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.

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.

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.

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?

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.

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.

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.

> 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.

> 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 :)

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.

Ok, I guess I'll look at the talk/ didn't know the guy was in the C++ committee. Still, I personally had to fight the other way around - to convince people that "no, smart pointers don't have significant overhead over raw pointers; and the optimizations/ custom smart pointers that you're doing now, to the extent that they have any effect at all, will likely be rendered obsolete by future compiler versions & libraries".

I could understand a CPP committee member expecting a certain compiler to optimize a certain situation; however, the blanket statement that "smart pointer X is zero-cost" is more than strange, given the fact that it's always bound to be implementation-dependent, and there isn't one single C++ compiler (or even a "canonical" one) so that you could make that claim, at all. I find it really suspicious.

[edit] I watched the part of the talk - I think he was a bit surprised that has favorite compiler didn't perform that optimization, and it took him some investigation to see why. I don't think he truly expects smart pointers to be "zero-cost abstractions", I think he was just surprised that his compiler didn't optimize better that particular situation and had to dig in to find why. I still find the whole presentation a bit of a misleading stunt - there _are_ zero-cost abstractions (for some definitions of "cost" of course; and/or in some situations). E.g. in rust [1]. And even in C++ - a local unique_ptr is _probably_ a zero-cost abstraction!

BTW.. in his example - just use move semantics, put the pointer as the 5th argument, and you'll probably have the same runtime cost (ie both the raw pointer and smart pointer methods will compile to same code; since the ABI no longer allows you to pass the raw pointer via registers, it goes to the stack, so you have the additional load that bothered him in the raw pointer case, too).

[1] https://medium.com/ingeniouslysimple/rust-zero-cost-abstract...

> BTW.. in his example - just use move semantics, put the pointer as the 5th argument, and you'll probably have the same runtime cost

He talks about this. C++ doesn't have destructive moves like rust does, which is the root cause of why you cannot make the unique_ptr cost zero. It'd take an ABI change to fix this. This is precisely why the scenario is interesting. A lot of people (including you) thought that "just use move semantics" would solve it.

Thing is... talks like these give a wide body of application developers ammunition to say "smart pointers are slow, we need to use raw pointers!". Which is incredibly damaging. In many real-life situations, the difference literally doesn't matter (one more memory move? pfft... I'd take the whole complicated, exception-safe function body over knowing that in case of an exception I won't have a memory leak)

There are indeed cases where this sort of difference in performance matters; But those are a vanishingly small group of people; and as far as those people are concerned - they typically look at assembly anyway.

> smart pointers are slow, we need to use raw pointers!

He addresses this in the QA at the end. Chandler explicitly says "I still believe you should use unique_ptr and pass it by value". If somebody watches this video and takes away "don't use smart pointers" then they weren't paying attention.

I've been wondering why they don't permit destructor elision after a move into a newly constructed parameter. I have a hard time seeing how this would break reasonable code, and even then, we can probably find a backward compatible workaround (or rely on a compiler flag). Any idea?

ABI. C++ defines rigorous calling conventions that don't permit this. There are some on the committee who want to permit ABI breaks to enable these sorts of optimizations (in this case, destructive-move) but the committee in general has been super hesitant to permit ABI breaks.

It won't break reasonable code. The problem is it breaks interop with compiled binaries.

I'm confused, this seems unrelated to ABI. It's entirely up to the caller whether to subsequently destroy the moved-from object or to skip doing so. Whether or not the caller does so is irrelevant to the call or how the parameter gets passed, and it wouldn't affect the moved-to object (whose destructor always runs). The destruction of the moved-from object (if not elided) happens long after the call has returned - at the end of the object's scope.

I think you're confusing my proposal with the pass-in-register proposal? Or maybe I'm missing something.