I'm also only familiar with this idiom in C++, but based on the description in the parent comment, I suspect that this is sometimes used in C too, in which case

I'm also only familiar with this idiom in C++, but based on the description in the parent comment, I suspect that this is sometimes used in C too, in which case you obviously can't use unique_ptr or private fields; maybe macros might be a way to avoid having to write a bunch of boilerplate to achieve a similar effect?