# Iterators

## Overview

A `basic_json` value is a container and allows access via iterators. Depending on the value type, `basic_json` stores zero or more values.

As for other containers, `begin()` returns an iterator to the first value and `end()` returns an iterator to the value following the last value. The latter iterator is a placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, `begin()` will return `end()`.

### Iteration order for objects

When iterating over objects, values are ordered with respect to the `object_comparator_t` type which defaults to `std::less`. See the [types documentation](https://json.nlohmann.me/features/types/#key-order) for more information.

Example

```
// create JSON object {"one": 1, "two": 2, "three": 3}
json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;

for (auto it = j.begin(); it != j.end(); ++it)
{
    std::cout << *it << std::endl;
}
```

Output:

```
1
3
2
```

The reason for the order is the lexicographic ordering of the object keys "one", "three", "two".

### Access object keys during iteration

The JSON iterators have two member functions, `key()` and `value()` to access the object key and stored value, respectively. When calling `key()` on a non-object iterator, an [invalid_iterator.207](https://json.nlohmann.me/home/exceptions/#jsonexceptioninvalid_iterator207) exception is thrown.

Example

```
// create JSON object {"one": 1, "two": 2, "three": 3}
json j;
j["one"] = 1;
j["two"] = 2;
j["three"] = 3;

for (auto it = j.begin(); it != j.end(); ++it)
{
    std::cout << it.key() << " : " << it.value() << std::endl;
}
```

Output:

```
one : 1
three : 3
two : 2
```

### Range-based for loops

C++11 allows using range-based for loops to iterate over a container.

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

For this reason, the `items()` 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 (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';
}
```

Note

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

Warning

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.

### Reverse iteration order

`rbegin()` and `rend()` return iterators in the reverse sequence.

Example

```
json j = {1, 2, 3, 4};

for (auto it = j.rbegin(); it != j.rend(); ++it)
{
    std::cout << *it << std::endl;
}
```

Output:

```
4
3
2
1
```

### Iterating strings and binary values

Note that "value" means a JSON value in this setting, not values stored in the underlying containers. That is, `*begin()` returns the complete string or binary array and is also safe if the underlying string or binary array is empty.

Example

```
json j = "Hello, world";
for (auto it = j.begin(); it != j.end(); ++it)
{
    std::cout << *it << std::endl;
}
```

Output:

```
"Hello, world"
```

## Iterator invalidation

| Operations | invalidated iterators |
| ---------- | --------------------- |
| `clear`    | all                   |