json/features/iterators/index.md

# 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()`](https://json.nlohmann.me/api/basic_json/begin/index.md) returns an iterator to the first value and [`end()`](https://json.nlohmann.me/api/basic_json/end/index.md) 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()`.

The range of `begin()` and `end()`: `begin()` refers to the first element, `end()` to the position after the last element, which must not be dereferenced.

### 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: iteration order of object values

```
// 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: access object keys with `key()` and `value()`

```
// 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()`](https://json.nlohmann.me/api/basic_json/items/index.md) 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()`](https://json.nlohmann.me/api/basic_json/rbegin/index.md) and [`rend()`](https://json.nlohmann.me/api/basic_json/rend/index.md) return iterators in the reverse sequence.

The range of `rbegin()` and `rend()` over the same elements: `rbegin()` refers to the last element and `rend()` to the position before the first one. Each reverse iterator stores an iterator to the element after the one it refers to.

Example: reverse iteration with `rbegin()` and `rend()`

```
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: iterate over a string value

```
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                   |

## See also

- [`begin`](https://json.nlohmann.me/api/basic_json/begin/index.md) - / [`end`](https://json.nlohmann.me/api/basic_json/end/index.md) - iterators to the first and one past the last element
- [`rbegin`](https://json.nlohmann.me/api/basic_json/rbegin/index.md) - / [`rend`](https://json.nlohmann.me/api/basic_json/rend/index.md) - reverse iterators
- [`cbegin`](https://json.nlohmann.me/api/basic_json/cbegin/index.md) - / [`cend`](https://json.nlohmann.me/api/basic_json/cend/index.md) - const iterators
- [`items`](https://json.nlohmann.me/api/basic_json/items/index.md) - iterate over keys and values
- [Object Order](https://json.nlohmann.me/features/object_order/index.md) - iteration order of object keys
- [Element Access](https://json.nlohmann.me/features/element_access/index.md) - access single elements
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - add and remove elements (including iterator invalidation rules)