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api/is_json_view/index.md

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+# nlohmann::is_json_view
+
+Defined in header `<nlohmann/json_view.hpp>`
+
+```
+template<typename T>
+struct is_json_view;
+```
+
+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.
+
+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.
+
+## Template parameters
+
+`T` : the type to check (without cv-qualifiers or references)
+
+## Notes
+
+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.
+
+## Examples
+
+Example
+
+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.
+
+```
+#include <iostream>
+#include <type_traits>
+#include <nlohmann/json_view.hpp>
+
+using nlohmann::json;
+
+namespace ns
+{
+struct person
+{
+    std::string name;
+    int age = 0;
+};
+
+// reads json values and views (of any document type), but nothing else
+template < typename J, typename std::enable_if < nlohmann::is_json_view<J>::value || std::is_same<J, json>::value, int >::type = 0 >
+void from_json(const J& j, person& p)
+{
+    p.name = j.at("name").template get<std::string>();
+    p.age = j.at("age").template get<int>();
+}
+} // namespace ns
+
+int main()
+{
+    std::cout << std::boolalpha
+              << nlohmann::is_json_view<nlohmann::json_view>::value << ' '
+              << nlohmann::is_json_view<nlohmann::ordered_json_editable_view>::value << ' '
+              << nlohmann::is_json_view<json>::value << '\n';
+
+    const nlohmann::json_document doc = nlohmann::json_document::parse(R"({"name": "Ada", "age": 36})");
+    const auto p = doc.root().get<ns::person>(); // reads the view, no json value is built
+    std::cout << p.name << ' ' << p.age << '\n';
+}
+```
+
+Output:
+
+```
+true true false
+Ada 36
+```
+
+## See also
+
+- [basic_json_view](https://json.nlohmann.me/api/basic_json_view/index.md) - a handle to one value of a document
+- [get](https://json.nlohmann.me/api/basic_json_view/get/index.md) - which conversions read the view directly
+- [view_mode](https://json.nlohmann.me/api/view_mode/index.md) - read-only or editable
+- [Writing generic code](https://json.nlohmann.me/features/json_view/#writing-generic-code) - functions and `from_json()` for views
+
+## Version history
+
+Added in version 3.13.0 unreleased.