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Niels Lohmann 08dcdaf9b8 Document element access, lookup, and iteration of json_view

- API pages for operator[], at, front, back, find, contains, count,
  begin, end, cbegin, cend, items, and type_name of basic_json_view,
  linked both ways with the basic_json pages
- the feature page and size() describe document order and duplicate
  keys
- the examples show when the view helps: reading a few fields of a large
  text, probing optional members, and members in source order

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

2026-09-30 21:01:33 +02:00

2.2 KiB

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nlohmann::basic_json_view::back

basic_json_view back() const;

Returns the last element of an array, the last member value of an object, or the value itself if it is primitive (as for BasicJsonType::back(), a primitive value is a range of one element).

Return value

The last element or member value. For a primitive value (number, string, boolean), the value itself.

Exception safety

Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.

Exceptions

Throws invalid_iterator.214 if the view is null or discarded, or if it is an empty array or object.

This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

Complexity

Linear in the size of the array or object: unlike front(), which only ever looks at the first element, back() has to walk every element to find where they end, since elements are not a fixed size in the index.

Notes

Unlike BasicJsonType::back(), which has undefined behavior for an empty array or object, back() throws invalid_iterator.214 in that case -- the same way it already does for #!json null and for a discarded view, where BasicJsonType::back() also throws.

Examples

??? example

The example below reads only the final status of a build log with `back()`. Even though `back()` is linear in
the number of events (unlike [`front()`](front.md), which is constant), it still avoids building a
`BasicJsonType` value for the events that are not needed.

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

Output:

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

Version history

Added in version 3.13.0.

2.2 KiB Raw Blame History