json/api/basic_json/operator_ne.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;  // (2)

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

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

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

1. Compares two JSON values for inequality. Returns #!cpp !(lhs == rhs) (until C++20) or #!cpp !(*this == rhs) (since C++20).

   • This means the comparison is simply the logical negation of operator==, including for special values like NaN and discarded.

2. Compares a JSON value and a scalar or a scalar and a JSON value for inequality 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 not equal

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Linear.

Notes

!!! note "Comparing NaN and discarded"

Since `operator!=` is defined as `!(a == b)`, the behavior for special values follows that of `operator==`:

- For `NaN` values: `NaN == NaN` yields `#!cpp false`, so `NaN != NaN` yields `#!cpp true`.
- For `discarded` values: `discarded == x` yields `#!cpp false` for any `x`, so `discarded != x` yields `#!cpp true`.

Examples

??? example

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

Output:

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

??? example

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

Output:

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

Version history

1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Changed in version 3.13.0 to remove special-casing for NaN and discarded values; operator!= now consistently means !(a == b).
2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Changed in version 3.13.0 to remove special-casing for NaN and discarded values; operator!= now consistently means !(a == b).