basic_json::operator==¶

bool operator==(const_reference lhs, const_reference rhs) noexcept;

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

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

Compares two JSON values for equality according to the following rules:

• Two JSON values are equal if (1) they are from the same type and (2) their stored values are the same according to their respective operator==.
• Integer and floating-point numbers are automatically converted before comparison. Note that two NaN values are always treated as unequal.
• Two JSON null values are equal.

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 the values lhs and rhs are equal

Exception safety¶

No-throw guarantee: this function never throws exceptions.

Complexity¶

Linear.

Notes¶

• Floating-point inside JSON values numbers are compared with json::number_float_t::operator== which is double::operator== by default. To compare floating-point while respecting an epsilon, an alternative comparison function could be used, for instance

  template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>
  inline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept
  {
      return std::abs(a - b) <= epsilon;
  }
  

  Or you can self-defined operator equal function like this:

  bool my_equal(const_reference lhs, const_reference rhs)
  {
      const auto lhs_type lhs.type();
      const auto rhs_type rhs.type();
      if (lhs_type == rhs_type)
      {
          switch(lhs_type)
              // self_defined case
              case value_t::number_float:
                  return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();
              // other cases remain the same with the original
              ...
      }
  ...
  }
  

• NaN values never compare equal to themselves or to other NaN values.

Example¶

#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 = 17.000000000000001L;
    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';
}

[1,2,3] == [1,2,4] false
{"A":"a","B":"b"} == {"A":"a","B":"b"} true
17 == 17.0 true
"foo" == "bar" false

Version history¶

• Added in version 1.0.0.