[mehrdadn]

With the caveat that I am NOT suggesting this justifies choosing C over C++, I just wanted to mention this talk about how "zero-cost abstraction" is an idealism, not necessarily a reality: https://www.youtube.com/watch?v=rHIkrotSwcc&t=17m30s

That said, I tried to reproduce something similar, and it seems the issue only occurs in my example due to external linkage (adding 'static' fixes it)... but I can't claim this will always resolve the issue: https://gcc.godbolt.org/z/1vbqo3

[bregma]

The "zero-cost abstraction" concept is that you don't pay for something you don't use. If you use a smart pointer, you pay for it. If you don't use a smart pointer, it's zero cost.

Any argument that smart pointers are not zero cost to the language because they have a cost when you use them is a classic straw man argument.

[mehrdadn]

I think there might be a terminology mixup here but he probably meant zero-overhead abstraction. And in any case, the point he's making is not a strawman argument.

[bregma]

OK, let's substitute "overhead" for "cost in the argument.

Premise one: C++ has zero overhead for most of its features, like smarts pointers: if you don't use them, they cost nothing.

Premise two: using features like smart pointers adds a cost to your C++ program.

Conclusion (due to the strawman logical fallacy): C++ is not zero overhead because there exists a feature that has a cost if you use it.

I'd like to see the reasoning that leads to the same conclusion without resorting to the strawman logical fallacy.

[mehrdadn]

He talked about more than just unique_ptr if you actually watch the video, but I don't even get why you're dying to have such a pointless argument here. When you see free lunches offered somewhere, do you start arguing with people how everyone is wrong and there is such a thing as a free lunch too? Is it so hard to actually take the point and just move on instead of dissecting it like a mathematical theorem?

[virgilp]

I watched only a tiny part of that talk but the speaker claims "there are no zero-cost abstractions" and justifies it with.... unique-ptr??? I mean, who doesn't know that smart pointers can be slower than raw pointers? They're not there because they're "zero-cost", but because the improvement in reliability & readability more than makes up for the runtime cost in the vast majority of situations.

[mehrdadn]

He's comparing unique_ptr to just plain old-fashioned new and delete. Is it obvious to you what the actual cost is in moving between these? It's not really obvious and probably not what you think. I would listen to the full section of the talk on unique_ptr.

[virgilp]

I developed an optimizing C++ compiler in my youth (one that was actually used in production, not just a pet project). I probably know the actual cost, and also know it may depend on compiler.

But that's not the point - the point is that smart pointers never claimed to be zero-cost abstractions, AFAIK.

[mehrdadn]

> But that's not the point - the point is that smart pointers never claimed to be zero-cost abstractions, AFAIK.

> I mean, who doesn't know that smart pointers can be slower than raw pointers?

Nobody... on what basis? If you're not in that set of people, that doesn't imply the set is small or empty. Googling suggests lots of people have given such advice, and, may I suggest, it's not because they were stupid.

[virgilp]

> Nobody... on what basis?

I don't understand the question (I didn't use the word "nobody"). I stand by the first claim (library writers & ISO C++ commitee never claimed that smart pointers are zero-cost abstractions; so why would people believe that? How did that claim start?).

The second (rhetoric) question... well, of course people would occasionally believe random stuff, for God's sake, we have (non-stupid!) people who believe COVID is a conspiracy. That said - there's no basis in the belief that smart pointers are zero-cost, so it's funny to make a talk "debunking" it - to me it looks similar to a talk that "COVID is real, guys!". I was genuinely surprised this talk is needed. I think if this is really a wide-spread belief, a more insightful talk would be about how it got to be a wide-spread belief :)

[UncleMeat]

Chandler is on the cpp committee and personally said he was surprised by this cost when passing unique ptr as a parameter, as were many on the committee.

So the evidence against your claim is literally within the talk.