Niels Lohmann
c8b380769c
operator[](idx), at(idx) and back() skipped the elements of an array one after the other, whatever they were. In an array none of whose elements is an array or object (numbers, strings, booleans, null) the elements are adjacent nodes, which the array node's own sizes tell: its subtree has exactly one node more than it has elements. Such an element is now the node idx after the array node; any other array is walked as before. Reading r[i] for every i of an array of 3000 integers: Apple M1: 28.8 M instructions, 3.1 ms -> 0.1 M instructions, 4 us Linux x86-64 Ir (whole run), GCC 13 / Clang 18: 84.1 M -> 3.1 M / 75.0 M -> 3.0 M Arrays that contain arrays are unchanged. Belongs to the PR "json_view: read-side speedups". Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.2 KiB
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.
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. Constant for an array none of whose elements 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 ofbasic_json
Version history
- Added in version 3.13.0.