json/docs/mkdocs/docs/api/basic_json/items.md

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';
}

If you need to name the type of the dereferenced element explicitly (e.g., to write a standalone function that takes it as a parameter, or to use items() with std::for_each), use decltype:

using element_type = decltype(*j_object.items().begin());

The per-element type (iteration_proxy_value) lives in the library's internal detail namespace and is intentionally unspecified as a stable, named type -- decltype is the supported way to obtain it, but its exact name/definition may change between versions.

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.

!!! danger "Lifetime issues"

Using `items()` on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See
[#2040](https://github.com/nlohmann/json/issues/2040) for more information.

Examples

??? example

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

```cpp
--8<-- "examples/items.cpp"
```

Output:

```json
--8<-- "examples/items.output"
```

See also

• begin returns an iterator to the first element
• end returns an iterator to one past the last element

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.

!!! warning "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 `#!cpp iterator_wrapper(j)` with `#!cpp j.items()`.

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