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# nlohmann::basic_json::is_discarded

    constexpr bool is_discarded() const noexcept;

This function returns `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 `false`; see `parse` for more information.

## Return value

`true` if type is discarded, `false` otherwise.

## Exception safety

No-throw guarantee: this member function never throws exceptions.

## Complexity

Constant.

## Notes

Comparisons

Discarded values are never compared equal with `operator==`. That is, checking whether a JSON value `j` is discarded will only work via:

    j.is_discarded()

because

    j == json::value_t::discarded

will always be `false`.

Removal during parsing with callback functions

When a value is discarded by a callback function (see `parser_callback_t`) 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 `[null, discarded, false]`, the array `[null, false]` is returned. Only if the top-level value is discarded, the return value of the `parse` call is discarded.

This function will always be `false` for JSON values after parsing. That is, discarded values can only occur during parsing, but will be removed when inside a structured value or replaced by null in other cases.

## Examples

Example

The following code exemplifies `is_discarded()` for all JSON types.

    #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:

    false
    false
    false
    false
    false
    false
    false
    false
    false

## Version history

- Added in version 1.0.0.

Last update: May 1, 2022
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