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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

7.2 KiB
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nlohmann::basic_json::swap

// (1)
void swap(reference other) noexcept (
    std::is_nothrow_move_constructible<value_t>::value &&
    std::is_nothrow_move_assignable<value_t>::value &&
    std::is_nothrow_move_constructible<json_value>::value &&
    std::is_nothrow_move_assignable<json_value>::value &&
    std::is_nothrow_move_constructible<json_base_class_t>::value &&
    std::is_nothrow_move_assignable<json_base_class_t>::value
);

// (2)
friend void swap(reference left, reference right) noexcept (
    std::is_nothrow_move_constructible<value_t>::value &&
    std::is_nothrow_move_assignable<value_t>::value &&
    std::is_nothrow_move_constructible<json_value>::value &&
    std::is_nothrow_move_assignable<json_value>::value &&
    std::is_nothrow_move_constructible<json_base_class_t>::value &&
    std::is_nothrow_move_assignable<json_base_class_t>::value
);

// (3)
void swap(array_t& other);

// (4)
void swap(object_t& other);

// (5)
void swap(string_t& other);

// (6)
void swap(binary_t& other);

// (7)
void swap(typename binary_t::container_type& other);
  1. Exchanges the contents of the JSON value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. If macro JSON_DIAGNOSTIC_POSITIONS is defined to #!cpp 1, the start_pos()/end_pos() diagnostic positions are exchanged along with the value. The json_base_class_t subobject is exchanged along with the value as well, the same way it is copied or moved by the copy/move constructors and assignment operators.
  2. Exchanges the contents of the JSON value from left with those of right. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. Implemented as a friend function callable via ADL. If macro JSON_DIAGNOSTIC_POSITIONS is defined to #!cpp 1, the start_pos()/end_pos() diagnostic positions are exchanged along with the value. The json_base_class_t subobject is exchanged along with the value as well, the same way it is copied or moved by the copy/move constructors and assignment operators.
  3. Exchanges the contents of a JSON array with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
  4. Exchanges the contents of a JSON object with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
  5. Exchanges the contents of a JSON string with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
  6. Exchanges the contents of a binary value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
  7. Exchanges the contents of a binary value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. Unlike version (6), no binary subtype is involved.

Parameters

other (in, out)
value to exchange the contents with
left (in, out)
value to exchange the contents with
right (in, out)
value to exchange the contents with

Exception safety

  1. No-throw guarantee: this function never throws exceptions.
  2. No-throw guarantee: this function never throws exceptions.
  3. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  4. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  5. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  6. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  7. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

Exceptions

  1. No-throw guarantee: this function never throws exceptions.
  2. No-throw guarantee: this function never throws exceptions.
  3. Throws type_error.310 if called on JSON values other than arrays; example: "cannot use swap(array_t&) with boolean"
  4. Throws type_error.310 if called on JSON values other than objects; example: "cannot use swap(object_t&) with boolean"
  5. Throws type_error.310 if called on JSON values other than strings; example: "cannot use swap(string_t&) with boolean"
  6. Throws type_error.310 if called on JSON values other than binaries; example: "cannot use swap(binary_t&) with boolean"
  7. Throws type_error.310 if called on JSON values other than binaries; example: "cannot use swap(binary_t::container_type&) with boolean"

Complexity

Constant.

Examples

??? example "Example: (1, 2) swap JSON values"

The example below shows how JSON values can be swapped with `swap()`.

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

Output:

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

??? example "Example: (3) swap array"

The example below shows how arrays can be swapped with `swap()`.

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

Output:

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

??? example "Example: (4) swap object"

The example below shows how objects can be swapped with `swap()`.

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

Output:

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

??? example "Example: (5) swap string"

The example below shows how strings can be swapped with `swap()`.

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

Output:

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

??? example "Example: (6) swap binary"

The example below shows how binary values can be swapped with `swap()`.

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

Output:

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

See also

Version history

  1. Since version 1.0.0. Exchanges the json_base_class_t subobject along with the value since version 3.13.0.
  2. Since version 1.0.0. Exchanges the json_base_class_t subobject along with the value since version 3.13.0.
  3. Since version 1.0.0.
  4. Since version 1.0.0.
  5. Since version 1.0.0.
  6. Since version 3.8.0.
  7. Since version 3.8.0.
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