Niels Lohmann
6a852ab96d
* Split the json_view feature page into sub-pages
The single features/json_view.md page grew to ~600 lines covering eight
topics. Split it into features/json_view/ with an overview page and
pages for ownership and lifetime, differences from basic_json, reading
values, generic code, editing, and images.
The new differences page collects what was spread over several sections
("What is the same", "What is different", "Duplicate keys", and notes in
the conversion and serialization sections) under one summary table, and
adds the parts of the basic_json interface a view does not have. The
other pages now link to it instead of restating the rules.
Internal links across the documentation point to the new pages.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
* Docs: fix stale comments and trim repeated text in json_view pages
- fig-jsonview.js/.css: point the header comments at the new pages
- reading.md: views' strings are valid as long as the document (and a
borrowed source text), as lifetime.md says
- reading.md, generic_code.md: link to differences.md instead of
restating the member-order and repeated-key rules
- differences.md: drop the third mention of ordering comparisons, name
rbegin()/rend() in the iterator row
- exceptions.md: link editable documents to editing.md
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---------
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.2 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 or its document 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 viewbasic_json_document::is_discarded- return whether the last parse failedBasicJsonType::is_discarded- the corresponding function ofbasic_json- Zero-copy JSON views - how views differ from basic_json
Version history
- Added in version 3.13.0.