json/docs/mkdocs/docs/api/basic_json_document/root.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.
2. Deleted: the view of a temporary document would dangle.

Return value

A view_type (i.e. #!cpp basic_json_view<BasicJsonType>) for the root value, or a discarded view if the document is discarded.

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 — in particular, it is invalidated by the next read() or shrink_to_fit() on this document. A view can dangle, like a #!cpp std::string_view; see Ownership and lifetime.

Examples

??? example

```cpp
--8<-- "examples/basic_json_document__root.cpp"
```

Output:

```json
--8<-- "examples/basic_json_document__root.output"
```

See also

• is_discarded - return whether the last parse failed
• materialize - build the BasicJsonType value of a subtree

Version history

• Added in version 3.13.0.