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Comment by RcouF1uZ4gsC | Hacker News Reader

This is a bit like saying there is no '+';

Because if you put in

    return 1+2+3;

And look at the assembly code, you will see that the compiler generated something like

    return 6;

The compiler is allowed to take advantage of the standard to substitute in more efficient code that does the same thing.

IIRC, for C++, it would actually be ok if std::vector was implemented completely as a compiler intrinsic with no actual header file. (No compiler I am aware of actually does it that way).

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Code that does

  #include <vector>

must compile, so that header must exist (whether it is stored in a file is the implementer’s choice. AFAIK, the standard carefully avoids the use of the term ‘header file’)

Also, I think code that doesn’t do that include must fail to compile when it tries to use std::vector. So, logically, that header must exist.

Well not really. The preprocessor is part of the compiler, so it only needs set a flag to tell the compiler proper to enable std::vector.

Is there more info to this, I remember this from Commmon Lisp (but details evade me) that the compiler can take benefit of certain specific functions and rely on them being... "open coded" - e.g. it can produce more efficient code by replacing these with something more suitable... http://www.sbcl.org/manual/#Open-Coding-and-Inline-Expansion

https://www.thecodingforums.com/threads/what-is-the-meaning-...

yeah but everyone knows that "there is no +"; It's an operator, and in C, anyways operators are special and expected to not necessarily do C-function-ey things, e.g, "take arguments of different types and add them successfully" not everyone is aware that C has "anointed functions" (including, I believe malloc) that the compiler is allowed to fiddle with.

> more efficient code that does the same thing

In this case, it produces a different result.

It produces a different ub, which is ub.

Furthermore observability would be defined in terms of the C abstract machine, “observing” by decompiling the program is out of scope.

oh right

> But what if you're not using C99 or newer?

UB - that takes all the fun out of it.

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