Niels Lohmann
13d2b3e224
Looking up the last member of a duplicate key cannot stop at a match, so every lookup scanned the whole object (1.6 to 3.4 times slower for small objects). operator[](key), at, find, value, contains, count, and JSON pointer resolution return the first member again, as yyjson and simdjson do; materialize() and get<map>() keep the last value, as parse(). The documentation says so in the feature page and on each lookup page, and explains how to get the value parse() would give. The integer index templates and the discarded chaining of operator[] stay. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
3.0 KiB
nlohmann::basic_json_view::items
/* unspecified */ items() const noexcept;
Returns a range of item values -- (key, value) pairs -- for use in range-based for loops.
The key of an array element is its index, converted to a string, as for
BasicJsonType::items().
The returned type is not part of the public API and may change between versions; use a range-based for loop (see the
example), or #!cpp decltype(v.items()) if you need to name it.
for (const auto& item : v.items())
{
std::cout << "key: " << item.key() << ", value: " << item.value() << '\n';
}
On C++17, item also supports structured bindings:
for (const auto [key, value] : v.items())
{
std::cout << "key: " << key << ", value: " << value << '\n';
}
Note the #!cpp const auto (by value), not #!cpp const auto&: unlike BasicJsonType::items(), whose elements are
references into an existing object, a view's item is produced on the fly for each step of the iteration, so there
is nothing for a reference to bind to.
Return value
A range whose iterators dereference to item and whose #!cpp begin()/#!cpp end() are
equivalent to basic_json_view::begin()/end(), in document order.
Exception safety
No-throw guarantee: this function never throws exceptions.
Complexity
Constant.
Notes
As for begin()/end(), items() visits every member of an object, including all
occurrences of a duplicate key -- unlike operator[], at, find,
contains, and count, which resolve to the first member with a given key. See the
Notes on duplicate keys of operator[].
!!! danger "Lifetime issues"
As for `BasicJsonType::items()`, calling `items()` on a temporary view (or a temporary document) is dangerous:
the range refers back to the document, so the document must outlive the loop. See
[#2040](https://github.com/nlohmann/json/issues/2040) for the `BasicJsonType` background.
Examples
??? example
The example below shows a settings object whose source text records every update to a key as a duplicate
member, in the order they happened. `items()` walks all of them, so the update history is visible, while
[`operator[]`](operator[].md) only ever sees the *first* one and [`materialize()`](materialize.md) -- like
[`BasicJsonType::parse()`](../basic_json/parse.md) -- keeps only the *last*.
```cpp
--8<-- "examples/basic_json_view__items.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__items.output"
```
See also
- begin, end - the iterators
items()is built on BasicJsonType::items- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.