json_view: view_mode enum instead of bool Editable, node_kind for node::kind (#5919)

* json_view: view_mode enum instead of bool Editable, node_kind for node::kind

basic_json_document and basic_json_view take a view_mode (read_only, the
default, or editable) instead of a bool: basic_json_document<json, true>
did not say what true means. The shortcut aliases are unchanged.

The kind of a node is a node_kind (the value_t values plus link, 10 as
before), which removes the static_cast<std::uint8_t>(value_t::...) of
every comparison. The node and image layouts are unchanged.

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

* json_view: node_kind in the node writers, spell out read_only, comment fixes

The builder and the edit functions take and store a node_kind instead of
casting a value_t; the numbering assertions and the comments now speak of
node_kind, and the link kind notes that it follows value_t::discarded.
The read-only aliases name view_mode::read_only like the editable ones.
view_mode.md says the mode is a compile-time property of the type.

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

* json_view: views of read-only documents convert to editable views

A read-only document has no edits, so a view of it reads correctly as a
view of an editable document before its first edit. The implicit
conversion lets a function taking a json_editable_view accept the views
of both modes. There is no conversion the other way: a read-only view of
an editable document would read wrong values after the next edit.

The mode comparisons in the cross-mode operator==/!= use an
is_other_mode trait, which astyle keeps compact.

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

---------

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

include/nlohmann/detail/view/builder.hpp

@@ -639,11 +639,11 @@ run_found:
             switch (cur())
             {
                 case '{':
-                    open(value_t::object);
+                    open(node_kind::object);
                     ++p;
                     goto obj_first;
                 case '[':
-                    open(value_t::array);
+                    open(node_kind::array);
                     ++p;
                     goto arr_first;
                 default:
@@ -665,11 +665,11 @@ run_found:
             if (NLOHMANN_VIEW_UNLIKELY(!string<true>())) { return false; }      \
             break;                                                              \
         case '{':                                                               \
-            open(value_t::object);                                              \
+            open(node_kind::object);                                              \
             ++p;                                                                \
             goto obj_first;                                                     \
         case '[':                                                               \
-            open(value_t::array);                                               \
+            open(node_kind::array);                                               \
             ++p;                                                                \
             goto arr_first;                                                     \
         case '-':                                                               \
@@ -886,17 +886,17 @@ root_done:
 
         NLOHMANN_VIEW_ALWAYS_INLINE bool literal_true()
         {
-            return literal("true", value_t::boolean, node_flags::is_true);
+            return literal("true", node_kind::boolean, node_flags::is_true);
         }
 
         NLOHMANN_VIEW_ALWAYS_INLINE bool literal_false()
         {
-            return literal("false", value_t::boolean, 0);
+            return literal("false", node_kind::boolean, 0);
         }
 
         NLOHMANN_VIEW_ALWAYS_INLINE bool literal_null()
         {
-            return literal("null", value_t::null, 0);
+            return literal("null", node_kind::null, 0);
         }
 
         /// skip whitespace (and comments); false on a malformed comment
@@ -962,7 +962,7 @@ indent_done:
 
         /// append a node: (kind, flags, extra, off) and the second word (len, or
         /// an integer's value); two stores on little-endian targets
-        NLOHMANN_VIEW_ALWAYS_INLINE node* push_node(value_t k, std::uint8_t flags, std::uint16_t extra, std::size_t off, std::uint64_t second)
+        NLOHMANN_VIEW_ALWAYS_INLINE node* push_node(node_kind k, std::uint8_t flags, std::uint16_t extra, std::size_t off, std::uint64_t second)
         {
             if (NLOHMANN_VIEW_UNLIKELY(out == cap))
             {
@@ -978,7 +978,7 @@ indent_done:
             std::memcpy(reinterpret_cast<unsigned char*>(n), &first, 8);
             std::memcpy(reinterpret_cast<unsigned char*>(n) + 8, &second, 8);
 #else
-            n->kind = static_cast<std::uint8_t>(k);
+            n->kind = k;
             n->flags = flags;
             n->extra = extra;
             n->off = static_cast<std::uint32_t>(off);
@@ -990,7 +990,7 @@ indent_done:
             return n;
         }
 
-        NLOHMANN_VIEW_ALWAYS_INLINE void open(value_t k)
+        NLOHMANN_VIEW_ALWAYS_INLINE void open(node_kind k)
         {
             // (base is read after push_node(), which moves the node array when it
             // grows; in one expression the order of the two is unspecified)
@@ -1017,7 +1017,7 @@ indent_done:
             max_depth = (std::max)(max_depth, depth);
             cur_idx = idx;
             cur_count = 0;
-            cur_is_object = k == value_t::object;
+            cur_is_object = k == node_kind::object;
         }
 
         NLOHMANN_VIEW_ALWAYS_INLINE void close()
@@ -1047,7 +1047,7 @@ indent_done:
 
         /// the literal `text` at p (N counts the terminating NUL)
         template<std::size_t N>
-        NLOHMANN_VIEW_ALWAYS_INLINE bool literal(const char (&text)[N], value_t k, std::uint8_t flags) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays): a string literal
+        NLOHMANN_VIEW_ALWAYS_INLINE bool literal(const char (&text)[N], node_kind k, std::uint8_t flags) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays): a string literal
         {
             const std::size_t n = N - 1;
             if (NLOHMANN_VIEW_UNLIKELY(e - p < static_cast<std::ptrdiff_t>(n) || std::memcmp(p, text, n) != 0))
@@ -1126,23 +1126,23 @@ indent_done:
                 is_float = true;
             }
 
