json/docs/mkdocs/docs/api/basic_json/front.md

nlohmann::basic_json::front

reference front();
const_reference front() const;

Returns a reference to the first element in the container. For a JSON container #!cpp c, the expression #!cpp c.front() is equivalent to #!cpp *c.begin().

Return value

In the case of a structured type (array or object), a reference to the first element is returned. In the case of number, string, boolean, or binary values, a reference to the value is returned.

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

If the JSON value is #!json null, exception invalid_iterator.214 is thrown.

Complexity

Constant.

Notes

!!! info "Precondition"

The array or object must not be empty. Calling `front` on an empty array or object yields undefined behavior.

Examples

??? example

The following code shows an example for `front()`.
 
```cpp
--8<-- "examples/front.cpp"
```

Output:

```json
--8<-- "examples/front.output"
```

See also

• back to access the last element

Version history

• Added in version 1.0.0.
• Adjusted code to return reference to binary values in version 3.8.0.