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Niels Lohmann 63c10a51fc Review and extend the documentation, and check it in CI (#5638 )

* Review and extend the documentation, and check it in CI

A review of all documentation pages found factual errors, dead links,
missing cross-references, and gaps in examples. This fixes them and adds
checks so the same problems are caught automatically.

Fixes:
- wrong signatures and version histories (operator!= C++20 member,
  binary() subtype type, get<PointerType>(), JSON_NO_THREAD_LOCAL, ...)
- stale descriptions (number parsing since #5283, UBJSON table, SAX
  example that no longer compiled, tsl::ordered_map advice)
- dead internal and external links; repology.org badges (the domain is
  suspended) replaced by badges that query the registries directly
- deprecation notes link the migration guide; the guide itself fixed

Additions:
- "See also" sections, cross-references, 25 runnable examples, 12
  Mermaid diagrams, new API pages for json_pointer::operator<=> and
  byte_container_with_subtype::operator==/!=
- landing page, guides for untrusted input and performance
- "unreleased" badge after versions newer than the latest release

Checks:
- strict documentation build (broken links/anchors fail it); CI and
  the publish workflow fetch the full history the build needs
- weekly external link check, Mermaid syntax check in CI
- check_structure.py: example titles, heading levels, alt texts,
  header links, docset index coverage; its unused-example check works
  again
- all examples produce the same output on every platform

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

* Keep the customer links that could not be fixed

A dead link on the customers page is still the evidence of where the
use of the library was documented. Keep the original URLs of the entries
without a working replacement (Marne, Cisco Webex Desk Camera, Philips
Hue, CyberArk) and exclude exactly these URLs from the link check.

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

* Correct the duplicate-key recipe's claim about SAX positions

The SAX interface's key() receives no position either; only parse_error()
does. Also note that the recipe does not report the path to the repeated
key (see discussion #5085).

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

* Say the library is available as a single header and mention json_fwd.hpp

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

* Correct documentation errors found while hunting for bugs

- patch/patch_inplace: list the JSON pointer errors parse_error.106-109
  and out_of_range.402/404, and quote the actual parse_error.105 message.
- unflatten: list parse_error.106/107/108 and out_of_range.404.
- to_bson: list out_of_range.415 (binary subtype above 255) and note
  that 412 and 415 are new in 3.13.0.
- to_string: state that string_t must be convertible to std::string, also
  in the StringType requirements table.
- JSON Lines: a `while (input >> j)` loop also throws after the last value
  for concatenated JSON values; show a loop that works for both.
- BON8: a string gets 0xFF only if nothing follows it in the message; a
  string at the end of an array or object is ended by 0xFE.
- custom_string_type.hpp: add operator+=(char), which the "Always
  required" list asks for (json_pointer::to_string, flatten, unflatten,
  and diff did not compile), and an ADL int_to_string for diff and items.

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

* Cache the release headers with functools.lru_cache

Codacy (Pylint) flagged the mutable default argument that header() used
as its cache. functools.lru_cache keeps the same memoization without it.
The script's output is unchanged.

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

---------

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

2026-10-02 11:32:15 +02:00

3.3 KiB

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nlohmann::basic_json::unflatten

basic_json unflatten() const;

The function restores the arbitrary nesting of a JSON value that has been flattened before using the flatten() function. The JSON value must meet certain constraints:

The value must be an object.
The keys must be JSON pointers (see RFC 6901)
The mapped values must be primitive JSON types.

Return value

the original JSON from a flattened version

Exception safety

Strong exception safety: if an exception occurs, the original value stays intact.

Exceptions

The function can throw the following exceptions:

Throws type_error.314 if value is not an object
Throws type_error.315 if object values are not primitive
Throws type_error.313 if a key (JSON pointer) leads to a conflicting nesting; example: "invalid value to unflatten"
Throws parse_error.106 if an array index in a key begins with '0'; example: "array index '01' must not begin with '0'"
Throws parse_error.107 if a key is not empty and does not begin with a slash (/); example: "JSON pointer must be empty or begin with '/' - was: 'a'"
Throws parse_error.108 if a tilde (~) in a key is not followed by 0 or 1; example: "escape character '~' must be followed with '0' or '1'"
Throws parse_error.109 if an array index in a key is not a number; example: "array index 'one' is not a number"
Throws out_of_range.404 if a level becomes an array (because one of its keys is 0) and another key at that level cannot be an array index; example: "unresolved reference token 'x'"

Complexity

Linear in the size of the JSON value.

Notes

Empty objects and arrays are flattened by flatten() to #!json null values and cannot unflattened to their original type.

A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are described by the same JSON pointers. A value is therefore restored as an array if and only if one of its keys is the reference token 0, and as an object otherwise: #!json {"2": 1} is restored unchanged, whereas #!json {"0": 1} is restored as #!json [1]. This decision does not depend on the order in which the flattened object is iterated.

Apart from these two cases, for a JSON value j, the following is always true: #!cpp j == j.flatten().unflatten().

Examples

??? example

The following code shows how a flattened JSON object is unflattened into the original nested JSON object.

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

Output:

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

Version history

Added in version 2.0.0.
Made the array/object decision independent of the object's iteration order in version 3.13.0.

3.3 KiB Raw Blame History