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59 lines
1.3 KiB
Markdown
59 lines
1.3 KiB
Markdown
# <small>nlohmann::basic_json::</small>front
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```cpp
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reference front();
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const_reference front() const;
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```
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Returns a reference to the first element in the container. For a JSON container `#!cpp c`, the expression
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`#!cpp c.front()` is equivalent to `#!cpp *c.begin()`.
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## Return value
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In case of a structured type (array or object), a reference to the first element is returned. In case of number, string,
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boolean, or binary values, a reference to the value is returned.
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## Exception safety
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Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
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## Exceptions
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If the JSON value is `#!json null`, exception
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[`invalid_iterator.214`](../../home/exceptions.md#jsonexceptioninvalid_iterator214) is thrown.
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## Complexity
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Constant.
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## Notes
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!!! danger
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Calling `front` on an empty array or object is undefined behavior and is **guarded by an assertion**!
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## Examples
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??? example
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The following code shows an example for `front()`.
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```cpp
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--8<-- "examples/front.cpp"
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```
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Output:
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```json
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--8<-- "examples/front.output"
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```
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## See also
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- [back](back.md) to access the last element
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## Version history
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- Added in version 1.0.0.
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- Adjusted code to return reference to binary values in version 3.8.0.
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