Generate single line json struture with cv::FileStorage

I try to write a one line json structure of a comple open cv struct.
I understand how to genere for my own objet one line with the help of “{:” syntaxe.
But when dumping cv::Rect2f for example multiple is produced

rather come up with your very own json (why that, at all ?) serialization.
(noone will mess with the library code, to fit it to your (very special) needs)