update ceph source to reef 18.1.2

[ceph.git] / ceph / src / jaegertracing / opentelemetry-cpp / third_party / nlohmann-json / doc / mkdocs / docs / features / iterators.md

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+# 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()`.
+
+![Illustration from cppreference.com](../images/range-begin-end.svg)
+
+### 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
+
+    ```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 key 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
+
+    ```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()` 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 <https://github.com/nlohmann/json/issues/2040> for more information.
+
+### Reverse iteration order
+
+`rbegin()` and `rend()` return iterators in the reverse sequence.
+    
+![Illustration from cppreference.com](../images/range-rbegin-rend.svg)
+
+??? example
+
+    ```cpp
+    json j = {1, 2, 3, 4};
+
+    for (auto it = j.begin(); it != j.end(); ++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 the underlying string or binary array is empty.
+
+??? example
+
+    ```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                   |