79 lines
2.7 KiB
Markdown
79 lines
2.7 KiB
Markdown
# nlohmann::basic_json_view::front
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```
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basic_json_view front() const;
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```
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Returns the first element of an array, the first member value of an object, or the value itself if it is primitive (as for [`BasicJsonType::front()`](https://json.nlohmann.me/api/basic_json/front/index.md), a primitive value is a range of one element).
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## Return value
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The first element or member value. For a primitive value (number, string, boolean), the value itself.
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## Exception safety
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Strong guarantee: there are no changes to the view or the document it refers to.
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## Exceptions
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Throws [`invalid_iterator.214`](https://json.nlohmann.me/home/exceptions/#jsonexceptioninvalid_iterator214) if the view is [null](https://json.nlohmann.me/api/basic_json_view/is_null/index.md) or [discarded](https://json.nlohmann.me/api/basic_json_view/is_discarded/index.md), or if it is an empty array or object.
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This exception does not carry a [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md) path: the view has no `BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with `JSON_DIAGNOSTICS` enabled.
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## Complexity
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Constant.
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## Notes
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Unlike [`BasicJsonType::front()`](https://json.nlohmann.me/api/basic_json/front/index.md), which has undefined behavior for an empty array or object, `front()` throws `invalid_iterator.214` for an empty array, an empty object, a null value, or a discarded view.
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## Examples
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Example
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The example below reads only the earliest entry of a build log with `front()`, without materializing the rest of the (possibly long) log.
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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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// the build log of a running job; front() reads just the earliest event
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// without materializing the (possibly long) rest of the log
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json_document log = json_document::parse(R"(["queued", "started", "compiling", "linking", "done"])");
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std::cout << log.root().front().materialize().dump() << '\n';
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// an empty log -- front() throws instead of the undefined behavior
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// basic_json::front() has for an empty array
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json_document empty_log = json_document::parse("[]");
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try
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{
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static_cast<void>(empty_log.root().front());
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}
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catch (const nlohmann::json::invalid_iterator& e)
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{
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std::cout << e.what() << '\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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"queued"
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[json.exception.invalid_iterator.214] cannot get value
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```
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## See also
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- [back](https://json.nlohmann.me/api/basic_json_view/back/index.md) - access the last element
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- [`BasicJsonType::front`](https://json.nlohmann.me/api/basic_json/front/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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