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

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

1. Compares two JSON values for inequality. Returns #!cpp !(lhs == rhs).

   • 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

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 "C++20"

Since C++20, `basic_json` declares no `operator!=`. The compiler rewrites `#!cpp a != b` as `#!cpp !(a == b)`
using [`operator==`](operator_eq.md), so the result is the same as described above.

!!! 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 "Example: (1) compare JSON values"

The example demonstrates comparing several JSON types.
    
```cpp
#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:

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

```

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

The example demonstrates comparing several JSON types against the null pointer (JSON `#!json null`).
    
```cpp
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create several JSON values
    json array = {1, 2, 3};
    json object = {{"A", "a"}, {"B", "b"}};
    json number = 17;
    json string = "foo";
    json null;

    // output values and comparisons
    std::cout << std::boolalpha;
    std::cout << array << " != nullptr " << (array != nullptr) << '\n';
    std::cout << object << " != nullptr " << (object != nullptr) << '\n';
    std::cout << number << " != nullptr " << (number != nullptr) << '\n';
    std::cout << string << " != nullptr " << (string != nullptr) << '\n';
    std::cout << null << " != nullptr " << (null != nullptr) << '\n';
}

```

Output:

```json
[1,2,3] != nullptr true
{"A":"a","B":"b"} != nullptr true
17 != nullptr true
"foo" != nullptr true
null != nullptr false

```

See also

• operator== comparison: equal
• operator<=> comparison: 3-way (C++20)

Version history

1. Added in version 1.0.0. Added a C++20 member function 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). Removed the C++20 member function in version 3.13.0; since C++20, the compiler rewrites a != b using operator==.
2. Added in version 1.0.0. Changed in version 3.13.0 to remove special-casing for NaN and discarded values; operator!= now consistently means !(a == b). Since C++20, the compiler rewrites a != b using operator==. Made conditionally #!cpp noexcept in version 3.13.0; before, a throwing conversion called std::terminate.