json/docs/mkdocs/docs/api/basic_json/operator_lt.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 whether one JSON value lhs is less than another JSON value rhs according to the following rules:

   • If either operand is discarded, the comparison yields #!cpp false.
   • If both operands have the same type, the values are compared using their respective operator<.
   • Integer and floating-point numbers are compared by their exact values, without converting the integer to floating-point. For instance, 9007199254740992.0 is less than 9007199254740993.
   • In case lhs and rhs have different types, the values are ignored and the order of the types is considered, which is:
     1. null
     2. boolean
     3. number (all types)
     4. object
     5. array
     6. string
     7. binary For instance, any boolean value is considered less than any string.

2. Compares whether a JSON value is less than a scalar or a scalar is less than a JSON value 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 lhs is less than rhs

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 NaN"

`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.

For arrays and objects, the elements are compared in order. If the comparison reaches a `NaN` and a number (or
another `NaN`) before any other difference, the result depends on the language standard. Before C++20, the pair
counts as equivalent and the following elements (or the sizes) decide: for instance, `[NaN] < [1, NaN]` and
`[1] <= [NaN]` are `#!cpp true`. Since C++20, the whole comparison is unordered, and `<`, `<=`, `>`, and `>=` all
yield `#!cpp false`. Otherwise (for example `[1, NaN] < [2, NaN]`, or objects whose keys differ), the result is the
same for all language standards. The same program can therefore order such values differently when it is compiled
with a different language standard.

!!! note "Operator overload resolution"

Since C++20 overload resolution will consider the _rewritten candidate_ generated from
[`operator<=>`](operator_spaceship.md).

Examples

??? example

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

Output:

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

See also

• operator<=> - comparison: 3-way

Version history

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