Accessing value with json pointer adds key if not existing

[sdaly2107]

• What is the issue you have?

When accessing a key using json pointer, if the key doesn't exist, it is added. Is this expected behaviour? If so, is there some alternative const accessor we can use? For now, we are working around this by copying the object.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

    json json;
    json["something"]["else"] = "me";
  
    auto dumpA = json.dump(4);
  
    auto value = json["/something/else"_json_pointer].get<std::string>();
    auto dumpB = json.dump(4);
    REQUIRE(dumpA == dumpB); //ok
  
    auto blah = json["/xyz"_json_pointer];
    auto dumpC = json.dump(4);
    REQUIRE(dumpB == dumpC); //fail, because new key has been added
  

• What is the expected behavior?

{
"something": {
"else": "me"
},
"xyz": null <-- didn't expect this to be added when accessing via json pointer
}

• And what is the actual behavior instead?

Key added.

• Which compiler and operating system are you using? Is it a supported compiler?

MSVC + Win 10
GCC + CentOS 7

• Did you use a released version of the library or the version from the develop branch?

Yes

• If you experience a compilation error: can you compile and run the unit tests?

[brouer]

I use code like this:

    try
    {
        last_page = j.at("isLastPage");
    } catch (...)
    {
        //"isLastPage" was not present
        last_page = true;
    }

Quote from README.md:

If you are not sure whether an element in an object exists, use checked access with the at() function.

[sdaly2107]

So looks like "at" might be the solution. I'd rather not be throwing and catching in normal paths though - would be nice if count() worked with json pointer too, then I could do if(count(json_pointer) then at(json_pointer).

[brouer]

I only try/catch around optional entries that might not be present, in which case I may provide my own default as shown.

I don't try/catch with entries I expect to be there. If that happens, I let the exception propagate back up to the caller, as I consider that a parse error.

YMMV.

[nickaein]

When accessing a key using json pointer, if the key doesn't exist, it is added. Is this expected behaviour?

I believe yes, as basic_json::operator[] documentation states. This mimics the behavior of operator[] on std::map which also adds an object when the key does not already exist.

If so, is there some alternative const accessor we can use? For now, we are working around this by copying the object.

AFAIK, this is not such API. To avoid copying, your best bet is probably a similar solution mentioned by @brouer. Here is documentation of basic_json::at(json_pointer&) that covers different scenarios.

I'd rather not be throwing and catching in normal paths though - would be nice if count() worked with json pointer too.

That's very good point. Though I think adding an overload to basic_json::contains() that accepts json_pointer would be more fitting. @nlohmann, @garethsb-sony Any thoughts?

[nlohmann]

I understand the issue. The implementation of JSON Pointers is a bit hairy. I agree that contains or count should be available for JSON Pointers too.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

I added a contains function for JSON pointers, see 258fa79. Feedback is greatly appreciated! @sdaly2107 @brouer @nickaein

[sdaly2107]

Looks good to me, thank you very much :D

[nlohmann]

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🔖 Release item

This issue/PR will be part of the next release of the library. This template helps preparing the release notes.

Type

• ✨ New Feature
• 🐛 Bug Fix
• ⚡️ Improvement
• 🔨 Further Change
• 🔥 Deprecated function

Description

• Add overload of contains to check if a JSON pointer is valid without throwing exceptions. Accessing value with json pointer adds key if not existing #1600

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