The C and POSIX standards guarantee that errno is never set to zero by any library function. The C++ standard has less to say about when errno is or isn't set, but libstdc++ follows the same rule and never sets it to zero.

On the other hand, there are few guarantees about when the C++ library sets errno on error, beyond what is specified for functions that come from the C library. For example, when std::stoi throws an exception of type std::out_of_range, errno may or may not have been set to ERANGE.

Parts of the C++ library may be implemented in terms of C library functions, which may result in errno being set with no explicit call to a C function. For example, on a target where operator new uses malloc a failed memory allocation with operator new might set errno to ENOMEM. Which C++ library functions can set errno in this way is unspecified because it may vary between platforms and between releases.