-            value_t kind = value_t::number_float;
+            node_kind kind = node_kind::number_float;
             if (!is_float)
             {
                 // integers that do not fit become floats, as in parse()
                 if (negative)
                 {
-                    kind = digits_exceed(int_start, int_digits, "9223372036854775808") ? value_t::number_float : value_t::number_integer;
+                    kind = digits_exceed(int_start, int_digits, "9223372036854775808") ? node_kind::number_float : node_kind::number_integer;
                 }
                 else
                 {
-                    kind = digits_exceed(int_start, int_digits, "18446744073709551615") ? value_t::number_float : value_t::number_unsigned;
+                    kind = digits_exceed(int_start, int_digits, "18446744073709551615") ? node_kind::number_float : node_kind::number_unsigned;
                 }
             }
             // parse() rejects floats that overflow; only numbers whose magnitude
             // could reach the largest FloatType (1e308 for double, 1e38 for
             // float) need the conversion
-            if (NLOHMANN_VIEW_UNLIKELY(static_cast<std::int64_t>(int_digits) + exponent > std::numeric_limits<FloatType>::max_exponent10 - 8 && kind == value_t::number_float))
+            if (NLOHMANN_VIEW_UNLIKELY(static_cast<std::int64_t>(int_digits) + exponent > std::numeric_limits<FloatType>::max_exponent10 - 8 && kind == node_kind::number_float))
             {
                 if (builder::float_overflows(s, p))
                 {
@@ -1152,7 +1152,7 @@ indent_done:
             }
             const auto layout = static_cast<std::uint16_t>((int_digits < 255 ? int_digits : 255) | ((frac_digits < 255 ? frac_digits : 255) << 8));
             auto second = static_cast<std::uint64_t>(p - s);
-            if (kind != value_t::number_float)
+            if (kind != node_kind::number_float)
             {
                 // integers are converted now, while their digits are in cache
                 const std::uint64_t m = int_digits <= 19 ? parse_upto19(int_start, static_cast<unsigned>(int_digits), e)
@@ -1175,7 +1175,7 @@ indent_done:
             p = scan_string_run<Value>(p, e);
             if (NLOHMANN_VIEW_LIKELY(cur() == '"'))
             {
-                push_node(value_t::string, 0, 0, static_cast<std::size_t>(s - b), static_cast<std::uint64_t>(p - s));
+                push_node(node_kind::string, 0, 0, static_cast<std::size_t>(s - b), static_cast<std::uint64_t>(p - s));
                 ++p;
                 return true;
             }
@@ -1185,7 +1185,7 @@ indent_done:
                 return false;
             }
             p = r.p;
-            push_node(value_t::string, node_flags::escaped, 0, r.start, r.len);
+            push_node(node_kind::string, node_flags::escaped, 0, r.start, r.len);
             return true;
         }
     };

include/nlohmann/detail/view/document_data.hpp

@@ -232,7 +232,7 @@ struct document_data
     /// following loads.
     static NLOHMANN_VIEW_ALWAYS_INLINE const node* deref(const node* n) noexcept
     {
-        return NLOHMANN_VIEW_LIKELY(n->kind != kind_link) ? n : follow_link(n);
+        return NLOHMANN_VIEW_LIKELY(n->kind != node_kind::link) ? n : follow_link(n);
     }
 
     static NLOHMANN_VIEW_NOINLINE const node* follow_link(const node* n) noexcept

include/nlohmann/detail/view/edit.hpp

@@ -26,12 +26,10 @@
 #include <nlohmann/detail/view/lookup.hpp>
 #include <nlohmann/detail/view/macro_scope.hpp>
 #include <nlohmann/detail/view/node.hpp>
+#include <nlohmann/detail/view/view_mode.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
-template<typename BasicJsonType, bool Editable>
-class basic_json_view;
-
 namespace detail
 {
 namespace view
@@ -120,15 +118,15 @@ class editor
     View set(const View& object, string_view_t key, V&& value)
     {
         node* const o = own(object);
-        if (o->kind != static_cast<std::uint8_t>(value_t::object) && o->kind != static_cast<std::uint8_t>(value_t::null))
+        if (o->kind != node_kind::object && o->kind != node_kind::null)
         {
             throw_type_error(exception_id::subscript_wrong_type, "cannot use operator[] with a string argument with ", object.type_name());
         }
         check_utf8(key.data(), key.size());
         const encoded e = encode(std::forward<V>(value));
-        if (o->kind == static_cast<std::uint8_t>(value_t::null))
+        if (o->kind == node_kind::null)
         {
-            become_empty(o, value_t::object);
+            become_empty(o, node_kind::object);
         }
         // an existing member: assign it (and drop later duplicates, so that
         // lookups, iteration, and materialize() agree)
@@ -171,7 +169,7 @@ class editor
     View set(const View& array, std::size_t idx, V&& value)
     {
         node* const a = own(array);
-        if (a->kind != static_cast<std::uint8_t>(value_t::array))
+        if (a->kind != node_kind::array)
         {
             throw_type_error(exception_id::subscript_wrong_type, "cannot use operator[] with a numeric argument with ", array.type_name());
         }
@@ -188,14 +186,14 @@ class editor
     View push_back(const View& array, V&& value)
     {
         node* const a = own(array);
-        if (a->kind != static_cast<std::uint8_t>(value_t::array) && a->kind != static_cast<std::uint8_t>(value_t::null))
+        if (a->kind != node_kind::array && a->kind != node_kind::null)
         {
             throw_type_error(exception_id::push_back_wrong_type, "cannot use push_back() with ", array.type_name());
         }
         const encoded e = encode(std::forward<V>(value));
-        if (a->kind == static_cast<std::uint8_t>(value_t::null))
+        if (a->kind == node_kind::null)
         {
-            become_empty(a, value_t::array);
+            become_empty(a, node_kind::array);
         }
         node* const slot = new_slot(e);
         node* const h = block_of(m_doc, a, 1);
@@ -212,7 +210,7 @@ class editor
     View insert(const View& array, std::size_t idx, V&& value)
     {
         node* const a = own(array);
-        if (a->kind != static_cast<std::uint8_t>(value_t::array))
+        if (a->kind != node_kind::array)
         {
             throw_type_error(exception_id::insert_wrong_type, "cannot use insert() with ", array.type_name());
         }
@@ -232,7 +230,7 @@ class editor
     std::size_t erase(const View& object, string_view_t key)
     {
         node* const o = own(object);
-        if (o->kind != static_cast<std::uint8_t>(value_t::object))
+        if (o->kind != node_kind::object)
         {
             throw_type_error(exception_id::erase_wrong_type, "cannot use erase() with ", object.type_name());
         }
@@ -250,7 +248,7 @@ class editor
     void erase(const View& array, std::size_t idx)
     {
         node* const a = own(array);
-        if (a->kind != static_cast<std::uint8_t>(value_t::array))
+        if (a->kind != node_kind::array)
         {
             throw_type_error(exception_id::erase_wrong_type, "cannot use erase() with ", array.type_name());
         }
@@ -324,11 +322,11 @@ class editor
     }
 
