Pragma once is a defacto standard and is supported by every compiler and platform you’re ever going to work with. If you need to support the targets it doesn’t

Pragma once is a defacto standard and is supported by every compiler and platform you’re ever going to work with. If you need to support the targets it doesn’t work with, then you’ll get a loud compile error. The old include guards are error prone, requiring a unique key per header. Getting that wrong can cause anything from a compile error, an obtuse linker error, or a runtime bug depending on what’s in the header. The chances of someone copy pasting a header and breaking the include guard are significantly higher than the chances of someone deciding that we need to support an esoteric embedded target overnight.