Niels Lohmann
08dcdaf9b8
- 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>
1.8 KiB
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 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
Constant.
Notes
Unlike BasicJsonType::front(), which has undefined behavior for an empty array or
object, front() throws invalid_iterator.214 in that case -- the same way it already does for #!json null and
for a discarded view, where BasicJsonType::front() also throws.
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 ofbasic_json
Version history
- Added in version 3.13.0.