     /// turn a null into an empty array/object in place
-    static void become_empty(node* n, value_t k) noexcept
+    static void become_empty(node* n, node_kind k) noexcept
     {
         const std::uint8_t linked = n->flags & node_flags::linked;
         *n = node{};
-        n->kind = static_cast<std::uint8_t>(k);
+        n->kind = k;
         n->flags = static_cast<std::uint8_t>(node_flags::is_new | linked);
         n->next = 1;
     }
@@ -405,8 +403,8 @@ class editor
     template<typename T>
     struct is_view : std::false_type {};
 
-    template<typename J, bool E>
-    struct is_view<basic_json_view<J, E>> : std::true_type {};
+    template<typename J, view_mode M>
+    struct is_view<basic_json_view<J, M>> : std::true_type {};
 
     template<typename V>
     encoded encode(V&& v)
@@ -421,8 +419,8 @@ class editor
     }
 
     /// a view of any document (copied; nothing is shared with it)
-    template<typename J, bool E>
-    encoded encode_impl(const basic_json_view<J, E>& v, encode_tag<0> /*view*/)
+    template<typename J, view_mode M>
+    encoded encode_impl(const basic_json_view<J, M>& v, encode_tag<0> /*view*/)
     {
         if (NLOHMANN_VIEW_UNLIKELY(v.m_node == nullptr))
         {
@@ -434,8 +432,8 @@ class editor
             r.scalar = copy_scalar(*v.m_doc, *v.m_node);
             return r;
         }
-        r.region = alloc_nodes(m_doc, count_nodes<E>(*v.m_doc, v.m_node));
-        fill_nodes<E>(*v.m_doc, v.m_node, r.region);
+        r.region = alloc_nodes(m_doc, count_nodes<M == view_mode::editable>(*v.m_doc, v.m_node));
+        fill_nodes<M == view_mode::editable>(*v.m_doc, v.m_node, r.region);
         edit_state_of(m_doc).regions.emplace(r.region, nullptr);
         return r;
     }
@@ -457,14 +455,14 @@ class editor
     encoded encode_impl(std::nullptr_t /*unused*/, encode_tag<2> /*null*/)
     {
         encoded r;
-        r.scalar = plain_node(value_t::null);
+        r.scalar = plain_node(node_kind::null);
         return r;
     }
 
     encoded encode_impl(bool b, encode_tag<3> /*boolean*/)
     {
         encoded r;
-        r.scalar = plain_node(value_t::boolean);
+        r.scalar = plain_node(node_kind::boolean);
         r.scalar.flags = static_cast<std::uint8_t>(r.scalar.flags | (b ? node_flags::is_true : 0));
         return r;
     }
@@ -493,10 +491,10 @@ class editor
         return encode_impl(BasicJsonType(std::forward<T>(x)), encode_tag<1> {});
     }
 
-    static node plain_node(value_t k) noexcept
+    static node plain_node(node_kind k) noexcept
     {
         node n{};
-        n.kind = static_cast<std::uint8_t>(k);
+        n.kind = k;
         n.flags = node_flags::is_new;
         return n;
     }
@@ -510,19 +508,19 @@ class editor
     template<typename T>
     node number_node(T x, std::integral_constant<int, 1> /*signed*/)
     {
-        return integer_node(static_cast<std::uint64_t>(static_cast<std::int64_t>(x)), value_t::number_integer);
+        return integer_node(static_cast<std::uint64_t>(static_cast<std::int64_t>(x)), node_kind::number_integer);
     }
 
     template<typename T>
     node number_node(T x, std::integral_constant<int, 2> /*unsigned*/)
     {
-        return integer_node(static_cast<std::uint64_t>(x), value_t::number_unsigned);
+        return integer_node(static_cast<std::uint64_t>(x), node_kind::number_unsigned);
     }
 
