nlohmann::basic_json::patch_inplace

void patch_inplace(const basic_json& json_patch);

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. This function applies a JSON patch in place and returns void.

Parameters

json_patch (in) : JSON patch document

Exception safety

No guarantees, value may be corrupted by an unsuccessful patch operation.

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

Unlike patch, patch_inplace applies the operation "in place" and no copy of the JSON value is created. That makes it faster for large documents by avoiding the copy. However, the JSON value might be corrupted if the function throws an exception.

Examples

Example: apply a JSON patch in place

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;

    // output original document
    std::cout << "Before\n" << std::setw(4) << doc << std::endl;

    // apply the patch
    doc.patch_inplace(patch);

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

Output:

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

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

Example: out_of_range.403 exception with a partially applied patch

The following code shows a patch whose first operation succeeds and whose second operation fails. Because patch_inplace applies each operation directly to the value, the first operation's effect is still visible after the exception is caught, unlike patch.

#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": 1,
          "b": 2
        }
    )"_json;

    // a patch whose second operation fails
    json patch = R"(
        [
          { "op": "replace", "path": "/a", "value": 99 },
          { "op": "remove", "path": "/nonexistent" }
        ]
    )"_json;

    // exception out_of_range.403
    try
    {
        doc.patch_inplace(patch);
    }
    catch (const json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // the first operation has already been applied to doc
    std::cout << std::setw(4) << doc << std::endl;
}

Output:

[json.exception.out_of_range.403] key 'nonexistent' not found
{
    "a": 99,
    "b": 2
}

Version history

Added in version 3.11.0.
Added out_of_range.411 and stopped relying on an internal assertion when an "add" operation's target location has a non-object/non-array parent in version 3.13.0 unreleased.
Added out_of_range.413 and stopped silently ignoring a "remove" operation whose target location has a non-object/non-array parent in version 3.13.0 unreleased.
Added out_of_range.414 and rejected a "move" operation whose "from" location is a proper prefix of its "path" location instead of silently producing a corrupted result in version 3.13.0 unreleased.

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