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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

2.0 KiB

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JSON_ASSERT

#define JSON_ASSERT(x) /* value */

This macro controls which code is executed for runtime assertions of the library.

Parameters

x (in): expression of a scalar type

Default definition

The default value is #!cpp assert(x).

#define JSON_ASSERT(x) assert(x)

Therefore, assertions can be switched off by defining NDEBUG.

Notes

The library uses numerous assertions to guarantee invariants and to abort in case of otherwise undefined behavior (e.g., when calling operator[] with a missing object key on a const object). See page runtime assertions for more information.
Defining the macro to code that does not call std::abort may leave the library in an undefined state.
The macro is undefined outside the library.

Examples

??? example "Example: default behavior"

The following code will trigger an assertion at runtime:

```cpp
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    const json j = {{"key", "value"}};
    auto v = j["missing"];
}
```

Output:

```
Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.
```

??? example "Example: user-defined behavior"

The assertion reporting can be changed by defining `JSON_ASSERT(x)` differently.

```cpp
#include <cstdio>
#include <cstdlib>
#define JSON_ASSERT(x) if(!(x)){fprintf(stderr, "assertion error in %s\n", __FUNCTION__); std::abort();}

#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    const json j = {{"key", "value"}};
    auto v = j["missing"];
}
```

Output:

```
assertion error in operator[]
```

Version history

Added in version 3.9.0.

2.0 KiB Raw Blame History