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Niels Lohmann
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* docs: new design for the documentation site Apply the brand to the Material for MkDocs site: Inter and JetBrains Mono served by the site (upright and real italic), brand colours, blue header band with the syntax pattern, wordmark in the header, new home page (hero, cards, animated feature tour), 404 page, polish for tabs, search, footer, focus rings, tables and code blocks, and theme features (code copy, previous/next footer, TOC follow, search suggestions). Draw the number railroad diagram, the parser callback events and the iterator illustrations as inline SVGs that follow the light and dark scheme. The generator scripts are in docs/mkdocs/scripts/figures; the feature slideshow (json-light.svg, json-dark.svg, replacing json.gif on the home page) is built by docs/mkdocs/scripts/slideshow. Decision nodes in five flowcharts become hexagons, which need far less room. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: add figures, table captions and a macro chooser - Figures with numbered captions: SAX stepper, json_view layout and edit stepper, exception map, JSON pointer and patch walkers, QA pipeline, scaling and binary size charts, roadmap lanes, release timeline, integration decision tree and namespace builder - Table captions (nj-table-caption) on 9 pages - Compile times: modules and PCH charts, filesystem section, re-measured Figure 1, links between features/modules.md and compile_times.md - Interactive macro chooser for features/arbitrary_types.md - Link check: exclude URLs that answer the plugin with 400/401/419 or are dead (customers.md) Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: macro chooser generates adl_serializer for non-default-constructible types Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: cross-link concept pages and API pages in See also sections Add See also links in both directions between the concept pages (features, home, integration) and the API pages, write API links in code font, give every entry a comment after a dash, and show the callback event figure on the parser callbacks page. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: address review comments on the new design - Home: add a "Latest release" button - Sponsors: update the list and group it by tier - Features: list all feature pages on the index page - Releases: add highlights per release, link the 4.0 roadmap, add the first commit to the timeline - Exceptions: prefix all example messages with the exception name - API: no code font for format names in the See also links of the to_/from_ pages - Merge Patch: show the document before, the patch, and the result - Replace "--" with typographic dashes, mark external links with an icon, center Mermaid diagrams Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
194 lines
9.5 KiB
Markdown
194 lines
9.5 KiB
Markdown
# JSON Pointer
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## Introduction
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The library supports **JSON Pointer** ([RFC 6901](https://tools.ietf.org/html/rfc6901)) as an alternative means to
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address structured values. A JSON Pointer is a string that identifies a specific value within a JSON document.
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Consider the following JSON document
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```json
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{
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"array": ["A", "B", "C"],
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"nested": {
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"one": 1,
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"two": 2,
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"three": [true, false]
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}
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}
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```
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Select a pointer (or a node of the tree) to walk it through this document; each segment of the pointer goes one level
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deeper. The table below lists the same pointers.
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<div class="nj-pointer-walk" data-nj-pointer-walk data-doc="{"array":["A","B","C"],"nested":{"one":1,"two":2,"three":[true,false]}}" role="group" aria-label="Pointer walker for the example document">
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<p class="nj-pointer-static">Highlighted: the path of the pointer <code>/nested/three/1</code>, which identifies <code>false</code>.</p>
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<ul class="nj-pointer-tree">
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<li data-p="" class="is-on"><span class="nj-pointer-n"><span class="nj-pointer-k">root</span> <span class="nj-pointer-t">object</span></span>
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<ul>
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<li data-p="/array" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">"array"</span> <span class="nj-pointer-t">array</span></span>
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<ul>
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<li data-p="/array/0"><span class="nj-pointer-n"><span class="nj-pointer-k">0</span>: <span class="nj-pointer-v nj-pointer-v--s">"A"</span></span></li>
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<li data-p="/array/1"><span class="nj-pointer-n"><span class="nj-pointer-k">1</span>: <span class="nj-pointer-v nj-pointer-v--s">"B"</span></span></li>
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<li data-p="/array/2"><span class="nj-pointer-n"><span class="nj-pointer-k">2</span>: <span class="nj-pointer-v nj-pointer-v--s">"C"</span></span></li>
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</ul>
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</li>
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<li data-p="/nested" class="is-on"><span class="nj-pointer-n"><span class="nj-pointer-k">"nested"</span> <span class="nj-pointer-t">object</span></span>
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<ul>
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<li data-p="/nested/one" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">"one"</span>: <span class="nj-pointer-v nj-pointer-v--n">1</span></span></li>
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<li data-p="/nested/two" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">"two"</span>: <span class="nj-pointer-v nj-pointer-v--n">2</span></span></li>
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<li data-p="/nested/three" class="is-on"><span class="nj-pointer-n"><span class="nj-pointer-k">"three"</span> <span class="nj-pointer-t">array</span></span>
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<ul>
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<li data-p="/nested/three/0" class="is-pass"><span class="nj-pointer-n"><span class="nj-pointer-k">0</span>: <span class="nj-pointer-v nj-pointer-v--b">true</span></span></li>
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<li data-p="/nested/three/1" class="is-on is-target"><span class="nj-pointer-n"><span class="nj-pointer-k">1</span>: <span class="nj-pointer-v nj-pointer-v--b">false</span></span></li>
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</ul>
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</li>
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</ul>
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</li>
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</ul>
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</li>
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</ul>
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</div>
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<p class="nj-caption" markdown>The highlighted nodes are the path from the root to the selected level, and the line below the tree shows the value the pointer resolves to. Select a segment in the pointer bar to stop at a shallower level, or a node of the tree to select its pointer.</p>
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Then every value inside the JSON document can be identified as follows:
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| JSON Pointer | JSON value |
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|-------------------|----------------------------------------------------------------------------------|
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| `` | `#!json {"array":["A","B","C"],"nested":{"one":1,"two":2,"three":[true,false]}}` |
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| `/array` | `#!json ["A","B","C"]` |
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| `/array/0` | `#!json A` |
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| `/array/1` | `#!json B` |
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| `/array/2` | `#!json C` |
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| `/nested` | `#!json {"one":1,"two":2,"three":[true,false]}` |
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| `/nested/one` | `#!json 1` |
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| `/nested/two` | `#!json 2` |
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| `/nested/three` | `#!json [true,false]` |
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| `/nested/three/0` | `#!json true` |
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| `/nested/three/1` | `#!json false` |
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Note `/` does not identify the root (i.e., the whole document), but an object entry with empty key `""`. See
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[RFC 6901](https://tools.ietf.org/html/rfc6901) for more information.
