81 lines
2.7 KiB
Markdown
81 lines
2.7 KiB
Markdown
# nlohmann::basic_json_document::root
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```
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// (1)
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view_type root() const& noexcept;
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// (2)
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view_type root() const&& = delete;
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```
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1. Returns a view of the root value of the document.
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1. Deleted: the view of a temporary document would dangle.
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## Return value
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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).
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## Exception safety
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No-throw guarantee: this function never throws exceptions.
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## Complexity
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Constant.
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## Notes
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**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:
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```
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auto v = json_document::parse(text).root(); // error: the document is destroyed at the end of the statement
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auto doc = json_document::parse(text); // OK: keep the document alive
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auto v = doc.root();
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```
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`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).
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## Examples
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Example
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```
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json_document = nlohmann::json_document;
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int main()
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{
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std::cout << std::boolalpha;
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json_document doc = json_document::parse(R"({"greeting": "hi"})");
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std::cout << doc.root().is_object() << '\n';
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// root() is a cheap handle, not a copy: repeated calls observe the same value
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std::cout << (doc.root().type() == doc.root().type()) << '\n';
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// the root of a failed parse (allow_exceptions == false) is discarded
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json_document failed = json_document::parse("{", false);
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std::cout << failed.root().is_discarded() << '\n';
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}
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```
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Output:
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```
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true
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true
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true
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```
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## See also
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- [is_discarded](https://json.nlohmann.me/api/basic_json_document/is_discarded/index.md) - return whether the last parse failed
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- [materialize](https://json.nlohmann.me/api/basic_json_view/materialize/index.md) - build the `BasicJsonType` value of a subtree
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## Version history
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- Added in version 3.13.0 unreleased.
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