Niels Lohmann
8a0679558f
- Copying is disabled because of its cost, not because of two documents borrowing the same buffer; the unique_ptr follows from move-only. - Inputs: streams, FILE*, and wide characters (strings, arrays, pointers) are all supported and owned; point to parse_copy() for owned copies. - The error replay only runs for failing input, builds no values, and is skipped with allow_exceptions = false. - read() writes copied inputs into the previous owned copy. - Views can dangle like std::string_view, and why there is no shared/weak pointer design. - The 4 GiB limit (16-byte nodes, 16-byte scanner steps), streams being read to the end first, which macros apply, why materialize() has no source positions, and wording nits (accept() vs basic_json::accept(), is_binary, is_discarded, see-also lists, size table, "back into"). Signed-off-by: Niels Lohmann <mail@nlohmann.me>
1.9 KiB
nlohmann::basic_json_document::root
// (1)
view_type root() const& noexcept;
// (2)
view_type root() const&& = delete;
- Returns a view of the root value of the document.
- 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
BasicJsonTypevalue of a subtree
Version history
- Added in version 3.13.0.