67 lines
3.6 KiB
Markdown
67 lines
3.6 KiB
Markdown
# Element Access
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There are many ways elements in a JSON value can be accessed:
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- unchecked access via [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md)
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- checked access via [`at`](https://json.nlohmann.me/features/element_access/checked_access/index.md)
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- access with default value via [`value`](https://json.nlohmann.me/features/element_access/default_value/index.md)
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- [iterators](https://json.nlohmann.me/features/iterators/index.md)
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- [JSON pointers](https://json.nlohmann.me/features/json_pointer/index.md)
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Testing whether a key or index exists before accessing it is also possible, with [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) or [`find`](https://json.nlohmann.me/api/basic_json/find/index.md) (which returns an iterator to the value, or `end()` if it is not found).
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```
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flowchart TD
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A["accessing a value"] --> B{"must it exist?"}
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B -->|"yes, missing is an error"| C["at() -- throws"]
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B -->|"yes, but checking is my job"| D["operator[] -- unchecked"]
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B -->|"no, a fallback is fine"| E["value() -- default value"]
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A --> F{"just testing first?"}
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F -->|"yes"| G["contains() / find()"]
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```
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## Does this key exist?
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Use [`contains`](https://json.nlohmann.me/api/basic_json/contains/index.md) to test for a key; it does not modify the value and does not throw:
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```
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json j = {{"name", "Mary"}, {"address", {{"city", "Berlin"}}}};
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j.contains("name"); // true
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j.contains("age"); // false
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j.contains("city"); // false: contains() only looks at the top level
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j["address"].contains("city"); // true
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j.contains(json::json_pointer("/address/city")); // true: a JSON pointer tests a nested path in one call
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```
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If the value is needed afterwards, [`find`](https://json.nlohmann.me/api/basic_json/find/index.md) tests and accesses with a single lookup:
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```
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if (auto it = j.find("name"); it != j.end())
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{
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std::cout << *it << '\n';
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}
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```
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`contains` also returns `true` for a key whose value is `null`; use `j.contains("k") && !j["k"].is_null()` if a `null` should count as missing. To read a value with a fallback instead, see [`value`](https://json.nlohmann.me/features/element_access/default_value/index.md).
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`operator[]` inserts missing keys
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Like `std::map::operator[]`, the non-const [`operator[]`](https://json.nlohmann.me/features/element_access/unchecked_access/index.md) adds a `null` value if the key is missing. Testing a key with it therefore *changes the object*:
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```
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json j = {{"foo", 1}};
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bool absent = j["bar"].is_null(); // true, but j is now {"bar": null, "foo": 1}
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```
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Use `contains`/`find` to test, and [`at`](https://json.nlohmann.me/features/element_access/checked_access/index.md) to access a value that must exist (it throws an [`out_of_range`](https://json.nlohmann.me/home/exceptions/#out-of-range) exception otherwise).
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## See also
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- [**operator[]**](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) - unchecked access
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- [**at**](https://json.nlohmann.me/api/basic_json/at/index.md) - checked access
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- [**value**](https://json.nlohmann.me/api/basic_json/value/index.md) - access with default value
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- [**contains**](https://json.nlohmann.me/api/basic_json/contains/index.md) and [**find**](https://json.nlohmann.me/api/basic_json/find/index.md) - test for existence
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- [**front**](https://json.nlohmann.me/api/basic_json/front/index.md) and [**back**](https://json.nlohmann.me/api/basic_json/back/index.md) - first and last element
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- [**items**](https://json.nlohmann.me/api/basic_json/items/index.md) - iterate over keys and values
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