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satelliteprogrammer 40a62947d3 support JSON Merge Patch (RFC 7396) diff creation (#4965 )

* support JSON Merge Patch (RFC 7396) diff creation

The JSON merge patch document format describes the set of modifications
to a resource's content, that more closely mimics the syntax of the
resource being modified.

However, and in contrast to JSON Patch (RFC 6902), a JSON Merge Patch
cannot express certain modifications, e.g., changing an array element
at a specific index, or setting a specific object value to null.
The null value in a JSON Merge Patch is used to remove the key from the
object.

The diff algorithm is not part of the RFC 7396, but it was tested
against all examples provided, plus additional cases on how null values
are handled.

Signed-off-by: Luís Murta <luis@murta.dev>

* improve algorithm efficiency

Signed-off-by: Luís Murta <luis@murta.dev>

* fix inconsistent nulls and empty arrays

Also add CHECKs for the patch, documentation and run `make amalgamate`.

Signed-off-by: Luís Murta <luis@murta.dev>

* documentation cleanups

Signed-off-by: Luís Murta <luis@murta.dev>

* add an iterative fallback to merge_diff

Bound the recursion depth like diff() and merge_patch(), and test values
nested past the bound.

Signed-off-by: Luís Murta <luis@murta.dev>

* address the remaining review comments

Signed-off-by: Luís Murta <luis@murta.dev>

* move multi-line raw strings out of CHECK macros

GCC 4.8/5, ICPC and NVHPC mis-tokenize multi-line raw string literals
inside macro arguments, breaking the build or the parsed JSON.

Signed-off-by: Luís Murta <luis@murta.dev>

---------

Signed-off-by: Luís Murta <luis@murta.dev>

2026-10-12 09:33:43 +02:00

1.4 KiB

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nlohmann::basic_json::diff

static basic_json diff(const basic_json& source,
                       const basic_json& target);

Creates a JSON Patch so that value source can be changed into the value target by calling patch function.

For two JSON values source and target, the following code always yields #!cpp true:

source.patch(diff(source, target)) == target;

Parameters

source (in): JSON value to compare from
target (in): JSON value to compare against

Return value

a JSON patch to convert the source to target

Exception safety

Strong guarantee: source and target are never modified.

Complexity

Linear in the lengths of source and target.

Notes

Currently, only remove, add, and replace operations are generated.

Examples

??? example

The following code shows how a JSON patch is created as a diff for two JSON values.
 
```cpp
--8<-- "examples/diff.cpp"
```

Output:

```json
--8<-- "examples/diff.output"
```

Version history

Added in version 2.0.0.

1.4 KiB Raw Blame History