82 lines
3.1 KiB
Markdown
82 lines
3.1 KiB
Markdown
# nlohmann::is_json_view
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Defined in header `<nlohmann/json_view.hpp>`
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```
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template<typename T>
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struct is_json_view;
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```
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A type trait: `is_json_view<T>::value` is `true` if `T` is a [`basic_json_view`](https://json.nlohmann.me/api/basic_json_view/index.md) of any `BasicJsonType` and any [`view_mode`](https://json.nlohmann.me/api/view_mode/index.md) -- [`json_view`](https://json.nlohmann.me/api/json_view/index.md), [`json_editable_view`](https://json.nlohmann.me/api/json_editable_view/index.md), [`ordered_json_view`](https://json.nlohmann.me/api/ordered_json_view/index.md), ... -- and `false` otherwise, for [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) as well.
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It is meant for constraining templates that accept views, in particular a `from_json()` that should read views directly: [`get<T>()`](https://json.nlohmann.me/api/basic_json_view/get/index.md) on a view calls a `from_json()` with the view only if that `from_json()` accepts views but not arbitrary types.
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## Template parameters
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`T` : the type to check (without cv-qualifiers or references)
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## Notes
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A view is recognized by its member type `basic_json_type` (a [`basic_json`](https://json.nlohmann.me/api/basic_json/index.md) specialization) and its [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) returning that type, so the trait needs no declaration of the view class.
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## Examples
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Example
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The example below shows the trait, and a `from_json()` constrained to `json` values and views: `get<T>()` on a view reads the view directly.
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```
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#include <iostream>
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#include <type_traits>
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#include <nlohmann/json_view.hpp>
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using nlohmann::json;
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namespace ns
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{
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struct person
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{
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std::string name;
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int age = 0;
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};
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// reads json values and views (of any document type), but nothing else
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template < typename J, typename std::enable_if < nlohmann::is_json_view<J>::value || std::is_same<J, json>::value, int >::type = 0 >
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void from_json(const J& j, person& p)
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{
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p.name = j.at("name").template get<std::string>();
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p.age = j.at("age").template get<int>();
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}
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} // namespace ns
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int main()
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{
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std::cout << std::boolalpha
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<< nlohmann::is_json_view<nlohmann::json_view>::value << ' '
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<< nlohmann::is_json_view<nlohmann::ordered_json_editable_view>::value << ' '
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<< nlohmann::is_json_view<json>::value << '\n';
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const nlohmann::json_document doc = nlohmann::json_document::parse(R"({"name": "Ada", "age": 36})");
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const auto p = doc.root().get<ns::person>(); // reads the view, no json value is built
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std::cout << p.name << ' ' << p.age << '\n';
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}
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```
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Output:
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```
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true true false
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Ada 36
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```
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## See also
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- [basic_json_view](https://json.nlohmann.me/api/basic_json_view/index.md) - a handle to one value of a document
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- [get](https://json.nlohmann.me/api/basic_json_view/get/index.md) - which conversions read the view directly
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- [view_mode](https://json.nlohmann.me/api/view_mode/index.md) - read-only or editable
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- [Writing generic code](https://json.nlohmann.me/features/json_view/#writing-generic-code) - functions and `from_json()` for views
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## Version history
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Added in version 3.13.0 unreleased.
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