     /// an integer with its canonical token in the edit arena
-    node integer_node(std::uint64_t bits, value_t k)
+    node integer_node(std::uint64_t bits, node_kind k)
     {
-        const bool negative = k == value_t::number_integer && static_cast<std::int64_t>(bits) < 0;
+        const bool negative = k == node_kind::number_integer && static_cast<std::int64_t>(bits) < 0;
         std::uint64_t magnitude = negative ? 0 - bits : bits;
         std::array<char, 24> buf{};
         char* p = buf.data() + buf.size(); // NOLINT(misc-const-correctness): digits are written through p
@@ -541,7 +539,7 @@ class editor
         n.flags = static_cast<std::uint8_t>(n.flags | node_flags::edited);
         n.off = append_text(m_doc, p, len);
         // number_length() adds one for the sign of number_integer nodes
-        n.extra = static_cast<std::uint16_t>(k == value_t::number_integer ? len - 1 : len);
+        n.extra = static_cast<std::uint16_t>(k == node_kind::number_integer ? len - 1 : len);
         set_integer_bits(n, bits);
         return n;
     }
@@ -563,7 +561,7 @@ class editor
         {
             text = BasicJsonType(x).dump();
         }
-        node n = plain_node(value_t::number_float);
+        node n = plain_node(node_kind::number_float);
         n.flags = static_cast<std::uint8_t>(n.flags | node_flags::edited);
         n.extra = 0xFFFFu; // (the digit layout is not recorded)
         n.off = append_text(m_doc, text.data(), text.size());
@@ -578,7 +576,7 @@ class editor
         {
             throw_out_of_range(exception_id::document_too_large, "strings of 4 GiB or more are not supported by json_document"); // LCOV_EXCL_LINE
         }
-        node n = plain_node(value_t::string);
+        node n = plain_node(node_kind::string);
         n.flags = static_cast<std::uint8_t>(n.flags | node_flags::edited);
         n.off = append_text(m_doc, s, len);
         n.len = static_cast<std::uint32_t>(len);
@@ -600,10 +598,10 @@ class editor
                 return string_node(from.str(n), n.len);
             case value_t::number_integer:
             case value_t::number_unsigned:
-                return integer_node(integer_bits(n), static_cast<value_t>(n.kind));
+                return integer_node(integer_bits(n), n.kind);
             case value_t::number_float:
             {
-                node r = plain_node(value_t::number_float);
+                node r = plain_node(node_kind::number_float);
                 r.flags = static_cast<std::uint8_t>(r.flags | node_flags::edited);
                 r.off = append_text(m_doc, from.str(n), n.len);
                 r.len = n.len;
@@ -612,7 +610,7 @@ class editor
             }
             case value_t::boolean:
             {
-                node r = plain_node(value_t::boolean);
+                node r = plain_node(node_kind::boolean);
                 r.flags = static_cast<std::uint8_t>(r.flags | (n.flags & node_flags::is_true));
                 return r;
             }
@@ -622,7 +620,7 @@ class editor
             case value_t::binary:
             case value_t::discarded:
             default:
-                return plain_node(value_t::null);
+                return plain_node(node_kind::null);
         }
     }
 
@@ -631,17 +629,17 @@ class editor
         switch (j.type())
         {
             case value_t::null:
-                return plain_node(value_t::null);
+                return plain_node(node_kind::null);
             case value_t::boolean:
             {
-                node r = plain_node(value_t::boolean);
+                node r = plain_node(node_kind::boolean);
                 r.flags = static_cast<std::uint8_t>(r.flags | (j.template get<bool>() ? node_flags::is_true : 0));
                 return r;
             }
             case value_t::number_integer:
-                return integer_node(static_cast<std::uint64_t>(static_cast<std::int64_t>(j.template get<number_integer_t>())), value_t::number_integer);
+                return integer_node(static_cast<std::uint64_t>(static_cast<std::int64_t>(j.template get<number_integer_t>())), node_kind::number_integer);
             case value_t::number_unsigned:
-                return integer_node(static_cast<std::uint64_t>(j.template get<number_unsigned_t>()), value_t::number_unsigned);
+                return integer_node(static_cast<std::uint64_t>(j.template get<number_unsigned_t>()), node_kind::number_unsigned);
             case value_t::number_float:
                 return float_node(j.template get<number_float_t>());
             case value_t::string:
@@ -681,7 +679,7 @@ class editor
             ++r;
             if (is_container(*n))
             {
-                open.push_back(frame{walk::first(d, n), walk::end(d, n), n->kind == static_cast<std::uint8_t>(value_t::object)});
+                open.push_back(frame{walk::first(d, n), walk::end(d, n), n->kind == node_kind::object});
             }
             // the next value: close finished containers, then step over the key
             for (;;)
@@ -728,9 +726,9 @@ class editor
             if (is_container(*n))
             {
                 node* const self = out++;
-                *self = plain_node(static_cast<value_t>(n->kind));
+                *self = plain_node(n->kind);
                 self->len = n->len;
-                open.push_back(frame{walk::first(d, n), walk::end(d, n), n->kind == static_cast<std::uint8_t>(value_t::object), self});
+                open.push_back(frame{walk::first(d, n), walk::end(d, n), n->kind == node_kind::object, self});
             }
             else
             {
@@ -819,7 +817,7 @@ class editor
             if (n->is_structured())
             {
                 node* const self = out++;
-                *self = plain_node(n->type());
+                *self = plain_node(static_cast<node_kind>(n->type()));
                 self->len = static_cast<std::uint32_t>(n->size());
                 open.push_back(frame{n->cbegin(), n->cend(), n->is_object(), self});
             }

include/nlohmann/detail/view/edit_storage.hpp

@@ -27,7 +27,7 @@
 // live in chunks that never move; strings and number tokens written by edits
 // live in the edit arena. An array/object whose elements change gets
 // node_flags::moved: its elements then live in a separate sequence (a header
-// node, then the entries), whose entries link to the values (kind_link).
+// node, then the entries), whose entries link to the values (node_kind::link).
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -185,7 +185,7 @@ inline node* block_of(document_data& d, node* n, std::size_t extra)
         return nh;
     }
     reserve_moved(d); // (so that set_moved() below cannot throw: the links are marked before)
-    const bool object = n->kind == static_cast<std::uint8_t>(value_t::object);
+    const bool object = n->kind == node_kind::object;
     const std::size_t used = 1 + (static_cast<std::size_t>(n->len) * (object ? 2 : 1));
     const std::size_t cap = used + extra;
     node* const h = alloc_nodes(d, cap);
@@ -251,7 +251,7 @@ inline node* find_parent(const document_data& d, const node* target)
         {
             return nullptr; // LCOV_EXCL_LINE (the value is inside c)
         }
-        const bool object = c->kind == static_cast<std::uint8_t>(value_t::object);
+        const bool object = c->kind == node_kind::object;
         const node* down = nullptr;
         for (const node* p = d.first_child_edited(c), *e = d.child_end_edited(c); p != e;)
         {

