Comment by benreesman

You'll see a fairly even split amongst S-tier, "possibly headed for standardization" level libraries. I'd say there's a skew for `#ifndef` in projects that are more "aspires to the standard library" and for `#pragma once` in projects that are more focused on like a very specific vertical.

`#pragma once` seems to be far preferred for internal code, there's an argument for being strictly conforming if you're putting out a library. I've converted stuff to `#ifndef` before sharing it, but I think heavy C++ people usually type `#pragma once` in the privacy of their own little repository.

- `spdlog`: `#pragma once` https://github.com/gabime/spdlog/blob/v1.x/include/spdlog/as...

- `absl`: `#ifndef` https://github.com/abseil/abseil-cpp/blob/master/absl/base/a...

- `zpp_bits`: `#ifndef` https://github.com/eyalz800/zpp_bits/blob/main/zpp_bits.h

- `stringzilla` `#ifndef` https://github.com/ashvardanian/StringZilla/blob/main/includ...