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)

Compares two JSON values for inequality. Returns !(lhs == rhs).
- This means the comparison is simply the logical negation of operator==, including for special values like NaN and discarded.
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.
No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is 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¶

C++20

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

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

Examples¶

Example: (1) compare JSON values

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] true
{"A":"a","B":"b"} != {"A":"a","B":"b"} false
17 != 17.0 false
"foo" != "bar" true

Example: (2) compare JSON values with nullptr

The example demonstrates comparing several JSON types against the null pointer (JSON null).

#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';
}