# nlohmann::basic_json_view::back

```
basic_json_view back() const;
```

Returns the last element of an array, the last member value of an object, or the value itself if it is primitive (as for [`BasicJsonType::back()`](https://json.nlohmann.me/api/basic_json/back/index.md), a primitive value is a range of one element).

## Return value

The last 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`](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.

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.

## Complexity

Linear in the size of the array or object: unlike [`front()`](https://json.nlohmann.me/api/basic_json_view/front/index.md), which only ever looks at the first element, `back()` has to walk every element to find where they end, since elements are not a fixed size in the index.

## Notes

Unlike [`BasicJsonType::back()`](https://json.nlohmann.me/api/basic_json/back/index.md), which has undefined behavior for an empty array or object, `back()` 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 final status of a build log with `back()`. Even though `back()` is linear in the number of events (unlike [`front()`](https://json.nlohmann.me/api/basic_json_view/front/index.md), which is constant), it still avoids building a `BasicJsonType` value for the events that are not needed.

```
#include <iostream>
#include <nlohmann/json_view.hpp>

using json_document = nlohmann::json_document;

int main()
{
    // the same build log; back() reads only the final status. It is linear
    // in the number of events (unlike front(), which is constant), but
    // still far less work than materializing the whole array
    json_document log = json_document::parse(R"(["queued", "started", "compiling", "linking", "done"])");
    std::cout << log.root().back().materialize().dump() << '\n';

    // an empty log -- back() throws instead of the undefined behavior
    // basic_json::back() has for an empty array
    json_document empty_log = json_document::parse("[]");
    try
    {
        static_cast<void>(empty_log.root().back());
    }
    catch (const nlohmann::json::invalid_iterator& e)
    {
        std::cout << e.what() << '\n';
    }
}
```

Output:

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

## See also

- [front](https://json.nlohmann.me/api/basic_json_view/front/index.md) - access the first element
- [`BasicJsonType::back`](https://json.nlohmann.me/api/basic_json/back/index.md) - the corresponding function of `basic_json`

## Version history

- Added in version 3.13.0 unreleased.