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nlohmann::basic_json::operator<

// until C++20
bool operator<(const_reference lhs, const_reference rhs) noexcept;   // (1)

template<typename ScalarType>
bool operator<(const_reference lhs, const ScalarType rhs) noexcept;  // (2)

template<typename ScalarType>
bool operator<(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
  1. Compares whether one JSON value lhs is less than another JSON value rhs according to the following rules:

    • If either operand is discarded, the comparison yields false.
    • If both operands have the same type, the values are compared using their respective operator<.
    • Integer and floating-point numbers are automatically converted before comparison.
    • In case lhs and rhs have different types, the values are ignored and the order of the types is considered, which is:
      1. null
      2. boolean
      3. number (all types)
      4. object
      5. array
      6. string
      7. binary For instance, any boolean value is considered less than any string.
  2. Compares wether a JSON value is less than a scalar or a scalar is less than a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

Template parameters

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

Parameters

lhs (in)
first value to consider
rhs (in)
second value to consider

Return value

whether lhs is less than 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 false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

Operator overload resolution

Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

Examples

Example

The example demonstrates comparing several JSON types.

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

using json = nlohmann::json;

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_1 = 17;
    json number_2 = 17.0000000000001L;
    json string_1 = "foo";
    json string_2 = "bar";

    // output values and comparisons
    std::cout << std::boolalpha;
    std::cout << array_1 << " == " << array_2 << " " << (array_1 < array_2) << '\n';
    std::cout << object_1 << " == " << object_2 << " " << (object_1 < object_2) << '\n';
    std::cout << number_1 << " == " << number_2 << " " << (number_1 < number_2) << '\n';
    std::cout << string_1 << " == " << string_2 << " " << (string_1 < string_2) << '\n';
}

Output:

[1,2,3] == [1,2,4] true
{"A":"a","B":"b"} == {"A":"a","B":"b"} false
17 == 17.0000000000001 true
"foo" == "bar" false

See also

Version history

  1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
  2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.

Last update: September 18, 2022