89 lines
2.5 KiB
Markdown
89 lines
2.5 KiB
Markdown
# <small>nlohmann::basic_json_document::</small>root
|
|
|
|
```cpp
|
|
// (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`](index.md#member-types) (i.e. `#!cpp basic_json_view<BasicJsonType>`) for the root value, or a
|
|
discarded view if the document is [discarded](is_discarded.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:
|
|
|
|
```cpp
|
|
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](../basic_json_view/index.md) -- in particular, it is invalidated by the next
|
|
[`read()`](read.md) or [`shrink_to_fit()`](shrink_to_fit.md) on this document. A view can dangle, like a
|
|
`#!cpp std::string_view`; see [Ownership and lifetime](../../features/json_view.md#ownership-and-lifetime).
|
|
|
|
## Examples
|
|
|
|
??? example
|
|
|
|
```cpp
|
|
#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:
|
|
|
|
```json
|
|
true
|
|
true
|
|
true
|
|
|
|
```
|
|
|
|
## See also
|
|
|
|
- [is_discarded](is_discarded.md) - return whether the last parse failed
|
|
- [materialize](../basic_json_view/materialize.md) - build the `BasicJsonType` value of a subtree
|
|
|
|
## Version history
|
|
|
|
- Added in version 3.13.0.
|