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(), 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 if the view is null or discarded, or if it is an empty array or object.

This exception does not carry a JSON_DIAGNOSTICS 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(), 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(), 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()`](front.md), which is constant), it still avoids building a
`BasicJsonType` value for the events that are not needed.

```cpp
#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:

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

```

Version history

Added in version 3.13.0.

3.0 KiB Raw Blame History