Skip to content

# nlohmann::basic_json::items

    iteration_proxy<iterator> items() noexcept;
    iteration_proxy<const_iterator> items() const noexcept;

This 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.

For loop without `items()` function:

    for (auto it = j_object.begin(); it != j_object.end(); ++it)
    {
        std::cout << "key: " << it.key() << ", value:" << it.value() << '\n';
    }

Range-based for loop without `items()` function:

    for (auto it : j_object)
    {
        // "it" is of type json::reference and has no key() member
        std::cout << "value: " << it << '\n';
    }

Range-based for loop with `items()` function:

    for (auto& el : j_object.items())
    {
        std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
    }

The `items()` function also allows using structured bindings (C++17):

    for (auto& [key, val] : j_object.items())
    {
        std::cout << "key: " << key << ", value:" << val << '\n';
    }

## Return value

iteration proxy object wrapping the current value with an interface to use in range-based for loops

## Exception safety

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

## Complexity

Constant.

## Notes

When iterating over an array, `key()` will return the index of the element as string (see example). For primitive types (e.g., numbers), `key()` returns an empty string.

Lifetime issues

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.

## Examples

Example

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

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

    using json = nlohmann::json;

    int main()
    {
        // create JSON values
        json j_object = {{"one", 1}, {"two", 2}};
        json j_array = {1, 2, 4, 8, 16};

        // example for an object
        for (auto& x : j_object.items())
        {
            std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
        }

        // example for an array
        for (auto& x : j_array.items())
        {
            std::cout << "key: " << x.key() << ", value: " << x.value() << '\n';
        }
    }

Output:

    key: one, value: 1
    key: two, value: 2
    key: 0, value: 1
    key: 1, value: 2
    key: 2, value: 4
    key: 3, value: 8
    key: 4, value: 16

## Version history

- Added `iterator_wrapper` in version 3.0.0.
- Added `items` and deprecated `iterator_wrapper` in version 3.1.0.
- Added structured binding support in version 3.5.0.

Deprecation

This function replaces the static function `iterator_wrapper` which was introduced in version 1.0.0, but has been deprecated in version 3.1.0. Function `iterator_wrapper` will be removed in version 4.0.0. Please replace all occurrences of `iterator_wrapper(j)` with `j.items()`.

You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated function.

Last update: May 1, 2022
Back to top