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nlohmann
83 lines
3.6 KiB
Markdown
83 lines
3.6 KiB
Markdown
# Checked access: at
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## Overview
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The [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) member function performs checked access; that is, it returns a reference to the desired value if it exists and throws a [`basic_json::out_of_range` exception](https://json.nlohmann.me/home/exceptions/#out-of-range) otherwise.
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Read access
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Consider the following JSON value:
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```
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{
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"name": "Mary Smith",
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"age": 42,
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"hobbies": ["hiking", "reading"]
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}
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```
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Assume the value is parsed to a `json` variable `j`.
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| expression | value |
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| ----------------------- | --------------------------------------------------------------------- |
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| `j` | `{"name": "Mary Smith", "age": 42, "hobbies": ["hiking", "reading"]}` |
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| `j.at("name")` | `"Mary Smith"` |
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| `j.at("age")` | `42` |
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| `j.at("hobbies")` | `["hiking", "reading"]` |
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| `j.at("hobbies").at(0)` | `"hiking"` |
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| `j.at("hobbies").at(1)` | `"reading"` |
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The return value is a reference, so it can be used to modify the original value.
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Write access
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```
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j.at("name") = "John Smith";
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```
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This code produces the following JSON value:
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```
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{
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"name": "John Smith",
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"age": 42,
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"hobbies": ["hiking", "reading"]
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}
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```
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When accessing an invalid index (i.e., an index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.
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Accessing via invalid index or missing key
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```
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j.at("hobbies").at(3) = "cooking";
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```
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This code produces the following exception:
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```
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[json.exception.out_of_range.401] array index 3 is out of range
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```
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When [extended diagnostic messages](https://json.nlohmann.me/home/exceptions/#extended-diagnostic-messages) are enabled by defining [`JSON_DIAGNOSTICS`](https://json.nlohmann.me/api/macros/json_diagnostics/index.md), the exception further gives information where the key or index is missing or out of range.
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```
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[json.exception.out_of_range.401] (/hobbies) array index 3 is out of range
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```
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## Notes
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Exceptions
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- [`at`](https://json.nlohmann.me/api/basic_json/at/index.md) can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a [`basic_json::type_error`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error304) is thrown.
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- [`basic_json::out_of_range` exception](https://json.nlohmann.me/home/exceptions/#out-of-range) exceptions are thrown if the provided key is not found in an object or the provided index is invalid.
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## Summary
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| scenario | non-const value | const value |
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| --------------------------------- | ---------------------------------------------- | ---------------------------------------------- |
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| access to existing object key | reference to existing value is returned | const reference to existing value is returned |
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| access to valid array index | reference to existing value is returned | const reference to existing value is returned |
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| access to non-existing object key | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
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| access to invalid array index | `basic_json::out_of_range` exception is thrown | `basic_json::out_of_range` exception is thrown |
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