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204 lines
7.6 KiB
Markdown
204 lines
7.6 KiB
Markdown
# <small>nlohmann::basic_json::</small>patch
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```cpp
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basic_json patch(const basic_json& json_patch) const;
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```
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[JSON Patch](http://jsonpatch.com) defines a JSON document structure for expressing a sequence of operations to apply to
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a JSON document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from
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the patch.
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## Parameters
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`json_patch` (in)
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: JSON patch document
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## Return value
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patched document
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## Exception safety
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Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
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## Exceptions
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- Throws [`parse_error.104`](../../home/exceptions.md#jsonexceptionparse_error104) if the JSON patch does not consist of
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an array of objects.
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- Throws [`parse_error.105`](../../home/exceptions.md#jsonexceptionparse_error105) if the JSON patch is malformed (e.g.,
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mandatory attributes are missing); example: `"operation 'add' must have member 'path'"`.
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- Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if an array index is out of range.
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- Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in a "path" or
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"from" member begins with '0'; example: `"array index '01' must not begin with '0'"`.
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- Throws [`parse_error.107`](../../home/exceptions.md#jsonexceptionparse_error107) if a "path" or "from" member is not
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empty and does not begin with a slash (`/`); example: `"JSON pointer must be empty or begin with '/' - was: 'a'"`.
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- Throws [`parse_error.108`](../../home/exceptions.md#jsonexceptionparse_error108) if a tilde (`~`) in a "path" or
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"from" member is not followed by `0` or `1`; example: `"escape character '~' must be followed with '0' or '1'"`.
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- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in a "path" or
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"from" member is not a number; example: `"array index 'foo' is not a number"`.
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- Throws [`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402) if the array index `-` is used
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where an existing element is required (the "path" of "replace", the "from" of "move" and "copy"); example:
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`"array index '-' (3) is out of range"`.
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- Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if a JSON pointer inside the patch
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could not be resolved successfully in the current JSON value; example: `"key baz not found"`.
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- Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if a reference token of a JSON
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pointer inside the patch cannot be resolved, e.g., `-` in a "remove" operation or `1a` for an array; example:
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`"unresolved reference token '-'"`.
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- Throws [`out_of_range.405`](../../home/exceptions.md#jsonexceptionout_of_range405) if JSON pointer has no parent
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("add", "remove", "move")
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- Throws [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) if an "add" operation's target
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location has a parent that is neither an object nor an array.
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- Throws [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) if a "remove" operation's target
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location has a parent that is neither an object nor an array.
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- Throws [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) if a "move" operation's "from"
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location is a proper prefix of its "path" location.
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- Throws [`other_error.501`](../../home/exceptions.md#jsonexceptionother_error501) if "test" operation was
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unsuccessful.
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## Complexity
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Linear in the size of the JSON value and the length of the JSON patch. As usually the patch affects only a fraction of
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the JSON value, the complexity can usually be neglected.
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## Notes
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The application of a patch is atomic: Either all operations succeed and the patched document is returned or an exception
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is thrown. In any case, the original value is not changed: the patch is applied to a copy of the value.
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## Examples
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??? example "Example: apply a JSON patch"
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The following code shows how a JSON patch is applied to a value.
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```cpp
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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using namespace nlohmann::literals;
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int main()
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{
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// the original document
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json doc = R"(
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{
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"baz": "qux",
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"foo": "bar"
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}
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)"_json;
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// the patch
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json patch = R"(
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[
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{ "op": "replace", "path": "/baz", "value": "boo" },
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{ "op": "add", "path": "/hello", "value": ["world"] },
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{ "op": "remove", "path": "/foo"}
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]
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)"_json;
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// apply the patch
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json patched_doc = doc.patch(patch);
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// output original and patched document
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std::cout << std::setw(4) << doc << "\n\n"
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<< std::setw(4) << patched_doc << std::endl;
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}
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```
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Output:
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```json
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{
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"baz": "qux",
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"foo": "bar"
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}
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{
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"baz": "boo",
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"hello": [
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"world"
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]
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}
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```
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??? example "Example: out_of_range.414 exception"
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The following code shows how a "move" operation whose "from" location is a proper prefix of its "path" location is
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rejected, and how the original document is left unchanged because the patch is applied to a copy.
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```cpp
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#include <iostream>
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#include <iomanip>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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using namespace nlohmann::literals;
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int main()
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{
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// the original document
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json doc = R"(
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{
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"a": { "b": 1 }
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}
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)"_json;
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// a patch that tries to move "/a" into one of its own children
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json patch = R"(
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[
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{ "op": "move", "from": "/a", "path": "/a/b" }
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]
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)"_json;
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// exception out_of_range.414
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try
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{
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json patched_doc = doc.patch(patch);
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}
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catch (const json::out_of_range& e)
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{
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std::cout << e.what() << '\n';
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}
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// the original document is unchanged
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std::cout << std::setw(4) << doc << std::endl;
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}
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```
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Output:
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```json
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[json.exception.out_of_range.414] cannot move value: 'from' path '/a' is a proper prefix of 'path' '/a/b'
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{
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"a": {
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"b": 1
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}
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}
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```
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## See also
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- [RFC 6902 (JSON Patch)](https://tools.ietf.org/html/rfc6902)
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- [RFC 6901 (JSON Pointer)](https://tools.ietf.org/html/rfc6901)
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- [patch_inplace](patch_inplace.md) applies a JSON Patch without creating a copy of the document
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- [merge_patch](merge_patch.md) applies a JSON Merge Patch
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## Version history
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- Added in version 2.0.0.
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- Added [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) and stopped relying on an internal assertion when an "add" operation's
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target location has a non-object/non-array parent in version 3.13.0.
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- Added [`out_of_range.413`](../../home/exceptions.md#jsonexceptionout_of_range413) and stopped silently ignoring a "remove" operation whose target
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location has a non-object/non-array parent in version 3.13.0.
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- Added [`out_of_range.414`](../../home/exceptions.md#jsonexceptionout_of_range414) and rejected a "move" operation whose "from" location is a proper
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prefix of its "path" location instead of silently producing a corrupted result in version 3.13.0.
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- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) instead of
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[`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) for a one-character array index that is
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not a digit (e.g., `/x`) in version 3.13.0, as it already did for longer ones.
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