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[RFC 6901, section 4](https://tools.ietf.org/html/rfc6901#section-4) states that evaluation fails if an object has
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non-unique member names. A `basic_json` object holds one value per key (see [`object_t`](../api/basic_json/object_t.md)
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for how duplicate keys are handled while parsing), so evaluating a JSON Pointer never encounters duplicates.
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## JSON Pointer creation
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JSON Pointers can be created from a string:
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```cpp
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json::json_pointer p("/nested/one");
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```
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Furthermore, a user-defined string literal can be used to achieve the same result:
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```cpp
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auto p = "/nested/one"_json_pointer;
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```
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The escaping rules of [RFC 6901](https://tools.ietf.org/html/rfc6901) are implemented. See the
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[constructor documentation](../api/json_pointer/json_pointer.md) for more information.
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!!! note "URI fragment representation"
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[RFC 6901, section 6](https://tools.ietf.org/html/rfc6901#section-6) also defines a URI fragment identifier
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representation (`#/a%20b`). It is not supported: `#!cpp json::json_pointer("#/a")` throws
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[`parse_error.107`](../home/exceptions.md#jsonexceptionparse_error107). Strip the leading `#` and
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percent-decode the remainder before creating the JSON pointer.
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## Value access
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JSON Pointers can be used in the [`at`](../api/basic_json/at.md), [`operator[]`](../api/basic_json/operator%5B%5D.md),
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and [`value`](../api/basic_json/value.md) functions just like object keys or array indices.
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```cpp
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// the JSON value from above
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auto j = json::parse(R"({
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"array": ["A", "B", "C"],
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"nested": {
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"one": 1,
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"two": 2,
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"three": [true, false]
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}
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})");
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// access values
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auto val = j[""_json_pointer]; // {"array":["A","B","C"],...}
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auto val1 = j["/nested/one"_json_pointer]; // 1
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auto val2 = j.at(json::json_pointer("/nested/three/1")); // false
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auto val3 = j.value(json::json_pointer("/nested/four"), 0); // 0
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```
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To read a value with a fallback, use [`value`](../api/basic_json/value.md) with a JSON Pointer; to test for existence,
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use [`contains`](../api/basic_json/contains.md). Neither needs intermediate checks, see
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[Nested values](element_access/default_value.md#nested-values).
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!!! note "Creating intermediate levels that don't exist"
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See the [`operator[]` notes](../api/basic_json/operator%5B%5D.md#return-value) for how array vs. object is
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decided when a pointer creates intermediate levels that don't exist yet.
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## Flatten / unflatten
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The library implements a function [`flatten`](../api/basic_json/flatten.md) to convert any JSON document into a JSON
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object where each key is a JSON Pointer and each value is a primitive JSON value (i.e., a string, boolean, number, or
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null).
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```cpp
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// the JSON value from above
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auto j = json::parse(R"({
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"array": ["A", "B", "C"],
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"nested": {
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"one": 1,
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"two": 2,
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"three": [true, false]
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}
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})");
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// create flattened value
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auto j_flat = j.flatten();
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```
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The resulting value `j_flat` is:
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```json
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{
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"/array/0": "A",
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"/array/1": "B",
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"/array/2": "C",
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"/nested/one": 1,
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"/nested/two": 2,
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"/nested/three/0": true,
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"/nested/three/1": false
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}
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```
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The reverse function, [`unflatten`](../api/basic_json/unflatten.md) recreates the original value.
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```cpp
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auto j_original = j_flat.unflatten();
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```
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Each leaf of the document becomes one key of the flat object, and `unflatten()` reverses the mapping:
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<div class="nj-fig-wrap" markdown>
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--8<-- "images/json_pointer_flatten.svg"
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</div>
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<p class="nj-caption" markdown>Left: the example document as a tree; right: the flat object it becomes. `flatten()` converts from left to right, `unflatten()` from right to left.</p>
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## See also
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- [`json_pointer`](../api/json_pointer/index.md) - the JSON Pointer class
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- [Nested values](element_access/default_value.md#nested-values) - [`value`](../api/basic_json/value.md),
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[`contains`](../api/basic_json/contains.md), and [`at`](../api/basic_json/at.md) accept JSON Pointers
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- [`flatten`](../api/basic_json/flatten.md) - convert a value to an object of JSON Pointers and primitive values
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- [`unflatten`](../api/basic_json/unflatten.md) - restore a value from a flattened object
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- [JSON Patch](json_patch.md) - paths inside a patch are JSON Pointers
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- [JSON Merge Patch](merge_patch.md) - an alternative patch format that does not use JSON Pointer
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- [`operator""_json_pointer`](../api/operator_literal_json_pointer.md) - user-defined literal to create a JSON pointer
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- [Element Access](element_access/index.md) - other ways to access elements
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