Since the order of evaluation of the arguments at the call site is indeterminate, I need guarantees that filename.wstring() does not modify errno.

The C++ standard does not give you this guarantee.

• In well-behaved standard library implementations, calling p.wstring() on a std::filesystem::path p will never touch errno.

• However, technically, the C++ Standard does not guarantee errno is never touched by any standard library function, because library implementers are not explicitly forbidden from doing so.

• In practice, on all major standard libraries (libstdc++, libc++, MSVC), this function is implemented without calling any C library I/O routines.

@BoP Thanks, I know. This would require additional lines of code that I want to avoid if they can be avoided easily.

– j6t

I am afraid it is the only way. **(Lines of source code != more code in executable.)**