As for other containers, `begin()` returns an iterator to the first value and `end()` returns an iterator to the value following the last value. The latter iterator is a placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, `begin()` will return `end()`.
When iterating over objects, values are ordered with respect to the `object_comparator_t` type which defaults to `std::less`. See the [types documentation](types/index.md#key-order) for more information.
The JSON iterators have two member functions, `key()` and `value()` to access the object key and stored value, respectively. When calling `key()` on a non-object iterator, an [invalid_iterator.207](../home/exceptions.md#jsonexceptioninvalid_iterator207) exception is thrown.
For this reason, the `items()` function allows accessing `iterator::key()` and `iterator::value()` during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.
When iterating over an array, `key()` will return the index of the element as string. For primitive types (e.g., numbers), `key()` returns an empty string.
Using `items()` on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See <https://github.com/nlohmann/json/issues/2040> for more information.
Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe the underlying string or binary array is empty.