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# nlohmann::basic_json::back

    reference back();

    const_reference back() const;

Returns a reference to the last element in the container. For a JSON container `c`, the expression `c.back()` is equivalent to

    auto tmp = c.end();
    --tmp;
    return *tmp;

## Return value

In case of a structured type (array or object), a reference to the last element is returned. In 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 `null`, exception `invalid_iterator.214` is thrown.

## Complexity

Constant.

## Notes

Precondition

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

## Examples

Example

The following code shows an example for `back()`.

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create JSON values
        json j_boolean = true;
        json j_number_integer = 17;
        json j_number_float = 23.42;
        json j_object = {{"one", 1}, {"two", 2}};
        json j_object_empty(json::value_t::object);
        json j_array = {1, 2, 4, 8, 16};
        json j_array_empty(json::value_t::array);
        json j_string = "Hello, world";

        // call back()
        std::cout << j_boolean.back() << '\n';
        std::cout << j_number_integer.back() << '\n';
        std::cout << j_number_float.back() << '\n';
        std::cout << j_object.back() << '\n';
        //std::cout << j_object_empty.back() << '\n';  // undefined behavior
        std::cout << j_array.back() << '\n';
        //std::cout << j_array_empty.back() << '\n';   // undefined behavior
        std::cout << j_string.back() << '\n';

        // back() called on a null value
        try
        {
            json j_null;
            j_null.back();
        }
        catch (json::invalid_iterator& e)
        {
            std::cout << e.what() << '\n';
        }
    }

Output:

    true
    17
    23.42
    2
    16
    "Hello, world"
    [json.exception.invalid_iterator.214] cannot get value

## See also

- front to access the first element

## Version history

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

Last update: May 1, 2022
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