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nlohmann
106 lines
4.1 KiB
Markdown
106 lines
4.1 KiB
Markdown
# nlohmann::basic_json::value_t
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```
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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 functions [`is_null`](https://json.nlohmann.me/api/basic_json/is_null/index.md), [`is_object`](https://json.nlohmann.me/api/basic_json/is_object/index.md), [`is_array`](https://json.nlohmann.me/api/basic_json/is_array/index.md), [`is_string`](https://json.nlohmann.me/api/basic_json/is_string/index.md), [`is_boolean`](https://json.nlohmann.me/api/basic_json/is_boolean/index.md), [`is_number`](https://json.nlohmann.me/api/basic_json/is_number/index.md) (with [`is_number_integer`](https://json.nlohmann.me/api/basic_json/is_number_integer/index.md), [`is_number_unsigned`](https://json.nlohmann.me/api/basic_json/is_number_unsigned/index.md), and [`is_number_float`](https://json.nlohmann.me/api/basic_json/is_number_float/index.md)), [`is_discarded`](https://json.nlohmann.me/api/basic_json/is_discarded/index.md), [`is_binary`](https://json.nlohmann.me/api/basic_json/is_binary/index.md), [`is_primitive`](https://json.nlohmann.me/api/basic_json/is_primitive/index.md), and [`is_structured`](https://json.nlohmann.me/api/basic_json/is_structured/index.md) rely on it.
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## Notes
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Ordering
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The order of types is as follows:
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1. `null`
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1. `boolean`
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1. `number_integer`, `number_unsigned`, `number_float`
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1. `object`
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1. `array`
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1. `string`
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1. `binary`
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`discarded` is unordered.
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Types of numbers
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There are three enumerators for numbers (`number_integer`, `number_unsigned`, and `number_float`) to distinguish between different types of numbers:
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- [`number_unsigned_t`](https://json.nlohmann.me/api/basic_json/number_unsigned_t/index.md) for unsigned integers
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- [`number_integer_t`](https://json.nlohmann.me/api/basic_json/number_integer_t/index.md) for signed integers
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- [`number_float_t`](https://json.nlohmann.me/api/basic_json/number_float_t/index.md) for floating-point numbers or to approximate integers which do not fit into the limits of their respective type
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Comparison operators
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`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:
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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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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create JSON values
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json j_null;
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json j_boolean = true;
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json j_number_integer = -17;
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json j_number_unsigned = 42u;
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json j_number_float = 23.42;
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json j_object = {{"one", 1}, {"two", 2}};
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json j_array = {1, 2, 4, 8, 16};
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json j_string = "Hello, world";
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// call type()
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std::cout << std::boolalpha;
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std::cout << (j_null.type() == json::value_t::null) << '\n';
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std::cout << (j_boolean.type() == json::value_t::boolean) << '\n';
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std::cout << (j_number_integer.type() == json::value_t::number_integer) << '\n';
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std::cout << (j_number_unsigned.type() == json::value_t::number_unsigned) << '\n';
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std::cout << (j_number_float.type() == json::value_t::number_float) << '\n';
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std::cout << (j_object.type() == json::value_t::object) << '\n';
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std::cout << (j_array.type() == json::value_t::array) << '\n';
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std::cout << (j_string.type() == json::value_t::string) << '\n';
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}
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```
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Output:
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```
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true
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true
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true
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true
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true
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true
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true
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true
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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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