator(pos, std::distance(first, last), first.m_it.array_iterator, last.m_it.array_iterator);
     }
 
     /*!
@@ -5835,7 +5874,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // insert to array and return iterator
-        return set_parents(insert_iterator(pos, ilist.begin(), ilist.end()), static_cast<typename iterator::difference_type>(ilist.size()));
+        return insert_iterator(pos, static_cast<typename iterator::difference_type>(ilist.size()), ilist.begin(), ilist.end());
     }
 
     /*!

single_include/nlohmann/json.hpp

@@ -22406,8 +22406,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add element to array (move semantics)
+        const auto capacity = m_value.array->capacity();
         m_value.array->push_back(std::move(val));
-        set_parent(m_value.array->back());
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of last element is sufficient
+            set_parent(m_value.array->back());
+        }
+        else
+        {
+            // capacity has changed: update all elements' parents
+            set_parents();
+        }
         // if val is moved from, basic_json move constructor marks it null so we do not call the destructor
     }
 
@@ -22442,7 +22452,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add element to array
+        const auto capacity = m_value.array->capacity();
         m_value.array->push_back(val);
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of last element is sufficient
+            set_parent(m_value.array->back());
+        }
+        else
+        {
+            // capacity has changed: update all elements' parents
+            set_parents();
+        }
         set_parent(m_value.array->back());
     }
 
@@ -22597,12 +22618,18 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // add element to array (perfect forwarding)
-#ifdef JSON_HAS_CPP_17
+        const auto capacity = m_value.array->capacity();
-        return set_parent(m_value.array->emplace_back(std::forward<Args>(args)...));
-#else
         m_value.array->emplace_back(std::forward<Args>(args)...);
-        return set_parent(m_value.array->back());
+
-#endif
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of last element is sufficient
+            return set_parent(m_value.array->back());
+        }
+
+        // capacity has changed: update all elements' parents
+        set_parents();
+        return m_value.array->back();
     }
 
     /*!
@@ -22665,12 +22692,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @note: This uses std::distance to support GCC 4.8,
     ///        see https://github.com/nlohmann/json/pull/1257
     template<typename... Args>
-    iterator insert_iterator(const_iterator pos, Args&& ... args)
+    iterator insert_iterator(const_iterator pos, typename iterator::difference_type cnt, Args&& ... args)
     {
         iterator result(this);
         JSON_ASSERT(m_value.array != nullptr);
 
         auto insert_pos = std::distance(m_value.array->begin(), pos.m_it.array_iterator);
+        const auto capacity = m_value.array->capacity();
         m_value.array->insert(pos.m_it.array_iterator, std::forward<Args>(args)...);
         result.m_it.array_iterator = m_value.array->begin() + insert_pos;
 
@@ -22678,6 +22706,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // result.m_it.array_iterator = m_value.array->insert(pos.m_it.array_iterator, cnt, val);
         // but the return value of insert is missing in GCC 4.8, so it is written this way instead.
 
+        if (capacity == m_value.array->capacity())
+        {
+            // capacity has not changed: updating parent of inserted elements is sufficient
+            set_parents(result, cnt);
+        }
+        else
+        {
+            // capacity has changed: update all elements' parents
+            set_parents();
+        }
+
         return result;
     }
 
@@ -22715,7 +22754,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             }
 
             // insert to array and return iterator
-            return set_parents(insert_iterator(pos, val), static_cast<typename iterator::difference_type>(1));
+            return insert_iterator(pos, static_cast<typename iterator::difference_type>(1), val);
         }
 
         JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name()), *this));
@@ -22766,7 +22805,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             }
 
             // insert to array and return iterator
-            return set_parents(insert_iterator(pos, cnt, val), static_cast<typename iterator::difference_type>(cnt));
+            return insert_iterator(pos, static_cast<typename iterator::difference_type>(cnt), cnt, val);
         }
 
         JSON_THROW(type_error::create(309, "cannot use insert() with " + std::string(type_name()), *this));
@@ -22828,7 +22867,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // insert to array and return iterator
-        return set_parents(insert_iterator(pos, first.m_it.array_iterator, last.m_it.array_iterator), std::distance(first, last));
+        return insert_iterator(pos, std::distance(first, last), first.m_it.array_iterator, last.m_it.array_iterator);
     }
 
     /*!
@@ -22870,7 +22909,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         }
 
         // insert to array and return iterator
-        return set_parents(insert_iterator(pos, ilist.begin(), ilist.end()), static_cast<typename iterator::difference_type>(ilist.size()));
+        return insert_iterator(pos, static_cast<typename iterator::difference_type>(ilist.size()), ilist.begin(), ilist.end());
     }
 
     /*!

test/src/unit-diagnostics.cpp

@@ -109,4 +109,61 @@ TEST_CASE("Better diagnostics")
         j["/foo"] = {1, 2, 3};
         CHECK_THROWS_WITH_AS(j.unflatten(), "[json.exception.type_error.315] (/~1foo) values in object must be primitive", json::type_error);
     }
+
+    SECTION("Regression test for https://github.com/nlohmann/json/issues/2838")
+    {
+        // void push_back(basic_json&& val)
+        {
+            json j_arr = json::array();
+            j_arr.push_back(json::object());
+            j_arr.push_back(json::object());
+            json j_obj = json::object();
+            j_obj["key"] = j_arr;
+        }
+
+        // void push_back(const basic_json& val)
+        {
+            json j_arr = json::array();
+            auto object = json::object();
+            j_arr.push_back(object);
+            j_arr.push_back(object);
+            json j_obj = json::object();
+            j_obj["key"] = j_arr;
+        }
+
+        // reference emplace_back(Args&& ... args)
+        {
+            json j_arr = json::array();
+            j_arr.emplace_back(json::object());
+            j_arr.emplace_back(json::object());
+            json j_obj = json::object();
+            j_obj["key"] = j_arr;
+        }
+
+        // iterator insert(const_iterator pos, const basic_json& val)
+        {
+            json j_arr = json::array();
+            j_arr.insert(j_arr.begin(), json::object());
+            j_arr.insert(j_arr.begin(), json::object());
+            json j_obj = json::object();
+            j_obj["key"] = j_arr;
+        }
+
+        // iterator insert(const_iterator pos, size_type cnt, const basic_json& val)
+        {
+            json j_arr = json::array();
+            j_arr.insert(j_arr.begin(), 2, json::object());
+            json j_obj = json::object();
+            j_obj["key"] = j_arr;
+        }
+
+        // iterator insert(const_iterator pos, const_iterator first, const_iterator last)
+        {
+            json j_arr = json::array();
+            json j_objects = {json::object(), json::object()};
+            j_arr.insert(j_arr.begin(), j_objects.begin(), j_objects.end());
+            json j_obj = json::object();
+            j_obj["key"] = j_arr;
+        }
+    }
 }