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nlohmann
121 lines
4.4 KiB
Markdown
121 lines
4.4 KiB
Markdown
# JSON Pointer
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## Introduction
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The library supports **JSON Pointer** ([RFC 6901](https://tools.ietf.org/html/rfc6901)) as an alternative means to address structured values. A JSON Pointer is a string that identifies a specific value within a JSON document.
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Consider the following JSON document
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```
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{
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"array": ["A", "B", "C"],
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"nested": {
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"one": 1,
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"two": 2,
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"three": [true, false]
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}
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}
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```
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Then every value inside the JSON document can be identified as follows:
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| JSON Pointer | JSON value |
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| ----------------- | ------------------------------------------------------------------------- |
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| \`\` | `{"array":["A","B","C"],"nested":{"one":1,"two":2,"three":[true,false]}}` |
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| `/array` | `["A","B","C"]` |
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| `/array/0` | `A` |
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| `/array/1` | `B` |
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| `/array/2` | `C` |
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| `/nested` | `{"one":1,"two":2,"three":[true,false]}` |
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| `/nested/one` | `1` |
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| `/nested/two` | `2` |
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| `/nested/three` | `[true,false]` |
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| `/nested/three/0` | `true` |
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| `/nested/three/1` | `false` |
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Note `/` does not identify the root (i.e., the whole document), but an object entry with empty key `""`. See [RFC 6901](https://tools.ietf.org/html/rfc6901) for more information.
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## JSON Pointer creation
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JSON Pointers can be created from a string:
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```
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json::json_pointer p("/nested/one");
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```
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Furthermore, a user-defined string literal can be used to achieve the same result:
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```
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auto p = "/nested/one"_json_pointer;
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```
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The escaping rules of [RFC 6901](https://tools.ietf.org/html/rfc6901) are implemented. See the [constructor documentation](https://json.nlohmann.me/api/json_pointer/json_pointer/index.md) for more information.
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## Value access
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JSON Pointers can be used in the [`at`](https://json.nlohmann.me/api/basic_json/at/index.md), [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md), and [`value`](https://json.nlohmann.me/api/basic_json/value/index.md) functions just like object keys or array indices.
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```
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// the JSON value from above
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auto j = json::parse(R"({
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"array": ["A", "B", "C"],
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"nested": {
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"one": 1,
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"two": 2,
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"three": [true, false]
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}
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})");
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// access values
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auto val = j[""_json_pointer]; // {"array":["A","B","C"],...}
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auto val1 = j["/nested/one"_json_pointer]; // 1
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auto val2 = j.at(json::json_pointer("/nested/three/1")); // false
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auto val3 = j.value(json::json_pointer("/nested/four"), 0); // 0
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```
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## Flatten / unflatten
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The library implements a function [`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md) to convert any JSON document into a JSON object where each key is a JSON Pointer and each value is a primitive JSON value (i.e., a string, boolean, number, or null).
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```
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// the JSON value from above
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auto j = json::parse(R"({
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"array": ["A", "B", "C"],
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"nested": {
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"one": 1,
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"two": 2,
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"three": [true, false]
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}
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})");
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// create flattened value
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auto j_flat = j.flatten();
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```
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The resulting value `j_flat` is:
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```
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{
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"/array/0": "A",
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"/array/1": "B",
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"/array/2": "C",
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"/nested/one": 1,
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"/nested/two": 2,
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"/nested/three/0": true,
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"/nested/three/1": false
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}
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```
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The reverse function, [`unflatten`](https://json.nlohmann.me/api/basic_json/unflatten/index.md) recreates the original value.
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```
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auto j_original = j_flat.unflatten();
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```
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## See also
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- Class [`json_pointer`](https://json.nlohmann.me/api/json_pointer/index.md)
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- Function [`flatten`](https://json.nlohmann.me/api/basic_json/flatten/index.md)
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- Function [`unflatten`](https://json.nlohmann.me/api/basic_json/unflatten/index.md)
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- [JSON Patch](https://json.nlohmann.me/features/json_patch/index.md)
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