include/nlohmann/detail/view/image.hpp

@@ -117,7 +117,7 @@ inline void compact_nodes(const document_data& d, std::size_t arena_size, std::v
                 r.off = (v->flags & (node_flags::moved | node_flags::is_new)) != 0 ? 0 : v->off;
                 r.len = 0;  // counted below
                 r.next = 0; // set when the container is complete
-                stack.push_back(frame{d.first_child_edited(v), d.child_end_edited(v), out.size(), 0, v->kind == static_cast<std::uint8_t>(value_t::object)});
+                stack.push_back(frame{d.first_child_edited(v), d.child_end_edited(v), out.size(), 0, v->kind == node_kind::object});
                 break;
             case value_t::string:
                 r = string_node(*v);
@@ -138,7 +138,7 @@ inline void compact_nodes(const document_data& d, std::size_t arena_size, std::v
                     if (t[0] == 'n' || t[0] == 'i' || (v->len > 1 && t[1] == 'i'))
                     {
                         r = node{}; // nan and infinity: null, as dump() writes them
-                        r.kind = static_cast<std::uint8_t>(value_t::null);
+                        r.kind = node_kind::null;
                         break;
                     }
                     r.off = static_cast<std::uint32_t>(d.size + text_tail.size());
@@ -221,7 +221,7 @@ inline std::vector<std::uint8_t> save_image(const document_data& d)
     // extra of an object, and the same document saves to the same bytes
     for (std::size_t i = 0; i < count; ++i)
     {
-        if (nodes[i].kind == static_cast<std::uint8_t>(value_t::object) && nodes[i].extra != 0)
+        if (nodes[i].kind == node_kind::object && nodes[i].extra != 0)
         {
             std::memset(o + (i * sizeof(node)) + offsetof(node, extra), 0, sizeof(nodes[i].extra));
         }
@@ -337,7 +337,7 @@ inline bool float_token_finite(const unsigned char* s, std::size_t len, const nu
     if (static_cast<std::int64_t>(t.int_digits) + t.exponent > 300)
     {
         node n{};
-        n.kind = static_cast<std::uint8_t>(value_t::number_float);
+        n.kind = node_kind::number_float;
         n.len = static_cast<std::uint32_t>(len);
         const auto v = float_value<double>(reinterpret_cast<const char*>(s), n); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
         return v <= (std::numeric_limits<double>::max)() && v >= -(std::numeric_limits<double>::max)();
@@ -358,7 +358,7 @@ inline bool check_integer(const node& n, const unsigned char* text)
     }
     // the token's value is the stored one; number_integer nodes of edits can
     // be non-negative (as basic_json keeps the type of a value)
-    const bool integer = n.kind == static_cast<std::uint8_t>(value_t::number_integer);
+    const bool integer = n.kind == node_kind::number_integer;
     if (t.is_float || t.int_digits > 20 || (t.negative && !integer))
     {
         return false;
@@ -540,7 +540,7 @@ inline bool check_image(const node* nodes, std::size_t count, const unsigned cha
         {
             return false;
         }
-        if (n.kind != static_cast<std::uint8_t>(value_t::number_float))
+        if (n.kind != node_kind::number_float)
         {
             return (n.extra >> 8u) == 0;
         }
@@ -581,7 +581,7 @@ inline bool check_image(const node* nodes, std::size_t count, const unsigned cha
         const node& n = nodes[i];
         if (!stack.empty() && stack.back().object && stack.back().expect_key)
         {
-            if (n.kind != static_cast<std::uint8_t>(value_t::string) || !check_string(n))
+            if (n.kind != node_kind::string || !check_string(n))
             {
                 return false;
             }
@@ -622,7 +622,7 @@ inline bool check_image(const node* nodes, std::size_t count, const unsigned cha
                 {
                     // (the contents of float tokens are checked later; the token of an
                     // integer has at most 256 characters)
-                    if (n.kind == static_cast<std::uint8_t>(value_t::number_float))
+                    if (n.kind == node_kind::number_float)
                     {
                         floats.push_back(float_range{n.off, n.len, n.extra});
                     }
@@ -640,7 +640,7 @@ inline bool check_image(const node* nodes, std::size_t count, const unsigned cha
                 {
                     return false;
                 }
-                stack.push_back(frame{i + n.next, n.len, 0, n.kind == static_cast<std::uint8_t>(value_t::object), true});
+                stack.push_back(frame{i + n.next, n.len, 0, n.kind == node_kind::object, true});
                 complete = false;
                 break;
             }
@@ -729,7 +729,7 @@ inline void load_image(document_data& d, const std::uint8_t* image, std::size_t
     for (std::size_t i = 0; i < count; ++i)
     {
         const node& n = d.nodes[i];
-        if (n.kind == static_cast<std::uint8_t>(value_t::object) && n.len >= document_data::index_min_members)
+        if (n.kind == node_kind::object && n.len >= document_data::index_min_members)
         {
             d.large_objects.push_back(static_cast<std::uint32_t>(i));
         }

include/nlohmann/detail/view/lookup.hpp

@@ -182,7 +182,7 @@ const node* element_at(const document_data& d, const node* array, std::size_t id
 template<bool Editable>
 const node* last_child(const document_data& d, const node* container) noexcept
 {
-    const std::size_t value_offset = container->kind == static_cast<std::uint8_t>(value_t::object) ? 1 : 0;
+    const std::size_t value_offset = container->kind == node_kind::object ? 1 : 0;
     const node* const end = navigation<Editable>::end(d, container);
     const node* last = navigation<Editable>::first(d, container);
     if (value_offset == 0 && container->next == container->len + 1u && (!navigation<Editable>::editable() || (container->flags & node_flags::moved) == 0))

include/nlohmann/detail/view/materialize.hpp

@@ -101,7 +101,7 @@ BasicJsonType materialize(const document_data& d, const node* n)
             case value_t::object:
             case value_t::array:
             {
-                const bool object = n->kind == static_cast<std::uint8_t>(value_t::object);
+                const bool object = n->kind == node_kind::object;
                 if (object)
                 {
                     sax.start_object(n->len);

include/nlohmann/detail/view/node.hpp

@@ -41,9 +41,22 @@ struct node_flags
     static constexpr std::uint8_t linked = 32; ///< an entry of a moved sequence links to this value (editable documents): its extent in the parsed layout no longer matters
 };
 
