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74 lines
3.4 KiB
Markdown
74 lines
3.4 KiB
Markdown
# nlohmann::basic_json_view::begin
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```
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iterator begin() const noexcept;
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```
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Returns an iterator to the first element of an array, or the first member value of an object, in **document order** -- the order the values appear in the source text, not sorted by key. A primitive value iterates as a range of one element (itself), and a `null` value or a [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md) view iterates as an empty range.
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## Return value
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Iterator to the first element.
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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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For an object, iteration visits **every** member, including all occurrences of a duplicate key -- unlike [`operator[]`](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/index.md), [`at`](https://json.nlohmann.me/api/basic_json_view/at/index.md), [`find`](https://json.nlohmann.me/api/basic_json_view/find/index.md), and [`contains`](https://json.nlohmann.me/api/basic_json_view/contains/index.md), which all resolve to the *first* member with a given key. See the [Notes on duplicate keys](https://json.nlohmann.me/api/basic_json_view/operator%5B%5D/#notes) of `operator[]`.
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Because objects are iterated in document order rather than sorted by key, the order seen here can differ from what iterating the [`materialize()`](https://json.nlohmann.me/api/basic_json_view/materialize/index.md)d `BasicJsonType` value would produce: a `basic_json` object (`std::map`-backed by default) sorts its keys, while a view does not.
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An `ordered_json` value also keeps document order, but it keeps only one entry per key: as `parse()` does, it keeps the *last* value, at the position of the key's *first* occurrence. The view, in contrast, visits every member, including the duplicates.
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## Examples
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Example
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The example below iterates a log record's members with `begin()`/[`end()`](https://json.nlohmann.me/api/basic_json_view/end/index.md) and prints them in the order they were written. Materializing the record into a `BasicJsonType` object and iterating that would instead print the members sorted by key.
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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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// a log record: the fields matter in the order they were written, e.g.
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// to reproduce the record as it was logged. A nlohmann::json object
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// sorts its keys, so materializing and iterating it would instead print
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// them alphabetically ("level", "message", "time")
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json_document record = json_document::parse(R"({"time": "10:00:01", "level": "info", "message": "started"})");
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for (auto it = record.root().begin(); it != record.root().end(); ++it)
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{
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std::cout << it.key() << '=' << it->materialize().dump() << '\n';
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}
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}
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```
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Output:
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```
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time="10:00:01"
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level="info"
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message="started"
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```
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## See also
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- [end](https://json.nlohmann.me/api/basic_json_view/end/index.md) - returns an iterator to one past the last element
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- [cbegin](https://json.nlohmann.me/api/basic_json_view/cbegin/index.md) - returns a const iterator to the first element
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- [items](https://json.nlohmann.me/api/basic_json_view/items/index.md) - access iterator member functions in range-based for
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- [`BasicJsonType::begin`](https://json.nlohmann.me/api/basic_json/begin/index.md) - the corresponding function of `basic_json`
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## Version history
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- Added in version 3.13.0 unreleased.
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