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Niels Lohmann 6d0867a85f Make comparisons with scalars noexcept only when the conversion is (#5751 )

The comparison operators taking a scalar (==, !=, <, <=, >, >=, and
C++20's <=>) convert the scalar to a basic_json and compare, but were
unconditionally noexcept. When that conversion throws, the program
called std::terminate instead of propagating the exception, e.g. when
comparing a json with a string literal under memory pressure
(std::bad_alloc) or with an enum value not mapped by
NLOHMANN_JSON_SERIALIZE_ENUM_STRICT (out_of_range.410). clang-tidy
22.1 reports the latter as bugprone-exception-escape.

Declare the 16 scalar overloads
noexcept(std::is_nothrow_constructible<basic_json, ScalarType>::value):
they stay noexcept for numbers, Booleans, nullptr, and plain enums,
and are noexcept(false) for strings and enums whose to_json may throw.
The comparisons of two basic_json values are unchanged.

Restore the strict-enum comparisons removed from unit-conversions.cpp
in the previous PR, check that comparing an unmapped strict enum now
throws, and pin the new exception specifications in unit-noexcept.cpp.
Document the exception safety of overload (2) on all seven operator
pages. Ran make amalgamate.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-02 17:25:17 +02:00

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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 #!cpp !(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 #!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
--8<-- "examples/operator__notequal.cpp"
```

Output:

```json
--8<-- "examples/operator__notequal.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__notequal__nullptr_t.cpp"
```

Output:

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

Version history

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==.
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.

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