0eaea7ccba
* Lookups (operator[], at, find, contains, count, value, JSON pointers) return the last member of a duplicate key, as materialize() and parse() keep it. * operator[] on a discarded view returns a discarded view instead of throwing, so v["a"]["b"] is safe for a missing "a". * operator[] and at() take any integer type (not only int and size_t), fixing ambiguous calls with unsigned, long, std::int64_t, ... * Fix the operator[] documentation, which claimed a discarded view for a type mismatch where type_error.305 is thrown. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.1 KiB
nlohmann::basic_json_view::is_discarded
bool is_discarded() const noexcept;
Returns whether this view is invalid, i.e. does not refer to a value. This is the case for a default-constructed
view (see (constructor)), and for root() of a document
that is itself discarded -- in particular, the root of a failed
parse() with allow_exceptions set to #!cpp false.
A discarded view is also what operator[] returns for a missing key, an index out of range, or a
JSON pointer that cannot be resolved, and for any access on a view that is itself discarded (so a chain such as
#!cpp v["a"]["b"] is safe). at throws instead.
Return value
#!cpp true if the view is discarded, #!cpp false otherwise.
Exception safety
No-throw guarantee: this function never throws exceptions.
Complexity
Constant.
Notes
A basic_json_view is not convertible to #!cpp bool: such a conversion would mean "refers to a value", whereas
basic_json converts to the #!cpp bool it holds, so the same code would silently behave differently. Test
#!cpp !v.is_discarded() explicitly.
Examples
??? example
The example below classifies several parsed documents by the type of their root value, without materializing any
of them into a `BasicJsonType` value.
```cpp
--8<-- "examples/basic_json_view__type_predicates.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__type_predicates.output"
```
See also
- operator[] - access specified element; yields a discarded view where an element is missing
- (constructor) - the default constructor creates a discarded view
- is_discarded (basic_json_document) - return whether the last parse failed
BasicJsonType::is_discarded- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.