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>

docs/mkdocs/docs/api/basic_json_document/erase.md

@@ -12,7 +12,7 @@ void erase(view_type array, I idx);
 std::size_t erase(const json_pointer& ptr);
 ```
 
-Only an **editable** document (`#!cpp Editable == true`, e.g. [`json_editable_document`](../json_editable_document.md))
+Only an **editable** document (`#!cpp view_mode::editable`, e.g. [`json_editable_document`](../json_editable_document.md))
 has `erase`; calling it on a read-only `basic_json_document` fails to compile (`#!cpp static_assert`).
 
 1. Removes every member of `object` whose key is `key` (see [Notes](#notes) on duplicate keys) and returns how many

docs/mkdocs/docs/api/basic_json_document/index.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-template<typename BasicJsonType, bool Editable = false>
+template<typename BasicJsonType, view_mode Mode = view_mode::read_only>
 class basic_json_document;
 ```
 
@@ -20,9 +20,9 @@ is move-only: a copy would duplicate the whole index, the decoded-string buffer,
 enough in every use we know of. The index, the decoded strings, and the owned text live in one heap block that the
 document holds through a `#!cpp std::unique_ptr`; this is also why views survive a move of the document.
 
-With `#!cpp Editable == true`, the document also offers [`set`](set.md), [`push_back`](push_back.md),
+With `#!cpp Mode == view_mode::editable`, the document also offers [`set`](set.md), [`push_back`](push_back.md),
 [`insert`](insert.md), and [`erase`](erase.md) to change values in place, see [Edits](#edits) below. The source text
-itself is never written; a read-only document (`#!cpp Editable == false`, the default) does not carry any of the
+itself is never written; a read-only document (`#!cpp view_mode::read_only`, the default) does not carry any of the
 bookkeeping edits need, and calling any of them on one fails to compile (`#!cpp static_assert`).
 
 ## Template parameters
@@ -32,9 +32,10 @@ bookkeeping edits need, and calling any of them on one fails to compile (`#!cpp
     [`ordered_json`](../ordered_json.md). Only 64-bit `number_integer_t`/`number_unsigned_t` types are supported; this
     is checked with a `static_assert`.
 
-`Editable`
-:   whether the document supports [`set`](set.md), [`push_back`](push_back.md), [`insert`](insert.md), and
-    [`erase`](erase.md) (optional, `#!cpp false` by default). See [Edits](#edits) below.
+`Mode`
+:   a [`view_mode`](../view_mode.md): `#!cpp view_mode::editable` if the document supports [`set`](set.md),
+    [`push_back`](push_back.md), [`insert`](insert.md), and [`erase`](erase.md) (optional, `#!cpp view_mode::read_only`
+    by default). See [Edits](#edits) below.
 
 ## Specializations
 
@@ -47,7 +48,7 @@ bookkeeping edits need, and calling any of them on one fails to compile (`#!cpp
 
 ## Member types
 
-- **view_type** - the type of view returned by [`root()`](root.md) (`#!cpp basic_json_view<BasicJsonType, Editable>`)
+- **view_type** - the type of view returned by [`root()`](root.md) (`#!cpp basic_json_view<BasicJsonType, Mode>`)
 - **value_t** - the JSON type enumeration, see [`basic_json::value_t`](../basic_json/value_t.md)
 - **image_check** - how [`load()`](load.md) validates an image (`full`, `bounds`, `none`), see [`image_check`](load.md#image_check)
 
@@ -80,15 +81,15 @@ bookkeeping edits need, and calling any of them on one fails to compile (`#!cpp
 ### Edits
 
 - [**set**](set.md) - replace a value, or set an object member, an array element, or the value a JSON pointer refers
-  to (`#!cpp Editable` documents only)
-- [**push_back**](push_back.md) - append to an array (`#!cpp Editable` documents only)
-- [**insert**](insert.md) - insert an element into an array before a given position (`#!cpp Editable` documents only)
+  to (editable documents only)
+- [**push_back**](push_back.md) - append to an array (editable documents only)
+- [**insert**](insert.md) - insert an element into an array before a given position (editable documents only)
 - [**erase**](erase.md) - remove an object member, an array element, or the value a JSON pointer refers to
-  (`#!cpp Editable` documents only)
+  (editable documents only)
 
