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nlohmann::basic_json::operator<=>

// since C++20
class basic_json {
    std::partial_ordering operator<=>(const_reference rhs) const noexcept;  // (1)

    template<typename ScalarType>
    std::partial_ordering operator<=>(const ScalarType rhs) const noexcept; // (2)
};
  1. 3-way compares two JSON values producing a result of type std::partial_ordering according to the following rules:

    • Two JSON values compare with a result of std::partial_ordering::unordered if either value is discarded.
    • If both JSON values are of the same type, the result is produced by 3-way comparing their stored values using their respective operator<=>.
    • Integer and floating-point numbers are converted to their common type and then 3-way compared using their respective operator<=>. For instance, comparing an integer and a floating-point value will 3-way compare the first value converted to floating-point with the second value.
    • Otherwise, yields a result by comparing the type (see value_t).
  2. 3-way compares a JSON value and a scalar or a scalar and a JSON value by converting the scalar to a JSON value and 3-way comparing both JSON values (see 1).

Template parameters

ScalarType
a scalar type according to std::is_scalar<ScalarType>::value

Parameters

rhs (in)
second value to consider

Return value

the std::partial_ordering of the 3-way comparison of *this and rhs

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Linear.

Notes

Comparing NaN

  • NaN values are unordered within the domain of numbers. The following comparisons all yield std::partial_ordering::unordered:
    1. Comparing a NaN with itself.
    2. Comparing a NaN with another NaN.
    3. Comparing a NaN and any other number.

Examples

Example: (1) comparing JSON values

The example demonstrates comparing several JSON values.

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

using json = nlohmann::json;

const char* to_string(const std::partial_ordering& po)
{
    if (std::is_lt(po))
    {
        return "less";
    }
    else if (std::is_gt(po))
    {
        return "greater";
    }
    else if (std::is_eq(po))
    {
        return "equivalent";
    }
    return "unordered";
}

int main()
{
    // create several JSON values
    json array_1 = {1, 2, 3};
    json array_2 = {1, 2, 4};
    json object_1 = {{"A", "a"}, {"B", "b"}};
    json object_2 = {{"B", "b"}, {"A", "a"}};
    json number = 17;
    json string = "foo";
    json discarded = json(json::value_t::discarded);


    // output values and comparisons
    std::cout << array_1 << " <=> " << array_2 << " := " << to_string(array_1 <=> array_2) << '\n'; // *NOPAD*
    std::cout << object_1 << " <=> " << object_2 << " := " << to_string(object_1 <=> object_2) << '\n'; // *NOPAD*
    std::cout << string << " <=> " << number << " := " << to_string(string <=> number) << '\n'; // *NOPAD*
    std::cout << string << " <=> " << discarded << " := " << to_string(string <=> discarded) << '\n'; // *NOPAD*
}

Output:

[1,2,3] <=> [1,2,4] := less
{"A":"a","B":"b"} <=> {"A":"a","B":"b"} := equivalent
"foo" <=> 17 := greater
"foo" <=> <discarded> := unordered
Example: (2) comparing JSON values and scalars

The example demonstrates comparing several JSON values and scalars.

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

using json = nlohmann::json;

const char* to_string(const std::partial_ordering& po)
{
    if (std::is_lt(po))
    {
        return "less";
    }
    else if (std::is_gt(po))
    {
        return "greater";
    }
    else if (std::is_eq(po))
    {
        return "equivalent";
    }
    return "unordered";
}

int main()
{
    using float_limits = std::numeric_limits<json::number_float_t>;
    constexpr auto nan = float_limits::quiet_NaN();

    // create several JSON values
    json boolean = false;
    json number = 17;
    json string = "17";


    // output values and comparisons
    std::cout << std::boolalpha << std::fixed;
    std::cout << boolean << " <=> " << true << " := " << to_string(boolean <=> true) << '\n'; // *NOPAD*
    std::cout << number << " <=> " << 17.0 << " := " << to_string(number <=> 17.0) << '\n'; // *NOPAD*
    std::cout << number << " <=> " << nan << " := " << to_string(number <=> nan) << '\n'; // *NOPAD*
    std::cout << string << " <=> " << 17 << " := " << to_string(string <=> 17) << '\n'; // *NOPAD*
}

Output:

false <=> true := less
17 <=> 17.000000 := equivalent
17 <=> nan := unordered
"17" <=> 17 := greater

See also

Version history

  1. Added in version 3.11.0.
  2. Added in version 3.11.0.

Last update: October 2, 2022