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Document how editable json_documents use the node index

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The node index section on the architecture page says how edits are kept
(the edit buffer, moved elements, links, and new nodes), and the feature
page links to it.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-29 14:03:15 +02:00
parent 266c2249fe
commit ada8f17230
2 changed files with 19 additions and 3 deletions
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docs/mkdocs/docs/features/json_view.md
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@@ -243,7 +243,8 @@ elements are not touched, but an iterator that was walking the old layout no lon
[`get_string()`](../api/basic_json_view/get_string.md) is unaffected either way and stays valid across further
edits. See [`basic_json_document`'s Edits](../api/basic_json_document/index.md#edits) for the details, and
[`set`'s Exception safety](../api/basic_json_document/set.md#exception-safety) for what an edit guarantees if it
throws (the *basic* guarantee, not the strong one `dump()` and the read-only functions provide).
throws (the *basic* guarantee, not the strong one `dump()` and the read-only functions provide). How edits are kept in
the index is described in the [architecture overview](../home/architecture.md#node-index-of-json-views).
## Choosing between `json`, `ordered_json`, the SAX interface, and `json_view`
docs/mkdocs/docs/home/architecture.md
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@@ -194,8 +194,8 @@ packet-beta
| Bytes | Field | Type | Contents |
|-------|---------|------------|-------------------------------------------------------------------------------------------------------------------------------|
| 0 | `kind` | `uint8_t` | the type, numbered as [`value_t`](../api/basic_json/value_t.md): 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float |
| 1 | `flags` | `uint8_t` | bits 0-1: where a string's bytes are (0: the source text, 1: the buffer of decoded strings, for strings with escapes); bit 2: the value of a boolean |
| 0 | `kind` | `uint8_t` | the type, numbered as [`value_t`](../api/basic_json/value_t.md): 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below) |
| 1 | `flags` | `uint8_t` | bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bits 3 and 4: moved and new (see below) |
| 2-3 | `extra` | `uint16_t` | numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their hash index (1-based), or 0; otherwise 0 |
| 4-7 | `off` | `uint32_t` | where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object |
| 8-11 | `len` | `uint32_t` | strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements |
@@ -244,6 +244,21 @@ flowchart LR
All `flags` are 0. The integer's bytes 8-15 hold its value, 1; its `extra` says it has one digit. The float's `extra`
says it has one integer and one fraction digit, and its `len` is that of the token `2.5`.
Editable documents ([`json_editable_document`](../api/json_editable_document.md),
[`detail/view/edit.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/edit.hpp) and
[`detail/view/edit_storage.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/edit_storage.hpp))
never write the source text and never move or resize the parsed index, so views stay valid while the document is
edited:
- A new scalar is written over its node. Its text (a string, or the token of a number as `dump()` writes it) goes to
the edit buffer, which `flags` bits 0-1 then name.
- An array or object whose elements change gets the flag *moved* (bit 3): its elements then live in a separate
sequence (a header node, then the entries), whose number is in `off`. The entries are links (`kind` 10), whose bytes
8-15 hold the address of the value's node, so values never move.
- A node written by an edit gets the flag *new* (bit 4): it has no position in the source text.
- Views of read-only documents compile without any of this: how views walk the index is a template parameter
(`navigation<Editable>`).
## Input adapters
Input is read via **input adapters** that abstract a source. Every input adapter provides this interface: