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api/basic_json_document/root/index.md

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+# nlohmann::basic_json_document::root
+
+```
+// (1)
+view_type root() const& noexcept;
+
+// (2)
+view_type root() const&& = delete;
+```
+
+1. Returns a view of the root value of the document.
+1. Deleted: the view of a temporary document would dangle.
+
+## Return value
+
+A [`view_type`](https://json.nlohmann.me/api/basic_json_document/#member-types) (i.e. `basic_json_view<BasicJsonType>`) for the root value, or a discarded view if the document is [discarded](https://json.nlohmann.me/api/basic_json_document/is_discarded/index.md).
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+**Lifetime.** A view refers into the document, so the document must outlive it. `root()` can therefore only be called on a document that has a name (an lvalue); calling it on a temporary does not compile:
+
+```
+auto v = json_document::parse(text).root();  // error: the document is destroyed at the end of the statement
+
+auto doc = json_document::parse(text);       // OK: keep the document alive
+auto v = doc.root();
+```
+
+`root()` is a cheap handle into the document's index, not a copy of anything; call it as often as needed. The returned view is valid under the same conditions as any other view of the document -- see [Object inspection](https://json.nlohmann.me/api/basic_json_view/index.md) -- in particular, it is invalidated by the next [`read()`](https://json.nlohmann.me/api/basic_json_document/read/index.md) or [`shrink_to_fit()`](https://json.nlohmann.me/api/basic_json_document/shrink_to_fit/index.md) on this document. A view can dangle, like a `std::string_view`; see [Ownership and lifetime](https://json.nlohmann.me/features/json_view/#ownership-and-lifetime).
+
+## Examples
+
+Example
+
+```
+#include <iostream>
+#include <nlohmann/json_view.hpp>
+
+using json_document = nlohmann::json_document;
+
+int main()
+{
+    std::cout << std::boolalpha;
+
+    json_document doc = json_document::parse(R"({"greeting": "hi"})");
+    std::cout << doc.root().is_object() << '\n';
+
+    // root() is a cheap handle, not a copy: repeated calls observe the same value
+    std::cout << (doc.root().type() == doc.root().type()) << '\n';
+
+    // the root of a failed parse (allow_exceptions == false) is discarded
+    json_document failed = json_document::parse("{", false);
+    std::cout << failed.root().is_discarded() << '\n';
+}
+```
+
+Output:
+
+```
+true
+true
+true
+```
+
+## See also
+
+- [is_discarded](https://json.nlohmann.me/api/basic_json_document/is_discarded/index.md) - return whether the last parse failed
+- [materialize](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) - build the `BasicJsonType` value of a subtree
+
+## Version history
+
+- Added in version 3.13.0 unreleased.