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

nlohmann::basic_json_view::front

basic_json_view front() const;

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

Return value

The first 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

Constant.

Notes

Unlike BasicJsonType::front(), which has undefined behavior for an empty array or object, front() 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 earliest entry of a build log with `front()`, without materializing the rest
of the (possibly long) log.

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

Output:

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

See also

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

Version history

• Added in version 3.13.0.