api/basic_json_view/front.md

# <small>nlohmann::basic_json_view::</small>front

```cpp
basic_json_view front() const;
```

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()`](../basic_json/front.md), a primitive value is a range of one element).

## Return value

The first element or member value. For a primitive value (number, string, boolean), the value itself.

## Exception safety

Strong guarantee: there are no changes to the view or the document it refers to.

## Exceptions

Throws [`invalid_iterator.214`](../../home/exceptions.md#jsonexceptioninvalid_iterator214) if the view is
[null](is_null.md) or [discarded](is_discarded.md), or if it is an empty array or object.

This exception does not carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.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.

## Complexity

Constant.

## Notes

Unlike [`BasicJsonType::front()`](../basic_json/front.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.

## Examples

??? example

    The example below reads only the earliest entry of a build log with `front()`, without materializing the rest
    of the (possibly long) log.

    ```cpp
    #include <iostream>
    #include <nlohmann/json_view.hpp>
    
    using json_document = nlohmann::json_document;
    
    int main()
    {
        // the build log of a running job; front() reads just the earliest event
        // without materializing the (possibly long) rest of the log
        json_document log = json_document::parse(R"(["queued", "started", "compiling", "linking", "done"])");
        std::cout << log.root().front().materialize().dump() << '\n';
    
        // an empty log -- front() throws instead of the undefined behavior
        // basic_json::front() has for an empty array
        json_document empty_log = json_document::parse("[]");
        try
        {
            static_cast<void>(empty_log.root().front());
        }
        catch (const nlohmann::json::invalid_iterator& e)
        {
            std::cout << e.what() << '\n';
        }
    }
    
    ```

    Output:

    ```json
    "queued"
    [json.exception.invalid_iterator.214] cannot get value
    
    ```

## See also

- [back](back.md) - access the last element
- [`BasicJsonType::front`](../basic_json/front.md) - the corresponding function of `basic_json`

## Version history

- Added in version 3.13.0.