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