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Niels Lohmann
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* docs: new design for the documentation site Apply the brand to the Material for MkDocs site: Inter and JetBrains Mono served by the site (upright and real italic), brand colours, blue header band with the syntax pattern, wordmark in the header, new home page (hero, cards, animated feature tour), 404 page, polish for tabs, search, footer, focus rings, tables and code blocks, and theme features (code copy, previous/next footer, TOC follow, search suggestions). Draw the number railroad diagram, the parser callback events and the iterator illustrations as inline SVGs that follow the light and dark scheme. The generator scripts are in docs/mkdocs/scripts/figures; the feature slideshow (json-light.svg, json-dark.svg, replacing json.gif on the home page) is built by docs/mkdocs/scripts/slideshow. Decision nodes in five flowcharts become hexagons, which need far less room. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: add figures, table captions and a macro chooser - Figures with numbered captions: SAX stepper, json_view layout and edit stepper, exception map, JSON pointer and patch walkers, QA pipeline, scaling and binary size charts, roadmap lanes, release timeline, integration decision tree and namespace builder - Table captions (nj-table-caption) on 9 pages - Compile times: modules and PCH charts, filesystem section, re-measured Figure 1, links between features/modules.md and compile_times.md - Interactive macro chooser for features/arbitrary_types.md - Link check: exclude URLs that answer the plugin with 400/401/419 or are dead (customers.md) Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: macro chooser generates adl_serializer for non-default-constructible types Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: cross-link concept pages and API pages in See also sections Add See also links in both directions between the concept pages (features, home, integration) and the API pages, write API links in code font, give every entry a comment after a dash, and show the callback event figure on the parser callbacks page. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * docs: address review comments on the new design - Home: add a "Latest release" button - Sponsors: update the list and group it by tier - Features: list all feature pages on the index page - Releases: add highlights per release, link the 4.0 roadmap, add the first commit to the timeline - Exceptions: prefix all example messages with the exception name - API: no code font for format names in the See also links of the to_/from_ pages - Merge Patch: show the document before, the patch, and the result - Replace "--" with typographic dashes, mark external links with an icon, center Mermaid diagrams Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.2 KiB
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.