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Niels Lohmann 77acd4563c Add element access, iteration, values, and JSON pointers to json_view

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>

2026-10-07 16:42:26 +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