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nlohmann
152 lines
3.6 KiB
Markdown
152 lines
3.6 KiB
Markdown
# Iterators
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## Overview
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A `basic_json` value is a container and allows access via iterators. Depending on the value type, `basic_json` stores zero or more values.
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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()`.
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### Iteration order for objects
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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.
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Example
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```
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// create JSON object {"one": 1, "two": 2, "three": 3}
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json j;
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j["one"] = 1;
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j["two"] = 2;
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j["three"] = 3;
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for (auto it = j.begin(); it != j.end(); ++it)
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{
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std::cout << *it << std::endl;
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}
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```
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Output:
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```
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1
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3
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2
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```
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The reason for the order is the lexicographic ordering of the object keys "one", "three", "two".
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### Access object keys during iteration
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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.
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Example
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```
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// create JSON object {"one": 1, "two": 2, "three": 3}
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json j;
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j["one"] = 1;
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j["two"] = 2;
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j["three"] = 3;
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for (auto it = j.begin(); it != j.end(); ++it)
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{
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std::cout << it.key() << " : " << it.value() << std::endl;
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}
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```
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Output:
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```
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one : 1
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three : 3
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two : 2
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```
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### Range-based for loops
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C++11 allows using range-based for loops to iterate over a container.
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```
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for (auto it : j_object)
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{
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// "it" is of type json::reference and has no key() member
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std::cout << "value: " << it << '\n';
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}
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```
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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.
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```
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for (auto& el : j_object.items())
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{
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std::cout << "key: " << el.key() << ", value:" << el.value() << '\n';
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}
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```
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The items() function also allows using structured bindings (C++17):
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```
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for (auto& [key, val] : j_object.items())
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{
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std::cout << "key: " << key << ", value:" << val << '\n';
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}
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```
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Note
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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.
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Warning
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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.
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### Reverse iteration order
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`rbegin()` and `rend()` return iterators in the reverse sequence.
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Example
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```
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json j = {1, 2, 3, 4};
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for (auto it = j.rbegin(); it != j.rend(); ++it)
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{
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std::cout << *it << std::endl;
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}
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```
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Output:
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```
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4
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3
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2
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1
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```
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### Iterating strings and binary values
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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.
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Example
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```
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json j = "Hello, world";
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for (auto it = j.begin(); it != j.end(); ++it)
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{
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std::cout << *it << std::endl;
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}
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```
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Output:
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```
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"Hello, world"
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```
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## Iterator invalidation
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| Operations | invalidated iterators |
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| ---------- | --------------------- |
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| `clear` | all |
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