apply_patch() request to add flexibility

[andriichA]

Hi, recently I discovered that add op doesn't work when using apply_patch() (jsoncons/include/jsoncons_ext/jsonpatch/jsonpatch.hpp) function. I was debugging it and the problem lies in add_if_absent() due to hardcoded "false" value for create_if_missing arg.

No error; applied patch

Error; patch hasn't been applied

Enumerate the steps to reproduce the bug

1. Call apply_patch() in order to add a new json pointer-value with "add" op
2. Observe the error

Include a small, self-contained example if possible When I try to add a new json pointer-value with "add" op the function add_if_absent() (inside apply_patch()) with create_if_missing = false is called. Then resolve() is called where the new json pointer isn't found in "current" json and since create_if_missing is false the error is returned.

What compiler, architecture, and operating system?

• Compiler: gcc 9.4.0 (c++17)
• Architecture (e.g. x86, x64) x86_64
• Operating system: Linux 20.04.2

What jsoncons library version?

• [ ] Latest release 0.173.4
• [x] Other release 0.171.0
• [ ] master

[andriichA]

[andriichA]

So the only way for this to work is to check the root json and allocate new pointers by myself before I call apply_patch(), which is a waste if this lib already has this logic

When I've changed false value to true everything was fine (see the screenshot)

[danielaparker]

jsonpatch::apply_patch implements the JSONPatch RFC. The "add" operation only adds members to existing objects, it won't create new empty objects.

So, if your data is,

{ "foo": "bar"}

you can apply this patch

[
    { "op": "add", "path": "/baz", "value": "qux" }
]

but not this one

[
    { "op": "add", "path": "/baz/new", "value": "qux" }
]

Automatically adding the missing objects can be helpful, even if not allowed by JSONPatch. That's why we added some jsonpointer functions with the create_if_missing option, so you can do it as follows:

#include <jsoncons/json.hpp>
#include <jsoncons_ext/jsonpointer/jsonpointer.hpp>
#include <iostream>

using jsoncons::json;
namespace jsonpointer = jsoncons::jsonpointer;

int main()
{
    json data = json::parse(R"(
{ "foo": "bar"}
)");

    jsonpointer::add(data, "/baz/new", "qux", true);
    std::cout << jsoncons::pretty_print(data) << "\n";
}

Output:

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

[andriichA]

Thank you for the explanation!

[andriichA]

Hi, Is there a way to implement apply_patch() func with an argument where I could specify If I want to create the missing pointers?

I would like to avoid redundant code since we are using your beautiful lib and don't just copy the apply_patch() logic but with true val for create_if_missing arg

Thank you!

[danielaparker]

apply_patch implements the JSONPatch standard, RFC6902, it can't deviate from that. The way to supply missing name-object pairs through JSONPatch is to include multiple add operations explicitly in the patch.