 ## Edits
 
-An editable document (`#!cpp Editable == true`) can be changed after parsing, with [`set`](set.md),
+An editable document (`#!cpp view_mode::editable`) can be changed after parsing, with [`set`](set.md),
 [`push_back`](push_back.md), [`insert`](insert.md), and [`erase`](erase.md);
 [`json_editable_document`](../json_editable_document.md) and
 [`ordered_json_editable_document`](../ordered_json_editable_document.md) are the corresponding specializations. A few

docs/mkdocs/docs/api/basic_json_document/insert.md

@@ -5,7 +5,7 @@ template<typename I, typename V>
 view_type insert(view_type array, I idx, V&& value);
 ```
 
-Only an **editable** document (`#!cpp Editable == true`, e.g. [`json_editable_document`](../json_editable_document.md))
+Only an **editable** document (`#!cpp view_mode::editable`, e.g. [`json_editable_document`](../json_editable_document.md))
 has `insert`; calling it on a read-only `basic_json_document` fails to compile (`#!cpp static_assert`).
 
 Inserts `value` into `array` as a new element before position `idx`, which must not be past the end

docs/mkdocs/docs/api/basic_json_document/push_back.md

@@ -13,7 +13,7 @@ editable, and it does not have to be `array`'s own document -- which is copied s
 source document afterward; a `BasicJsonType` value; or anything `BasicJsonType` can be constructed from (numbers,
 strings, `#!cpp bool`, `#!cpp nullptr`, containers, ...).
 
-Only an **editable** document (`#!cpp Editable == true`, e.g. [`json_editable_document`](../json_editable_document.md))
+Only an **editable** document (`#!cpp view_mode::editable`, e.g. [`json_editable_document`](../json_editable_document.md))
 has `push_back`; calling it on a read-only `basic_json_document` fails to compile (`#!cpp static_assert`).
 
 ## Template parameters

docs/mkdocs/docs/api/basic_json_document/save.md

@@ -12,7 +12,7 @@ An edited document is written in its *current* state, with its values in documen
 parser would have produced them for that JSON text: a member [`set`](set.md) added goes at the end, an
 [`erase`](erase.md)d member leaves no trace, and a float that is not finite (NaN or positive/negative infinity)
 becomes null, the same substitution [`dump()`](../basic_json_view/dump.md) makes. The same document always saves to
-the same bytes -- also across `BasicJsonType` and `#!cpp Editable`, since the image reflects document order and
+the same bytes -- also across `BasicJsonType` and the [`view_mode`](../view_mode.md), since the image reflects document order and
 values only, not which specialization produced them.
 
 ## Return value

docs/mkdocs/docs/api/basic_json_document/set.md

@@ -18,7 +18,7 @@ template<typename V>
 view_type set(const json_pointer& ptr, V&& value);
 ```
 
-Only an **editable** document (`#!cpp Editable == true`, e.g. [`json_editable_document`](../json_editable_document.md))
+Only an **editable** document (`#!cpp view_mode::editable`, e.g. [`json_editable_document`](../json_editable_document.md))
 has `set`; calling it on a read-only `basic_json_document` fails to compile (`#!cpp static_assert`).
 
 1. Replaces the value `target` refers to with `value`.

docs/mkdocs/docs/api/basic_json_view/basic_json_view.md

@@ -1,20 +1,32 @@
 # <small>nlohmann::basic_json_view::</small>basic_json_view
 
 ```cpp
+// (1)
 basic_json_view() noexcept = default;
+
+// (2) only for Mode == view_mode::editable
+basic_json_view(const basic_json_view<BasicJsonType, view_mode::read_only>& other) noexcept;
 ```
 
-Creates an invalid (discarded) view: [`type()`](type.md) is `#!cpp value_t::discarded` and
-[`is_discarded()`](is_discarded.md) is `#!cpp true`.
+1. Creates an invalid (discarded) view: [`type()`](type.md) is `#!cpp value_t::discarded` and
+   [`is_discarded()`](is_discarded.md) is `#!cpp true`.
+2. Converts a view of a read-only document into a view of an editable document (implicitly), so that a function
+   taking a [`json_editable_view`](../json_editable_view.md) accepts both. The result is a view of the same value of
+   the same read-only document. See [`view_mode`](../view_mode.md) for why there is no conversion the other way.
 