-/// kind of an entry of an edited sequence that stands for a value stored
-/// elsewhere (the address of the value's node is kept in the len/next bytes)
-constexpr std::uint8_t kind_link = 10;
+/// the kind of a node: the value_t of its value, or link
+enum class node_kind : std::uint8_t
+{
+    null = static_cast<std::uint8_t>(value_t::null),
+    object = static_cast<std::uint8_t>(value_t::object),
+    array = static_cast<std::uint8_t>(value_t::array),
+    string = static_cast<std::uint8_t>(value_t::string), // NOLINT(build/include_what_you_use): an enumerator, not std::string
+    boolean = static_cast<std::uint8_t>(value_t::boolean),
+    number_integer = static_cast<std::uint8_t>(value_t::number_integer),
+    number_unsigned = static_cast<std::uint8_t>(value_t::number_unsigned),
+    number_float = static_cast<std::uint8_t>(value_t::number_float),
+    /// an entry of an edited sequence that stands for a value stored elsewhere
+    /// (the address of the value's node is kept in the len/next bytes); placed
+    /// after value_t::discarded, which no node ever has
+    link = static_cast<std::uint8_t>(value_t::discarded) + 1,
+};
 
 /// One entry of the flat index, in document order. An object's members are
 /// stored as key node followed by the value's subtree. Integers keep their
@@ -51,7 +64,7 @@ constexpr std::uint8_t kind_link = 10;
 /// always the next one, and the token length follows from `extra`).
 struct node
 {
-    std::uint8_t kind;   ///< value_t, or kind_link
+    node_kind kind;      ///< the kind of the value, or link
     std::uint8_t flags;  ///< node_flags
     std::uint16_t extra; ///< numbers: integer digits (low byte) and fraction digits (high byte), 255 = "many"; objects: number of the object's key hash index (1-based, 0 = none); otherwise 0
     std::uint32_t off;   ///< source offset (string content, number token, literal, bracket); arena offset if escaped/edited; number of the element sequence if moved
@@ -60,13 +73,13 @@ struct node
 };
 static_assert(sizeof(node) == 16, "node must stay 16 bytes");
 
-// the node kinds are value_t values; the test below (object and array are 1
-// and 2) and those of the number kinds depend on this numbering
-static_assert(static_cast<std::uint8_t>(value_t::null) == 0 && static_cast<std::uint8_t>(value_t::object) == 1
-              && static_cast<std::uint8_t>(value_t::array) == 2 && static_cast<std::uint8_t>(value_t::string) == 3
-              && static_cast<std::uint8_t>(value_t::boolean) == 4 && static_cast<std::uint8_t>(value_t::number_integer) == 5
-              && static_cast<std::uint8_t>(value_t::number_unsigned) == 6 && static_cast<std::uint8_t>(value_t::number_float) == 7,
-              "the node format depends on the numbering of value_t");
+// images store the node kinds as bytes, and the test below (object and array
+// are 1 and 2) and those of the number kinds depend on this numbering
+static_assert(static_cast<std::uint8_t>(node_kind::null) == 0 && static_cast<std::uint8_t>(node_kind::object) == 1
+              && static_cast<std::uint8_t>(node_kind::array) == 2 && static_cast<std::uint8_t>(node_kind::string) == 3
+              && static_cast<std::uint8_t>(node_kind::boolean) == 4 && static_cast<std::uint8_t>(node_kind::number_integer) == 5
+              && static_cast<std::uint8_t>(node_kind::number_unsigned) == 6 && static_cast<std::uint8_t>(node_kind::number_float) == 7,
+              "the node format depends on the numbering of node_kind");
 
 NLOHMANN_VIEW_ALWAYS_INLINE bool is_container(const node& n) noexcept
 {
@@ -86,7 +99,7 @@ inline void make_link(node& n, node* target) noexcept
 {
     target->flags = static_cast<std::uint8_t>(target->flags | node_flags::linked);
     n = node{};
-    n.kind = kind_link;
+    n.kind = node_kind::link;
     std::memcpy(reinterpret_cast<unsigned char*>(&n) + 8, static_cast<const void*>(&target), sizeof(const node*)); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
 }
 
@@ -116,8 +129,8 @@ NLOHMANN_VIEW_ALWAYS_INLINE void set_integer_bits(node& n, std::uint64_t v) noex
 /// token length of a number node
 NLOHMANN_VIEW_ALWAYS_INLINE std::uint32_t number_length(const node& n) noexcept
 {
-    return n.kind == static_cast<std::uint8_t>(value_t::number_float) ? n.len
-           : (n.extra & 0xFFu) + (n.kind == static_cast<std::uint8_t>(value_t::number_integer) ? 1u : 0u);
+    return n.kind == node_kind::number_float ? n.len
+           : (n.extra & 0xFFu) + (n.kind == node_kind::number_integer ? 1u : 0u);
 }
 
