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6b97599a27
* Add C++20 3-way comparison operator and fix broken comparisons Fixes #3207. Fixes #3409. * Fix iterators to meet (more) std::ranges requirements Fixes #3130. Related discussion: #3408 * Add note about CMake standard version selection to unit tests Document how CMake chooses which C++ standard version to use when building tests. * Update documentation * CI: add legacy discarded value comparison * Fix internal linkage errors when building a module
2.6 KiB
2.6 KiB
nlohmann::basic_json::value_t
enum class value_t : std::uint8_t {
null,
object,
array,
string,
boolean,
number_integer,
number_unsigned,
number_float,
binary,
discarded
};
This enumeration collects the different JSON types. It is internally used to distinguish the stored values, and the
functions is_null, is_object, is_array, is_string,
is_boolean, is_number (with is_number_integer,
is_number_unsigned, and is_number_float),
is_discarded, is_binary, is_primitive, and
is_structured rely on it.
Notes
!!! note "Ordering"
The order of types is as follows:
1. `null`
2. `boolean`
3. `number_integer`, `number_unsigned`, `number_float`
4. `object`
5. `array`
6. `string`
7. `binary`
`discarded` is unordered.
!!! note "Types of numbers"
There are three enumerators for numbers (`number_integer`, `number_unsigned`, and `number_float`) to distinguish
between different types of numbers:
- [`number_unsigned_t`](number_unsigned_t.md) for unsigned integers
- [`number_integer_t`](number_integer_t.md) for signed integers
- [`number_float_t`](number_float_t.md) for floating-point numbers or to approximate integers which do not fit
into the limits of their respective type
!!! warning "Comparison operators"
`operator<` and `operator<=>` (since C++20) are overloaded and compare according to the ordering described above.
Until C++20 all other relational and equality operators yield results according to the integer value of each
enumerator.
Since C++20 some compilers consider the _rewritten candidates_ generated from `operator<=>` during overload
resolution, while others do not.
For predictable and portable behavior use:
- `operator<` or `operator<=>` when wanting to compare according to the order described above
- `operator==` or `operator!=` when wanting to compare according to each enumerators integer value
Examples
??? example
The following code how `type()` queries the `value_t` for all JSON types.
```cpp
--8<-- "examples/type.cpp"
```
Output:
```json
--8<-- "examples/type.output"
```
Version history
- Added in version 1.0.0.
- Added unsigned integer type in version 2.0.0.
- Added binary type in version 3.8.0.