-This is the only constructor a caller can use directly. Every other view is obtained from a
+## Parameters
+
+`other` (in)
+:   a view of a read-only document with the same `BasicJsonType`
+
+Apart from these, a view is obtained from a
 [`basic_json_document`](../basic_json_document/index.md), via [`root()`](../basic_json_document/root.md) or by
 navigating into a container with [`operator[]`](operator[].md), [`at`](at.md), [`front`](front.md), [`back`](back.md),
 [`find`](find.md), or iteration.
 
 ## Exception safety
 
-No-throw guarantee: this constructor never throws exceptions.
+No-throw guarantee: these constructors never throw exceptions.
 
 ## Complexity
 
@@ -45,6 +57,7 @@ placeholder for "no value yet" and later be assigned a real view.
 
 - [is_discarded](is_discarded.md) - return whether the view is invalid
 - [root](../basic_json_document/root.md) - the view of a document's root value
+- [view_mode](../view_mode.md) - whether a document and its views can be edited
 
 ## Version history
 

docs/mkdocs/docs/api/basic_json_view/index.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-template<typename BasicJsonType, bool Editable = false>
+template<typename BasicJsonType, view_mode Mode = view_mode::read_only>
 class basic_json_view;
 ```
 
@@ -37,7 +37,7 @@ Moving the document itself does not invalidate its views: the index is heap-allo
 `#!cpp operator ValueType()`), and no `#!cpp get_ptr()` or `#!cpp get_ref()`; see [`get`](get.md).
 
 `basic_json_view` itself is always read-only -- it never has a `set` or `push_back` of its own. A view of an
-**editable** document (`#!cpp Editable == true`) sees every edit made through
+**editable** document (`#!cpp view_mode::editable`) sees every edit made through
 [`basic_json_document::set`](../basic_json_document/set.md) and
 [`basic_json_document::push_back`](../basic_json_document/push_back.md): once a value is changed, every view that
 still refers to it -- including ones taken before the change -- reads the new value. See
@@ -49,9 +49,10 @@ still refers to it -- including ones taken before the change -- reads the new va
 :   a specialization of [`basic_json`](../basic_json/index.md), matching the
     [`basic_json_document`](../basic_json_document/index.md) the view was taken from.
 
-`Editable`
-:   whether the view is of an editable document, matching the [`basic_json_document`](../basic_json_document/index.md)
-    it was taken from (optional, `#!cpp false` by default). See [Edits](../basic_json_document/index.md#edits).
+`Mode`
+:   a [`view_mode`](../view_mode.md): whether the view is of an editable document, matching the
+    [`basic_json_document`](../basic_json_document/index.md) it was taken from (optional, `#!cpp view_mode::read_only` by
+    default). See [Edits](../basic_json_document/index.md#edits).
 
 ## Specializations
 

docs/mkdocs/docs/api/json_document.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using json_document = basic_json_document<json>;
+using json_document = basic_json_document<json, view_mode::read_only>;
 ```
 
 This type is a [`basic_json_document`](basic_json_document/index.md) of the default [`json`](json.md)

docs/mkdocs/docs/api/json_editable_document.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using json_editable_document = basic_json_document<json, true>;
+using json_editable_document = basic_json_document<json, view_mode::editable>;
 ```
 
 This type is an **editable** [`basic_json_document`](basic_json_document/index.md) of the default

docs/mkdocs/docs/api/json_editable_view.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using json_editable_view = basic_json_view<json, true>;
+using json_editable_view = basic_json_view<json, view_mode::editable>;
 ```
 
 This type is a [`basic_json_view`](basic_json_view/index.md) of a value of a

docs/mkdocs/docs/api/json_view.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using json_view = basic_json_view<json>;
+using json_view = basic_json_view<json, view_mode::read_only>;
 ```
 
 This type is a [`basic_json_view`](basic_json_view/index.md) of a value of a [`json_document`](json_document.md).

docs/mkdocs/docs/api/ordered_json_document.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using ordered_json_document = basic_json_document<ordered_json>;
+using ordered_json_document = basic_json_document<ordered_json, view_mode::read_only>;
 ```
 