 /// estimated number of nodes for an input of `size` bytes (one node per ~12

include/nlohmann/detail/view/serializer.hpp

@@ -874,7 +874,7 @@ class view_serializer
                 case value_t::object:
                 case value_t::array:
                 {
-                    const bool object = n->kind == static_cast<std::uint8_t>(value_t::object);
+                    const bool object = n->kind == node_kind::object;
                     room(8);
                     if (n->len == 0)
                     {

include/nlohmann/detail/view/view_mode.hpp

@@ -0,0 +1,52 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <cstdint> // uint8_t
+#include <type_traits> // false_type, true_type
+
+#include <nlohmann/json.hpp>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+
+/// whether a basic_json_document and its views can be edited
+enum class view_mode : std::uint8_t
+{
+    read_only, ///< parsed once, then only read (the default)
+    editable,  ///< also has set(), push_back(), insert(), and erase()
+};
+
+template<typename BasicJsonType, view_mode Mode>
+class basic_json_view;
+
+namespace detail
+{
+namespace view
+{
+
+/// whether two view modes differ
+template<view_mode A, view_mode B>
+struct is_other_mode : std::true_type {};
+
+template<view_mode A>
+struct is_other_mode<A, A> : std::false_type {};
+
+/// whether a view of mode From converts implicitly to one of mode To: a
+/// read-only document has no edits, so a view of it is read correctly as a
+/// view of an editable document that has not been edited yet
+template<view_mode From, view_mode To>
+struct is_widening_mode : std::false_type {};
+
+template<>
+struct is_widening_mode<view_mode::read_only, view_mode::editable> : std::true_type {};
+
+}  // namespace view
+}  // namespace detail
+
+NLOHMANN_JSON_NAMESPACE_END

include/nlohmann/json_view.hpp

@@ -84,6 +84,7 @@
 #include <nlohmann/detail/view/serializer.hpp>
 #include <nlohmann/detail/view/string_ref.hpp>
 #include <nlohmann/detail/view/value.hpp>
+#include <nlohmann/detail/view/view_mode.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
@@ -93,13 +94,16 @@ NLOHMANN_JSON_NAMESPACE_BEGIN
 Trivially copyable (two pointers). Valid as long as the document is alive and
 has not been re-parsed, and as long as a borrowed source text is alive.
 */
-template<typename BasicJsonType, bool Editable = false>
+template<typename BasicJsonType, view_mode Mode = view_mode::read_only>
 class basic_json_view
 {
+    /// whether this is a view of an editable document
+    static constexpr bool is_editable = Mode == view_mode::editable;
+
     using node = detail::view::node;
     using document_data = detail::view::document_data;
     /// how the index is walked (with edits only for editable documents)
-    using navigation = detail::view::navigation<Editable>;
+    using navigation = detail::view::navigation<is_editable>;
 
   public:
     using value_t = detail::value_t;
@@ -120,6 +124,15 @@ class basic_json_view
     /// an invalid view (type() == value_t::discarded)
     basic_json_view() noexcept = default;
 
+    /// a view of a read-only document as a view of an editable one, so that a
+    /// function taking an editable view accepts both (a read-only document has
+    /// no edits; the view reads it as an editable document before its first
+    /// edit, at the cost of one check per access)
+    template<view_mode M, detail::enable_if_t<detail::view::is_widening_mode<M, Mode>::value, int> = 0>
+    basic_json_view(const basic_json_view<BasicJsonType, M>& other) noexcept // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+        : m_doc(other.m_doc), m_node(other.m_node)
+    {}
+
     //////////
     // type //
     //////////
@@ -446,7 +459,7 @@ class basic_json_view
         if (is_structured() && m_node->len != 0)
         {
             return unedited() ? basic_json_view(m_doc, detail::view::last_child<false>(*m_doc, m_node) + (is_object() ? 1 : 0))
-                   : basic_json_view(m_doc, navigation::value(detail::view::last_child<Editable>(*m_doc, m_node) + (is_object() ? 1 : 0)));
+                   : basic_json_view(m_doc, navigation::value(detail::view::last_child<is_editable>(*m_doc, m_node) + (is_object() ? 1 : 0)));
         }
         return front();
     }
@@ -504,7 +517,7 @@ class basic_json_view
     /// more than once); 0 if this is not an object
     size_type count(string_view_t key) const
     {
-        return is_object() ? detail::view::count_members<Editable>(*m_doc, m_node, key.data(), key.size()) : 0;
+        return is_object() ? detail::view::count_members<is_editable>(*m_doc, m_node, key.data(), key.size()) : 0;
     }
 
     size_type count(const char* key) const
@@ -646,7 +659,7 @@ class basic_json_view
         }
         else
         {
-            detail::view::view_serializer<BasicJsonType, Editable>(*m_doc, out, estimate, style).dump(m_node);
+            detail::view::view_serializer<BasicJsonType, is_editable>(*m_doc, out, estimate, style).dump(m_node);
         }
         return out;
     }
@@ -682,14 +695,14 @@ class basic_json_view
     }
 
     /// a view of an editable document compares with one of a read-only document
-    template < bool E, typename std::enable_if < E != Editable, int >::type = 0 >
-    friend bool operator==(const basic_json_view& a, const basic_json_view<BasicJsonType, E>& b)
+    template<view_mode M, detail::enable_if_t<detail::view::is_other_mode<M, Mode>::value, int> = 0>
+    friend bool operator==(const basic_json_view& a, const basic_json_view<BasicJsonType, M>& b)
     {
-        return detail::view::equal<BasicJsonType>(side(a), detail::view::view_side<BasicJsonType, basic_json_view<BasicJsonType, E>>(b));
+        return detail::view::equal<BasicJsonType>(side(a), detail::view::view_side<BasicJsonType, basic_json_view<BasicJsonType, M>>(b));
     }
 
-    template < bool E, typename std::enable_if < E != Editable, int >::type = 0 >
-    friend bool operator!=(const basic_json_view& a, const basic_json_view<BasicJsonType, E>& b)
+    template<view_mode M, detail::enable_if_t<detail::view::is_other_mode<M, Mode>::value, int> = 0>
+    friend bool operator!=(const basic_json_view& a, const basic_json_view<BasicJsonType, M>& b)
     {
         return !(a == b);
     }
@@ -727,7 +740,7 @@ class basic_json_view
         {
             return BasicJsonType(value_t::discarded);
         }
-        return unedited() ? detail::view::materialize<BasicJsonType, false>(*m_doc, m_node) : detail::view::materialize<BasicJsonType, Editable>(*m_doc, m_node);
+        return unedited() ? detail::view::materialize<BasicJsonType, false>(*m_doc, m_node) : detail::view::materialize<BasicJsonType, is_editable>(*m_doc, m_node);
     }
 
