Fix json::update() with merge_objects==true error

[DanielHwangbo]

Description

This PR fixes a crash when using update() on an ordered_json object when JSON_DIAGNOSTICS is enabled.

The Issue

When update() inserts new elements into an ordered_json object, the underlying std::vector may reallocate. This invalidates the m_parent pointers of existing children, causing assert_invariant to fail immediately because the children point to the old memory address.

The Fix

I have updated the update() function to call set_parents() after insertion operations when diagnostics are enabled. This ensures that if the container moves in memory, all children are correctly updated to point to the new parent address.

Reproduction Code

#define JSON_DIAGNOSTICS 1
#include "nlohmann/json.hpp"
#include <iostream>

int main() {
    using json = nlohmann::ordered_json;
    json j1 = { {"numbers", {{"one", 1}}} };
    json j2 = { {"numbers", {{"two", 2}}}, {"string", "t"} };

    // This triggers reallocation and assertion failure without the fix
    j1.update(j2, true); 
    std::cout << j1.dump() << std::endl;
}

[coveralls]

coverage: 99.176% (-0.02%) from 99.191%
when pulling f7e256e on DanielHwangbo:issue4813
into a0e9fb1 on nlohmann:develop.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @DanielHwangbo
Please read and follow the Contribution Guidelines.

[nlohmann]

Maybe related: #4813

[DanielHwangbo]

Hi nlohmann,

I have fixed the Amalgamation error and would like to request a code review again!

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @DanielHwangbo
Please read and follow the Contribution Guidelines.

[gregmarr]

You have a LOT of unrelated formatting changes in this file. Did your editor do an auto-format?

[gregmarr]

What is the reason for this change?

[gregmarr]

I recommend reverting all of the whitespace changes and the changes at line 3491 to introduce ref. Then you also need to amalgamate and add the file to the commit. I don't know if it didn't work before, or if you didn't add the file, but there are no changes to the amalgamated header.

[gregmarr]

I think the comment should go inside the #if.

[gregmarr]

I think the comment should go inside the #if.

[github-actions]

This pull request has been marked as stale because it has had no activity for 30 days. While we won’t close it automatically, we encourage you to update or comment if it is still relevant. Keeping pull requests active and up-to-date helps us review and merge changes more efficiently. Thank you for your contributions!

[gregmarr]

@DanielHwangbo Are you still pursuing this PR?