Address review comments on operator[](json_pointer) rollback

- undo() shrinks the array with resize() instead of erase()
- comments say what "will be" done in get_unchecked_create
- tests: avoid a clang-tidy performance-unnecessary-copy-initialization
  hit (unit-alt-string) and the word "access" in section names, which
  Flawfinder flags as race/access

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/json_pointer.hpp

@@ -810,7 +810,7 @@ class json_pointer
                 case change::array_grown:
                 {
                     auto& elements = *m_value->m_data.m_value.array;
-                    elements.erase(std::next(elements.begin(), static_cast<typename BasicJsonType::difference_type>(m_old_size)), elements.end());
+                    elements.resize(m_old_size);
                     break;
                 }
                 case change::none:
@@ -1271,7 +1271,7 @@ class json_pointer
                         // explicitly treat "-" as index beyond the end
                         return array_append(*ptr);
                     }
-                    // an element is appended in get_unchecked_create
+                    // an element will be appended in get_unchecked_create
                     break;
                 }
                 if (token + 1 == end)
@@ -1282,14 +1282,14 @@ class json_pointer
                 const auto idx = array_index<BasicJsonType>(*token);
                 if (idx >= ptr->m_data.m_value.array->size())
                 {
-                    // the array is padded in get_unchecked_create
+                    // the array will be padded in get_unchecked_create
                     break;
                 }
                 ptr = &(*ptr->m_data.m_value.array)[idx];
             }
             else if (ptr->is_null())
             {
-                // the null value is converted in get_unchecked_create
+                // the null value will be converted in get_unchecked_create
                 break;
             }
             else

single_include/nlohmann/json.hpp

@@ -24875,7 +24875,7 @@ class json_pointer
                 case change::array_grown:
                 {
                     auto& elements = *m_value->m_data.m_value.array;
-                    elements.erase(std::next(elements.begin(), static_cast<typename BasicJsonType::difference_type>(m_old_size)), elements.end());
+                    elements.resize(m_old_size);
                     break;
                 }
                 case change::none:
@@ -25336,7 +25336,7 @@ class json_pointer
                         // explicitly treat "-" as index beyond the end
                         return array_append(*ptr);
                     }
-                    // an element is appended in get_unchecked_create
+                    // an element will be appended in get_unchecked_create
                     break;
                 }
                 if (token + 1 == end)
@@ -25347,14 +25347,14 @@ class json_pointer
                 const auto idx = array_index<BasicJsonType>(*token);
                 if (idx >= ptr->m_data.m_value.array->size())
                 {
-                    // the array is padded in get_unchecked_create
+                    // the array will be padded in get_unchecked_create
                     break;
                 }
                 ptr = &(*ptr->m_data.m_value.array)[idx];
             }
             else if (ptr->is_null())
             {
-                // the null value is converted in get_unchecked_create
+                // the null value will be converted in get_unchecked_create
                 break;
             }
             else

tests/src/unit-alt-string.cpp

@@ -385,9 +385,9 @@ TEST_CASE("alternative string type")
         CHECK(j.dump() == R"({"a":{"b":[1,null,null,{"x":[{"y":1}]},4],"z":2},"c":[{"q":3}]})");
 
         // a failing access leaves the value unchanged
-        alt_json k = before;
+        j = before;
-        CHECK_THROWS_AS(k[alt_json::json_pointer("/a/n/99999999999999999999999")], alt_json::out_of_range&);
+        CHECK_THROWS_AS(j[alt_json::json_pointer("/a/n/99999999999999999999999")], alt_json::out_of_range&);
-        CHECK(k == before);
+        CHECK(j == before);
     }
 
     SECTION("operator/(std::size_t)")

tests/src/unit-json_pointer.cpp

@@ -613,13 +613,13 @@ TEST_CASE("JSON pointers")
                                  "[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'", json::parse_error&);
         }
 
-        SECTION("a failing access leaves the value unchanged")
+        SECTION("a failing operator[] leaves the value unchanged")
         {
             check_strong_guarantee_cases<json>();
             check_strong_guarantee_cases<nlohmann::ordered_json>();
         }
 
-        SECTION("a successful access still creates the same values")
+        SECTION("a successful operator[] still creates the same values")
         {
             json j = {{"a", {1}}};
             j["/a/3/x/-/y"_json_pointer] = 1;