     /// byte offset of this value in the source text (for strings: of the
@@ -740,8 +753,8 @@ class basic_json_view
     }
 
   private:
-    template<typename, bool> friend class basic_json_document;
-    template<typename, bool> friend class basic_json_view;
+    template<typename, view_mode> friend class basic_json_document;
+    template<typename, view_mode> friend class basic_json_view;
     template<typename, typename> friend class detail::view::editor;
     friend iterator;
 
@@ -757,11 +770,11 @@ class basic_json_view
     }
 
     /// the template argument as a runtime condition: testing it directly in
-    /// `Editable && ...` makes the condition constant, which MSVC reports as
+    /// `is_editable && ...` makes the condition constant, which MSVC reports as
     /// C4127
     static bool editable() noexcept
     {
-        return Editable;
+        return is_editable;
     }
 
     /// the number of source bytes of this value (estimated for values with
@@ -810,7 +823,7 @@ class basic_json_view
             const node* const k = detail::view::find_member<false>(*m_doc, m_node, key.data(), key.size());
             return k != nullptr ? basic_json_view(m_doc, k + 1) : basic_json_view();
         }
-        const node* const k = detail::view::find_member<Editable>(*m_doc, m_node, key.data(), key.size());
+        const node* const k = detail::view::find_member<is_editable>(*m_doc, m_node, key.data(), key.size());
         return k != nullptr ? basic_json_view(m_doc, navigation::value(k + 1)) : basic_json_view();
     }
 
@@ -824,7 +837,7 @@ class basic_json_view
     /// the member of this object with @a key (the last one if the key occurs more than once), or nullptr
     NLOHMANN_VIEW_ALWAYS_INLINE const node* find_key(string_view_t key) const
     {
-        return find_key(key, std::integral_constant<bool, Editable> {});
+        return find_key(key, std::integral_constant<bool, is_editable> {});
     }
 
     NLOHMANN_VIEW_ALWAYS_INLINE const node* find_key(string_view_t key, std::true_type /*editable*/) const
@@ -846,7 +859,7 @@ class basic_json_view
         {
             return basic_json_view(m_doc, detail::view::element_at<false>(*m_doc, m_node, idx));
         }
-        return basic_json_view(m_doc, navigation::value(detail::view::element_at<Editable>(*m_doc, m_node, idx)));
+        return basic_json_view(m_doc, navigation::value(detail::view::element_at<is_editable>(*m_doc, m_node, idx)));
     }
 
     // --- get() dispatch ---
@@ -952,16 +965,19 @@ Borrowed parses keep a pointer to the caller's text, which must outlive the
 document. Owned parses (parse_copy, rvalue std::string, streams, and inputs
 that are not contiguous byte ranges) keep their own copy.
 */
-template<typename BasicJsonType, bool Editable = false>
+template<typename BasicJsonType, view_mode Mode = view_mode::read_only>
 class basic_json_document
 {
+    /// whether the document can be edited
+    static constexpr bool is_editable = Mode == view_mode::editable;
+
     using document_data = detail::view::document_data;
 
     static_assert(sizeof(typename BasicJsonType::number_integer_t) == 8 && sizeof(typename BasicJsonType::number_unsigned_t) == 8,
                   "json_view supports 64-bit integer types only");
 
   public:
-    using view_type = basic_json_view<BasicJsonType, Editable>;
+    using view_type = basic_json_view<BasicJsonType, Mode>;
     using value_t = detail::value_t;
 
     /// an empty (discarded) document
@@ -1357,7 +1373,7 @@ class basic_json_document
 
     detail::view::editor<BasicJsonType, view_type> editor()
     {
-        static_assert(Editable, "only an editable document can be edited: use basic_json_document<BasicJsonType, true> (json_editable_document)");
+        static_assert(is_editable, "only an editable document can be edited: use basic_json_document<BasicJsonType, view_mode::editable> (json_editable_document)");
         if (NLOHMANN_VIEW_UNLIKELY(!m_data || m_data->discarded))
         {
             detail::view::throw_invalid_iterator(detail::exception_id::iterator_from_other_value, "view does not belong to this document");
@@ -1634,21 +1650,21 @@ class basic_json_document
 };
 
 /// a parsed JSON text for json
-using json_document = basic_json_document<json>;
+using json_document = basic_json_document<json, view_mode::read_only>;
 /// a value of a json_document
-using json_view = basic_json_view<json>;
+using json_view = basic_json_view<json, view_mode::read_only>;
 /// a parsed JSON text for ordered_json
-using ordered_json_document = basic_json_document<ordered_json>;
+using ordered_json_document = basic_json_document<ordered_json, view_mode::read_only>;
 /// a value of an ordered_json_document
-using ordered_json_view = basic_json_view<ordered_json>;
+using ordered_json_view = basic_json_view<ordered_json, view_mode::read_only>;
 /// an editable parsed JSON text for json
-using json_editable_document = basic_json_document<json, true>;
+using json_editable_document = basic_json_document<json, view_mode::editable>;
 /// a value of a json_editable_document
-using json_editable_view = basic_json_view<json, true>;
+using json_editable_view = basic_json_view<json, view_mode::editable>;
 /// an editable parsed JSON text for ordered_json
-using ordered_json_editable_document = basic_json_document<ordered_json, true>;
+using ordered_json_editable_document = basic_json_document<ordered_json, view_mode::editable>;
 /// a value of an ordered_json_editable_document
-using ordered_json_editable_view = basic_json_view<ordered_json, true>;
+using ordered_json_editable_view = basic_json_view<ordered_json, view_mode::editable>;
 
 NLOHMANN_JSON_NAMESPACE_END