# 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()`](../api/basic_json/begin.md) returns an iterator to the first value and [`end()`](../api/basic_json/end.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()`.
A row of fifteen elements followed by a dashed past-the-last position. begin() points to the first element and end() points to the past-the-last position. begin end Past-the-last element

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](types/index.md#key-order) for more information. ??? example "Example: iteration order of object values" ```cpp // 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: ```json 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](../home/exceptions.md#jsonexceptioninvalid_iterator207) exception is thrown. ??? example "Example: access object keys with `key()` and `value()`" ```cpp // 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: ```json one : 1 three : 3 two : 2 ``` ### Range-based for loops C++11 allows using range-based for loops to iterate over a container. ```cpp 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()`](../api/basic_json/items.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. ```cpp 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): ```cpp 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()`](../api/basic_json/rbegin.md) and [`rend()`](../api/basic_json/rend.md) return iterators in the reverse sequence.
Two rows of sixteen cells. The upper row shows the reversed sequence: rend() points to a dashed position before the first element and rbegin() to the last element. The lower row shows the forward sequence: begin() points to the first element and end() to a dashed position after the last element. A reverse iterator stores an iterator to the next element, so rend() corresponds to begin() and rbegin() to end(). rend rbegin Reverse past-the-last element A reverse iterator stores an iterator to the next element begin end Past-the-last element

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 "Example: reverse iteration with `rbegin()` and `rend()`" ```cpp json j = {1, 2, 3, 4}; for (auto it = j.rbegin(); it != j.rend(); ++it) { std::cout << *it << std::endl; } ``` Output: ```json 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 "Example: iterate over a string value" ```cpp json j = "Hello, world"; for (auto it = j.begin(); it != j.end(); ++it) { std::cout << *it << std::endl; } ``` Output: ```json "Hello, world" ``` ## Iterator invalidation | Operations | invalidated iterators | |------------|-----------------------| | `clear` | all | ## See also - [`begin`](../api/basic_json/begin.md) - / [`end`](../api/basic_json/end.md) - iterators to the first and one past the last element - [`rbegin`](../api/basic_json/rbegin.md) - / [`rend`](../api/basic_json/rend.md) - reverse iterators - [`cbegin`](../api/basic_json/cbegin.md) - / [`cend`](../api/basic_json/cend.md) - const iterators - [`items`](../api/basic_json/items.md) - iterate over keys and values - [Object Order](object_order.md) - iteration order of object keys - [Element Access](element_access/index.md) - access single elements - [Modifying values](modifying_values.md) - add and remove elements (including iterator invalidation rules)