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. If the top-level value itself is discarded by the callback, the parse call returns a null value.

After a successful parse, this function always returns false: discarded values can only occur during parsing and are either removed when inside a structured value or replaced by null at the top level. The exception is parsing with allow_exceptions set to false: a parse error then yields a discarded value for which this function returns true (see parse).

Examples¶

Example: is_discarded() for ordinary JSON values

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

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 false, and a parser callback that discards the top-level value (which is replaced by null and therefore does not remain discarded).

#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:

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.