 This type is a [`basic_json_document`](basic_json_document/index.md) of the [`ordered_json`](ordered_json.md)

docs/mkdocs/docs/api/ordered_json_editable_document.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using ordered_json_editable_document = basic_json_document<ordered_json, true>;
+using ordered_json_editable_document = basic_json_document<ordered_json, view_mode::editable>;
 ```
 
 This type is an **editable** [`basic_json_document`](basic_json_document/index.md) of the

docs/mkdocs/docs/api/ordered_json_editable_view.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using ordered_json_editable_view = basic_json_view<ordered_json, true>;
+using ordered_json_editable_view = basic_json_view<ordered_json, view_mode::editable>;
 ```
 
 This type is a [`basic_json_view`](basic_json_view/index.md) of a value of an

docs/mkdocs/docs/api/ordered_json_view.md

@@ -3,7 +3,7 @@
 <small>Defined in header `<nlohmann/json_view.hpp>`</small>
 
 ```cpp
-using ordered_json_view = basic_json_view<ordered_json>;
+using ordered_json_view = basic_json_view<ordered_json, view_mode::read_only>;
 ```
 
 This type is a [`basic_json_view`](basic_json_view/index.md) of a value of an

docs/mkdocs/docs/api/view_mode.md

@@ -0,0 +1,88 @@
+# <small>nlohmann::</small>view_mode
+
+<small>Defined in header `<nlohmann/json_view.hpp>`</small>
+
+```cpp
+enum class view_mode : std::uint8_t
+{
+    read_only,
+    editable
+};
+```
+
+The second template parameter of [`basic_json_document`](basic_json_document/index.md) and
+[`basic_json_view`](basic_json_view/index.md): whether a document can be changed after parsing, and whether a view is
+of such a document.
+
+`read_only`
+:   the default: the document is parsed once and then only read. It carries none of the bookkeeping edits need, and
+    calling [`set`](basic_json_document/set.md), [`push_back`](basic_json_document/push_back.md),
+    [`insert`](basic_json_document/insert.md), or [`erase`](basic_json_document/erase.md) on it fails to compile.
+
+`editable`
+:   the document also offers [`set`](basic_json_document/set.md), [`push_back`](basic_json_document/push_back.md),
+    [`insert`](basic_json_document/insert.md), and [`erase`](basic_json_document/erase.md); its views see every edit,
+    see [Edits](basic_json_document/index.md#edits).
+
+The shortcuts name both modes: [`json_document`](json_document.md) and [`json_view`](json_view.md) are read-only,
+[`json_editable_document`](json_editable_document.md) and [`json_editable_view`](json_editable_view.md) are editable
+(and likewise for `ordered_json`).
+
+## Notes
+
+The mode is a compile-time property of the document's and the view's type, not a setting of a call. A read-only view
+reads a container in place, while a view of an editable document follows the links that edits leave behind. A view of
+a read-only document converts implicitly to a view of an editable document of the same `BasicJsonType`: a read-only
+document has no edits, so the converted view reads it like an editable document before its first edit (at the cost of
+one check per access). There is no conversion the other way, because a read-only view of an editable document would
+read wrong values after the next edit.
+
+A function that takes a [`json_editable_view`](json_editable_view.md) therefore accepts the views of both modes. A
+function template deduces the mode from its argument instead, without conversions, so it names the modes it accepts in
+its signature:
+
+```cpp
+// views of editable documents only
+template<typename BasicJsonType>
+void f(basic_json_view<BasicJsonType, view_mode::editable> v);
+
+// views of read-only documents only
+template<typename BasicJsonType>
+void f(basic_json_view<BasicJsonType, view_mode::read_only> v);
+
+// either
+template<typename BasicJsonType, view_mode Mode>
+void f(basic_json_view<BasicJsonType, Mode> v);
+```
+
+A function that takes a [`json_view`](json_view.md) accepts neither an [`ordered_json_view`](ordered_json_view.md)
+nor a [`json_editable_view`](json_editable_view.md). Views of the two modes compare with
+[`operator==`](basic_json_view/operator_eq.md).
+
+## Examples
+
+??? example
+
+    The example below shows a function template that accepts the views of both modes, one that accepts only views of
+    editable documents, and a function taking a `json_editable_view` that also accepts a view of a read-only document.
+
+    ```cpp
+    --8<-- "examples/view_mode.cpp"
+    ```
+
+    Output:
+
+    ```
+    --8<-- "examples/view_mode.output"
+    ```
+
+## See also
+
+- [basic_json_document](basic_json_document/index.md) - a parsed JSON text
+- [basic_json_view](basic_json_view/index.md) - a handle to one value of a document
+- [json_editable_document](json_editable_document.md) - an editable document of `json`
+- [(constructor)](basic_json_view/basic_json_view.md) - the conversion from a read-only to an editable view
+
+## Version history
+
+Added in version 3.13.0.

