Skip to content

# 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

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 = 17.000000000000001L;
        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';
    }

Output:

    [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.