mirror of https://github.com/nlohmann/json.git synced 2026-09-30 22:15:19 +00:00

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Niels Lohmann 54455afa40 Document json_document and json_view

- API pages for basic_json_document and basic_json_view, one per member,
  and for the four aliases, each with an example
- features/json_view.md: the problem the view solves, ownership and
  lifetime, what matches basic_json::parse() and what differs, and when
  to choose json, ordered_json, SAX, or the view
- the examples show why one would use the view, not only how: borrowed
  vs. owned input, reading a few fields and materializing one subtree,
  reusing a document across many messages
- registered in the mkdocs navigation, llms.txt, the docset, the
  exceptions page (out_of_range.416), architecture.md, the integration
  page, and the README; the yyjson credit is added to the README and
  license.md

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

2026-09-29 14:02:43 +02:00

1.5 KiB

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nlohmann::basic_json_view::empty

bool empty() const noexcept;

Checks whether size() is 0, as BasicJsonType::empty() would for the same value.

Return value

The return value depends on the type and is defined as follows:

Value type	return value
null	`#!cpp true`
discarded	`#!cpp true`
boolean	`#!cpp false`
string	`#!cpp false`
number	`#!cpp false`
object	`#!cpp object_t::empty()`
array	`#!cpp array_t::empty()`

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Constant.

Notes

As for BasicJsonType::empty(), this does not return whether a string value is empty -- it is #!cpp false for any string, regardless of its length.

Examples

??? example

The example below uses [`size()`](size.md) and `empty()` to decide whether a parsed message is worth acting on,
without materializing it into a `BasicJsonType` value.

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

Output:

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

Version history

Added in version 3.13.0.

1.5 KiB Raw Blame History