docs/mkdocs/docs/examples/ordered_json_editable_document.cpp

@@ -5,7 +5,7 @@ using nlohmann::ordered_json_editable_document;
 
 int main()
 {
-    // ordered_json_editable_document is basic_json_document<nlohmann::ordered_json, true>
+    // ordered_json_editable_document is basic_json_document<nlohmann::ordered_json, nlohmann::view_mode::editable>
     ordered_json_editable_document doc = ordered_json_editable_document::parse(R"({"z": 1, "a": 2, "m": 3})");
     doc.set(doc.root(), "b", 4); // set() always appends a new member at the end
 

docs/mkdocs/docs/examples/view_mode.cpp

@@ -0,0 +1,36 @@
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using nlohmann::basic_json_view;
+using nlohmann::view_mode;
+
+// accepts the views of read-only and of editable documents, of json and of ordered_json
+template<typename BasicJsonType, view_mode Mode>
+std::size_t count_members(basic_json_view<BasicJsonType, Mode> v)
+{
+    return v.is_object() ? v.size() : 0;
+}
+
+// accepts only the views of editable documents
+template<typename BasicJsonType>
+bool is_edited(basic_json_view<BasicJsonType, view_mode::editable> v)
+{
+    return v.is_string() && v.get_string() == "edited";
+}
+
+// a function (no template) taking a json_editable_view also accepts the view of a read-only document
+std::size_t size_of(nlohmann::json_editable_view v)
+{
+    return v.size();
+}
+
+int main()
+{
+    const nlohmann::json_document doc = nlohmann::json_document::parse(R"({"a": 1, "b": 2})");
+    nlohmann::json_editable_document edoc = nlohmann::json_editable_document::parse(R"({"a": 1})");
+    edoc.set(edoc.root()["a"], "edited");
+
+    std::cout << count_members(doc.root()) << ' ' << count_members(edoc.root()) << '\n';
+    std::cout << std::boolalpha << is_edited(edoc.root()["a"]) << '\n';
+    std::cout << size_of(doc.root()) << ' ' << size_of(edoc.root()) << '\n';
+}

docs/mkdocs/docs/examples/view_mode.output

@@ -0,0 +1,3 @@
+2 1
+true
+2 1

docs/mkdocs/docs/features/json_view.md

@@ -252,14 +252,14 @@ the stream's `width`/`fill` for indentation.
 ## Editing a document
 
 Everything above is read-only: a `json_document`/`json_view` lets you look at a parsed text without copying it, but
-not change it. [`basic_json_document<BasicJsonType, true>`](../api/basic_json_document/index.md) -- more conveniently
+not change it. [`basic_json_document<BasicJsonType, view_mode::editable>`](../api/basic_json_document/index.md) -- more conveniently
 spelled [`json_editable_document`](../api/json_editable_document.md) or
 [`ordered_json_editable_document`](../api/ordered_json_editable_document.md) -- also lets you
 [`set`](../api/basic_json_document/set.md) a value, [`push_back`](../api/basic_json_document/push_back.md) onto or
 [`insert`](../api/basic_json_document/insert.md) into an array, and [`erase`](../api/basic_json_document/erase.md)
 an object member or an array element, still without ever building a `basic_json` tree for parts you do not touch.
 
-`#!cpp Editable` defaults to `#!cpp false`, so `json_document`/`ordered_json_document` are unaffected -- they carry
+The [mode](../api/view_mode.md) defaults to `#!cpp view_mode::read_only`, so `json_document`/`ordered_json_document` are unaffected -- they carry
 none of the bookkeeping edits need, and calling `set`/`push_back`/`insert`/`erase` on one is a compile error, not a
 runtime one.
 

docs/mkdocs/mkdocs.yml

@@ -411,6 +411,7 @@ nav:
       - 'ordered_json_editable_view': api/ordered_json_editable_view.md
       - 'ordered_json_view': api/ordered_json_view.md
       - 'ordered_map': api/ordered_map.md
+      - 'view_mode': api/view_mode.md
       - macros:
           - 'Overview': api/macros/index.md
           - 'JSON_ASSERT': api/macros/json_assert.md