nlohmann::basic_json::patch¶

basic_json patch(const basic_json& json_patch) const;

JSON Patch defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from the patch.

Parameters¶

json_patch (in): JSON patch document

Return value¶

patched document

Exception safety¶

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions¶

Throws parse_error.104 if the JSON patch does not consist of an array of objects.
Throws parse_error.105 if the JSON patch is malformed (e.g., mandatory attributes are missing); example: "operation 'add' must have member 'path'".
Throws out_of_range.401 if an array index is out of range.
Throws parse_error.106 if an array index in a "path" or "from" member begins with '0'; example: "array index '01' must not begin with '0'".
Throws parse_error.107 if a "path" or "from" member is not empty and does not begin with a slash (/); example: "JSON pointer must be empty or begin with '/' - was: 'a'".
Throws parse_error.108 if a tilde (~) in a "path" or "from" member is not followed by 0 or 1; example: "escape character '~' must be followed with '0' or '1'".
Throws parse_error.109 if an array index in a "path" or "from" member is not a number; example: "array index 'foo' is not a number".
Throws out_of_range.402 if the array index - is used where an existing element is required (the "path" of "replace", the "from" of "move" and "copy"); example: "array index '-' (3) is out of range".
Throws out_of_range.403 if a JSON pointer inside the patch could not be resolved successfully in the current JSON value; example: "key baz not found".
Throws out_of_range.404 if a reference token of a JSON pointer inside the patch cannot be resolved, e.g., - in a "remove" operation or 1a for an array; example: "unresolved reference token '-'".
Throws out_of_range.405 if JSON pointer has no parent ("add", "remove", "move")
Throws out_of_range.411 if an "add" operation's target location has a parent that is neither an object nor an array.
Throws out_of_range.413 if a "remove" operation's target location has a parent that is neither an object nor an array.
Throws out_of_range.414 if a "move" operation's "from" location is a proper prefix of its "path" location.
Throws other_error.501 if "test" operation was unsuccessful.

Complexity¶

Linear in the size of the JSON value and the length of the JSON patch. As usually the patch affects only a fraction of the JSON value, the complexity can usually be neglected.

Notes¶

The application of a patch is atomic: Either all operations succeed and the patched document is returned or an exception is thrown. In any case, the original value is not changed: the patch is applied to a copy of the value.

Examples¶

Example: apply a JSON patch

The following code shows how a JSON patch is applied to a value.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // the original document
    json doc = R"(
        {
          "baz": "qux",
          "foo": "bar"
        }
    )"_json;

    // the patch
    json patch = R"(
        [
          { "op": "replace", "path": "/baz", "value": "boo" },
          { "op": "add", "path": "/hello", "value": ["world"] },
          { "op": "remove", "path": "/foo"}
        ]
    )"_json;

    // apply the patch
    json patched_doc = doc.patch(patch);

    // output original and patched document
    std::cout << std::setw(4) << doc << "\n\n"
              << std::setw(4) << patched_doc << std::endl;
}

Output:

{
    "baz": "qux",
    "foo": "bar"
}

{
    "baz": "boo",
    "hello": [
        "world"
    ]
}

Example: out_of_range.414 exception

The following code shows how a "move" operation whose "from" location is a proper prefix of its "path" location is rejected, and how the original document is left unchanged because the patch is applied to a copy.

#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // the original document
    json doc = R"(
        {
          "a": { "b": 1 }
        }
    )"_json;

    // a patch that tries to move "/a" into one of its own children
    json patch = R"(
        [
          { "op": "move", "from": "/a", "path": "/a/b" }
        ]
    )"_json;

    // exception out_of_range.414
    try
    {
        json patched_doc = doc.patch(patch);
    }
    catch (const json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // the original document is unchanged
    std::cout << std::setw(4) << doc << std::endl;
}