api/basic_json_document/root/index.md

# 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 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.