C++ : Pass-By-Reference Or Die

Before today's Post, I'm on a mission folks, to get 1000 subs on YouTube.  If only 5% of viewers here subscribed we've have met this target in one month...

I've just had group code review of one of my personal projects, and been rather surprised by the vitriol levelled at one of my practices.... Pass by Reference.

The reviewer, one of a group of peers, has had major issues with the project (my personal) insistance on passing by reference wherever possible, in C++ this takes the form of an additional ampersand on parameter definitions; maybe this was the chaps problem, he has to type an ampersand?

So his problem?  Well, without the actual code we'll simplify and use the Compiler Explorer (from Godbolt.org) and we'll take up their basic square function example, it starts up thus:

Giving the assembler:

On the right, and this chap had taken time to prepare a whole slide show of functions, usually simple, and present them at this code review, showing this kind of thing.  His point... Well the very same C++ but with a pass by reference:

Turns up more lines of assembler:

He's got me right, right, I'm taking more time, I'm slowing everything down, by my not taking a copy of everything and using less memory I'm slowing things down....

This is where the sort of power play turned, I allowed him to present everything, I never interjected, never spoke, I allowed him to speak to the whole group.  We've hired a venue for this, we're meeting live for the first time.  This has to be good.... A couple of the chaps who can already see the fault in the complainers logic were smirking, but we let him finish.

Triumphant, he has won the day, he will not carry the torch of coding standard gods...  WRONG.

I pulled over the presentation laptop, opened godbolt.org myself... Added the ampersand to the "num" and let it produce the above assembler... The chap was smirking completely from ear to ear, he knew he had me...

And then I typed three characters....

-O2

Yes, I told the compiler to optimize, and this happened...

Remarkably small code wouldn't you say?  I still haven't spoken, but I turn the laptop back to the presenter and just sit there.

There's a noticable snigger from those in the know, older-wiser heads then my own I hasten to add.  But this young chap is now looking from me to the screen to the overhead projection and back with a mix of fury and completely puzzlement, he'd checked everything, he's dotted every j and crossed every t, he had me down pat, he wanted to usurp me.

Except, he's never ever, been willing to listen, to learn or to experiment, "code runs, that'll do" is very much his style (and Kyle if you're reading, yes I'm talking about you) but getting code to run is not enough, understanding the code you've written is often only just enough, but getting it to run everywhere, the same way, that's an art.  Debug, Release, Optimised, Unoptimised, automatically profiled, link database and continune they're all subtly different.  Just listing one thing out, the only thing you've looked at; because it backs up your point of view; is not enough you have to look around and see the holistic picture.

And optimised without a pass-by-reference?

Spookily similar code in this case, but often times pass-by-reference is prefered, using const-correctness is prefered it communicates a meaning.

For instance in the "square" function above, how does the caller know that the parameter "num" is not altered in value?  How does the caller know it returns the new value only?  It could be returning an error status code and the parameter altered in value!  You don't know, but making the parameter const and a reference you start to communicate more firmly the intent of your code.