json/api/basic_json/operator_eq.md

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(/* see below */); // (2)

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

// since C++20
class basic_json {
    bool operator==(const_reference rhs) const noexcept;              // (1)

    template<typename ScalarType>
    bool operator==(ScalarType rhs) const noexcept(/* see below */); // (2)
};

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

   • Two JSON values are equal if (1) neither value is discarded, and (2) they are of the same type and their stored values are the same according to their respective operator==.
   • Integer and floating-point numbers are automatically converted before comparison.

2. Compares a JSON value and a scalar or a scalar and a JSON value for equality 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 the values lhs/*this and rhs are equal

Exception safety

1. No-throw guarantee: this function never throws exceptions.
2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and #!cpp nullptr; the function is #!cpp noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.

Complexity

Linear.

Notes

!!! note "Comparing special values"

- `NaN` values are unordered within the domain of numbers.
  The following comparisons all yield `#!cpp false`:
    1. Comparing a `NaN` with itself.
    2. Comparing a `NaN` with another `NaN`.
    3. Comparing a `NaN` and any other number.
- JSON `#!cpp null` values are all equal.
- Discarded values never compare equal to themselves.

!!! note "Comparing floating-point numbers"

Floating-point numbers 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](https://github.com/mariokonrad/marnav/blob/master/include/marnav/math/floatingpoint.hpp#L34-#L39)
could be used, for instance

```cpp
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 define your own equality function like this:

```cpp
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
            ...
    }
...
}
```

!!! note "Comparing different basic_json specializations"

Comparing different `basic_json` specializations can have surprising effects. For instance, the result of comparing
the JSON objects

```json
{
   "version": 1,
   "type": "integer"
}
```

and

```json
{
   "type": "integer",
   "version": 1
}
```

depends on whether [`nlohmann::json`](../json.md) or [`nlohmann::ordered_json`](../ordered_json.md) is used:

 ```cpp
 --8<-- "examples/operator__equal__specializations.cpp"
 ```
 
 Output:
 
 ```json
 --8<-- "examples/operator__equal__specializations.output"
 ```

Examples

??? example "Example: (1) compare JSON values"

The example demonstrates comparing several JSON types.
    
```cpp
--8<-- "examples/operator__equal.cpp"
```

Output:

```json
--8<-- "examples/operator__equal.output"
```

??? example "Example: (2) compare JSON values with #!cpp nullptr"

The example demonstrates comparing several JSON types against the null pointer (JSON `#!json null`).
    
```cpp
--8<-- "examples/operator__equal__nullptr_t.cpp"
```

Output:

```json
--8<-- "examples/operator__equal__nullptr_t.output"
```

See also

• operator!= compare for inequality
• operator<=> comparison: 3-way (C++20)

Version history

1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Made conditionally #!cpp noexcept in version 3.13.0; before, a throwing conversion called std::terminate.