85 lines
2.6 KiB
Markdown
85 lines
2.6 KiB
Markdown
# <small>nlohmann::basic_json_view::</small>basic_json_view
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```cpp
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// (1)
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basic_json_view() noexcept = default;
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// (2) only for Mode == view_mode::editable
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basic_json_view(const basic_json_view<BasicJsonType, view_mode::read_only>& other) noexcept;
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```
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1. Creates an invalid (discarded) view: [`type()`](type.md) is `#!cpp value_t::discarded` and
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[`is_discarded()`](is_discarded.md) is `#!cpp true`.
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2. Converts a view of a read-only document into a view of an editable document (implicitly), so that a function
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taking a [`json_editable_view`](../json_editable_view.md) accepts both. The result is a view of the same value of
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the same read-only document. See [`view_mode`](../view_mode.md) for why there is no conversion the other way.
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## Parameters
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`other` (in)
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: a view of a read-only document with the same `BasicJsonType`
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Apart from these, a view is obtained from a
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[`basic_json_document`](../basic_json_document/index.md), via [`root()`](../basic_json_document/root.md) or by
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navigating into a container with [`operator[]`](operator[].md), [`at`](at.md), [`front`](front.md), [`back`](back.md),
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[`find`](find.md), or iteration.
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## Exception safety
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No-throw guarantee: these constructors never throw exceptions.
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## Complexity
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Constant.
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## Notes
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`basic_json_view` is trivially copyable (it holds two pointers), so a default-constructed view can be used as a
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placeholder for "no value yet" and later be assigned a real view.
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## Examples
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??? example
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The example below shows the default constructor and that a `basic_json_view` is a small, copyable handle.
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```cpp
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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int main()
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{
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std::cout << std::boolalpha;
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// the default constructor is the only public one: it creates an invalid
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// (discarded) view, useful as a "no value yet" placeholder
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nlohmann::json_view v;
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std::cout << v.is_discarded() << '\n';
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// views are trivially copyable handles (two pointers); the document owns
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// the actual data
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nlohmann::json_view copy = v;
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std::cout << copy.is_discarded() << '\n';
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}
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```
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Output:
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```json
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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`](is_discarded.md) - return whether the view is invalid
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- [`root`](../basic_json_document/root.md) - the view of a document's root value
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- [`view_mode`](../view_mode.md) - whether a document and its views can be edited
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- [Zero-copy JSON views](../../features/json_view.md#ownership-and-lifetime) - how long a view stays valid
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## Version history
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- Added in version 3.13.0.
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