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