I don't think the comment 'we do not recommend general use of these functions' applies, becaue this is not one of those undocumented internal-use builtins.

Should this be best practice? In gcc project they say:

"GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and are not documented here because they may change from time to time; we do not recommend general use of these functions."

https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

There's another kata that forbids use of string , this one's too late to change

There's another kata that forbids use of string , this one's too late to change

Not a suggestion (As long as it solves the tasks within the time constraint, it is a valid solution)

kata hint != kata suggestion

Maybe you're not eligible yet to become a contributor (1K honor at least iirc) (not an issue)

Language? Does this happen only in this kata? It works fine for me.

I cannot edit the kata at all. Is this kata closed or is something else wrong?

Is this kata somehow closed? I can't edit the solution

Awesome, this should be considered best practice for C++ prior to C++11. So in my opinion every c++ developer should be aware of this solution too. For example you can't use std::to_string in c++99.