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
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON values
    json j_null;
    json j_boolean = true;
    json j_number_integer = 17;
    json j_number_unsigned_integer = 12345678987654321u;
    json j_number_float = 23.42;
    json j_object = {{"one", 1}, {"two", 2}};
    json j_array = {1, 2, 4, 8, 16};
    json j_string = "Hello, world";
    json j_binary = json::binary({1, 2, 3});

    // call is_discarded()
    std::cout << std::boolalpha;
    std::cout << j_null.is_discarded() << '\n';
    std::cout << j_boolean.is_discarded() << '\n';
    std::cout << j_number_integer.is_discarded() << '\n';
    std::cout << j_number_unsigned_integer.is_discarded() << '\n';
    std::cout << j_number_float.is_discarded() << '\n';
    std::cout << j_object.is_discarded() << '\n';
    std::cout << j_array.is_discarded() << '\n';
    std::cout << j_string.is_discarded() << '\n';
    std::cout << j_binary.is_discarded() << '\n';
}

```

Output:

```json
false
false
false
false
false
false
false
false
false

```

??? 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
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // parsing invalid JSON without exceptions yields a discarded value
    json j_invalid = json::parse("[1,2,3", nullptr, false);

    // a callback that discards the top-level value does not leave it
    // "discarded" -- it is replaced by null instead
    json j_discarded_by_callback = json::parse("[1,2,3]", [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)
    {
        return event != json::parse_event_t::array_start;
    });

    std::cout << std::boolalpha;
    std::cout << "j_invalid.is_discarded()               = " << j_invalid.is_discarded() << '\n';
    std::cout << "j_discarded_by_callback                = " << j_discarded_by_callback << '\n';
    std::cout << "j_discarded_by_callback.is_discarded() = " << j_discarded_by_callback.is_discarded() << '\n';
}

```

Output:

```json
j_invalid.is_discarded()               = true
j_discarded_by_callback                = null
j_discarded_by_callback.is_discarded() = false

```

Version history

Added in version 1.0.0.

4.8 KiB Raw Blame History