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Comment by maccard | Hacker News Reader

Pragma once is widely enough accepted that anyone who argues against its use for that purpose better either be able to tell me the compilers it doesn’t work on, or I’m going to assume they’re being pedantic for the hell of it.

And honestly, anyone who relies on the scenarios Pragma once fails in (Compiling off network shared and symlinks) should really fix those monstrous issues instead. The places Pragma once trips up in are likely to be issues for detecting modifications in incremental builds anyway.

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There used to be a couple holdouts, but all major C++ compilers have supported it for years (https://en.wikipedia.org/wiki/Pragma_once).

Prarma once works in the vast majority of cases. However there are some rare ones where it fails. Every attempt to fix those last ones breaks some other rare case or profilings shows significant compile time increases.

use pragma once where it works in internal code but never in headers you ship to someone else is a simple rule that should work well enough.

> However there are some rare ones where it fails.

As I said in my previous comment, those cases are very very often cases where the compialtion model is broken, and it's held together by luck.

> Every attempt to fix those last ones breaks some other rare case

My experience (and I have experiences of this) have been that the cases where pragma once fails, other tools (source control, built tools) cause "random" problems that usually are hand waved away with "oh that problem, just run this other makefile target that someone has put together to clear that file and rebuild".

> or profilings shows significant compile time increases.

Again, my experience here has been that the compile time change is usually a result of a broken project model, and that there are other gotchas like "oh you can only include files a.h and c.h but if you include b.h then you'll break the internal build tool". Also, that taking the hit to make your build correct unlocks optimisations that weren't possible before.

Also, the projects that have these kinds of warts are usually allergic to doing anything new whatsoever, making any changes or improvements, upgrading compilers and libraries. I suspect using C++17 is enough to scare them off in many cases.

If it's good enough for QT or LLVM, it's good enough for me.

It isn't luck - I know exactly what I did to my package manage to get that file installed into two different locatian. I know exactly which -I options I passed to the compiler causes that one file to be found in different locations depending on quotes or brackets to be used. And it works great if I don't use pragma.

i also know why I had to do that aweful abuse to what anyone sane would call wholesome. I don't like it but there are other things going on and I don't want to talk about it anymore.

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