json/docs/mkdocs/docs/api/basic_json_view/back.md

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 guarantee: 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.

As for BasicJsonType, with JSON_DIAGNOSTICS the exception names the JSON Pointer path of the value within the document, and with JSON_DIAGNOSTIC_POSITIONS its bytes in the source text (when materialize() sets them).

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. Constant if no element of the array is an array or object, since the elements are then adjacent in the index.

Notes

Unlike BasicJsonType::back(), which has undefined behavior for an empty array or object, back() throws invalid_iterator.214 for an empty array, an empty object, a null value, or a discarded view.

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"
```

See also

• front - access the first element
• BasicJsonType::back - the corresponding function of basic_json

Version history

• Added in version 3.13.0.