Niels Lohmann
a47170346a
* 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>
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3.0 KiB
nlohmann::basic_json::is_discarded
constexpr bool is_discarded() const noexcept;
This function returns #!cpp true for a JSON value if either:
- the value was discarded during parsing with a callback function (see
parser_callback_t), or - the value is the result of parsing invalid JSON with parameter
allow_exceptionsset to#!cpp false; seeparsefor more information.
Return value
#!cpp true if type is discarded, #!cpp false otherwise.
Exception safety
No-throw guarantee: this member function never throws exceptions.
Complexity
Constant.
Notes
!!! note "Comparisons"
Discarded values are never compared equal with [`operator==`](operator_eq.md). That is, checking whether a JSON
value `j` is discarded will only work via:
```cpp
j.is_discarded()
```
because
```cpp
j == json::value_t::discarded
```
will always be `#!cpp false`.
!!! note "Removal during parsing with callback functions"
When a value is discarded by a callback function (see [`parser_callback_t`](parser_callback_t.md)) during parsing,
then it is removed when it is part of a structured value. For instance, if the second value of an array is discarded,
instead of `#!json [null, discarded, false]`, the array `#!json [null, false]` is returned. If the top-level value
itself is discarded by the callback, the `parse` call returns a `#!json null` value.
After a successful parse, this function always returns #!cpp false: discarded values can only occur during parsing and
are either removed when inside a structured value or replaced by #!json null at the top level. The exception is parsing
with allow_exceptions set to #!cpp false: a parse error then yields a discarded value for which this function returns
#!cpp true (see parse).
Examples
??? example "Example: is_discarded() for ordinary JSON values"
The following code exemplifies `is_discarded()` for all JSON types.
```cpp
--8<-- "examples/is_discarded.cpp"
```
Output:
```json
--8<-- "examples/is_discarded.output"
```
??? example "Example: discarded values from parsing"
The following code shows the two situations in which a discarded value can be observed: parsing invalid JSON with
`allow_exceptions` set to `#!cpp false`, and a parser callback that discards the top-level value (which is replaced
by `#!json null` and therefore does *not* remain discarded).
```cpp
--8<-- "examples/is_discarded__parse.cpp"
```
Output:
```json
--8<-- "examples/is_discarded__parse.output"
```
See also
basic_json_view::is_discarded- the corresponding check on a zero-copy view, which is#!cpp trueif the view refers to no value- Types - the JSON value types and how they are stored
Version history
- Added in version 1.0.0.