Niels Lohmann
0adb7783a4
Give basic_json_view the read-only access functions of basic_json: operator[] and at() with keys and indices, front()/back(), find(), contains(), count(), begin()/end() and cbegin()/cend(), items() with structured bindings from C++17 on, and type_name(). Exceptions have the ids and messages of the const functions of basic_json. Where basic_json has undefined behavior the view answers safely: operator[] with a missing key or an out-of-range index returns a discarded view, and front()/back() of an empty container throw invalid_iterator.214. Objects are iterated in document order, and all members are visited; duplicate-key lookups find the first member (as yyjson and simdjson do), while parse(), materialize(), and the map conversions keep the last value, as parse() does. Keys of up to 16 bytes are compared with two overlapping loads. Add value conversions: get<T>()/get_to() for arithmetic types, bool, nullptr_t, strings (std::basic_string copied, string_view_t without a copy), BasicJsonType, views, std::vector, and maps with string keys; get_string() for the string without a copy; number_token() for the number exactly as written in the source; value() with keys and JSON pointers; and operator[]/at()/ contains() with JSON pointers. Everything else, including types with from_json(), goes through materialize() of that subtree. get<T>() of arithmetic types is inlined down to the conversion, so reading an integer needs no call. detail::json_pointer_access exposes a pointer's reference tokens to code outside basic_json. 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.