Does it work with all supported formats?
Could you please add simple test for that in this file?

It looks like empty key is not possible at least for YAML (syntax issue) and XML (empty tag). I propose to add explicit check to code to not allow empty keys. @alalek @vpisarev what do you think?

I tried FileStorage with empty key string and it throws assertion as expected. So empty key name is not allowed.

C++ code:

    std::string fname = std::string("test.json");
    FileStorage fs(tempfile(fname.c_str()), cv::FileStorage::WRITE);
    fs.write("", 10);
    fs.release();

Ouput:

unknown file: Failure
C++ exception with description "OpenCV(4.5.0-dev) /home/alexander/Projects/OpenCV/opencv-master2/modules/core/src/persistence_json.cpp:161: error: (-5:Bad argument) An attempt to add element without a key to a map, or add element with key to sequence in function 'writeScalar'

The same behavior for writeStruct call:

    std::string fname = std::string("test.json");
    FileStorage fs(tempfile(fname.c_str()), cv::FileStorage::WRITE);
    fs.startWriteStruct("", cv::FileNode::MAP);
    fs.endWriteStruct();
    fs.release();

Output:

unknown file: Failure
C++ exception with description "OpenCV(4.5.0-dev) /home/alexander/Projects/OpenCV/opencv-master2/modules/core/src/persistence_json.cpp:161: error: (-5:Bad argument) An attempt to add element without a key to a map, or add element with key to sequence in function 'writeScalar'

The PR is closed as proposed documentation changes do not correspond to real OpenCV behavior.

It looks like empty key is not possible at least for YAML (syntax issue) and XML (empty tag). I propose to add explicit check to code to not allow empty keys. @alalek @vpisarev what do you think?

Passing an empty key is only possible, if you write to a sequence (i.e. called fs.startWriteStruct("sequence", FileNode::SEQ) before). I rephrased the comment to make this clearer. I also provided a test.

The changes are not reflected in the closed PR, they can be viewed here. Can the PR be reopend, should I open a new one, or is still something amiss?

Please create new PR. Github does not allow to re-open this one as force push is done.