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Niels Lohmann a47170346a docs: new design for the documentation site (#5934 )

* 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>

2026-10-27 07:41:42 +01:00

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nlohmann::basic_json::merge_diff

static basic_json merge_diff(const basic_json& source,
                             const basic_json& target);

Creates a JSON Merge Patch so that value source can be changed into the value target by calling the merge_patch function. The patch contains only what differs: changed and added members with their new values, and removed members with the value #!json null. Nested objects are compared member by member; any other value that differs, including an array, is replaced as a whole.

For two JSON values source and target, where target contains no object member whose value is #!json null, the following code always yields #!cpp true (for ordered_json, see the notes below):

basic_json patched = source;
patched.merge_patch(merge_diff(source, target));
patched == target;

Parameters

source (in): JSON value to compare from
target (in): JSON value to compare against

Return value

a JSON Merge Patch to convert the source to target

Exception safety

Strong guarantee: source and target are never modified.

Complexity

Linear in the total number of values (including nested ones) in source and target, times the cost of a key-based object operation (looking up a key, or adding one to the patch): logarithmic for json, and linear for ordered_json (see ordered_map complexity). For ordered_json, diffing two objects with n and m members therefore takes O((n+m)²).

Notes

A merge patch uses #!json null to remove a member, so it cannot set a member to #!json null. As in RFC 7396, a #!json null member of target is treated as absent: the patch removes the member from source instead of setting it to #!json null. A member that is #!json null in both source and target is unchanged and left out of the patch, so source keeps it.
A merge patch cannot reorder object members, and merge_patch appends added members at the end. For ordered_json, the patched value therefore has the same members as target, but not necessarily in the same order, and may compare unequal to it. The patch lists changed and removed members in the order of source, followed by added members in the order of target.

Examples

??? example

The following code shows how a JSON Merge Patch is created as a diff for two JSON values.

```cpp
--8<-- "examples/merge_diff.cpp"
```

Output:

```json
--8<-- "examples/merge_diff.output"
```

Version history

Added in version 3.13.0.

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