json/docs/mkdocs/docs/api/basic_json/is_discarded.md

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_exceptions set to #!cpp false; see parse for 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 true if the view refers to no value

Version history

• Added in version 1.0.0.