[mattnewport]

Two other goals for C++ are 'zero-cost abstractions' and 'leaving no room for a lower level language'. It does better on both of these goals than Common Lisp and they are important reasons for its popularity (along with backwards compatibility and easy interop with C APIs).

[MichaelSalib]

Zero-cost abstractions only exist in a world where you don't highly value language simplicity and comprehensibility.

Simplicity and comprehensibility were things the committee had to give up in order to pretend they had "zero-cost" abstractions. Nothing in life comes free: everything, including all abstractions, comes at some cost.

[steveklabnik]

> Nothing in life comes free: everything, including all abstractions, comes at some cost.

Yes. As a slogan, it is imprecise. But it's always been talking about a very specific kind of cost: runtime costs. You're 100% right about there always being some kind of cost, but the slogan doesn't disagree with you.

(Some prefer "zero-overhead principle" instead to make this a bit more clear.)

[MichaelSalib]

(1) I love your writing.

(2) Even reducing it to runtime costs, it seems a bit nonsensical. Are C++ exceptions a zero cost abstraction? All the googlers I argued with about them would insist that they have unacceptably high runtime costs.

OK, but templates are surely zero (runtime) cost abstractions, right? Unless you start to worry about duplicate code blowing out your instruction cache but if that's a problem, no profiler in the world will ever be able to tell you, so I guess you'll never know just how costly the abstraction is, so you might as well continue believing it is zero...?

[mattnewport]

Zero cost abstraction is not the same thing as a free lunch. It's a goal that using the abstraction will have no runtime cost relative to implementing the same or equivalent functionality manually. It goes hand in hand in C++ with the "don't pay for what you don't use" principle (again talking about runtime performance cost).

When it comes to exceptions, it's generally true on x64 that you don't pay for what you don't use (there's no performance penalty to exception handling if you don't throw) although that hasn't always been true for all platforms and implementations. It's also generally true that you couldn't implement that kind of non local flow control more efficiently yourself, although the value of that guarantee is a little questionable with exception handling.

I'd argue that no language has a really good story for error handling. It's kind of tragic that we have yet to find a good way to deal with errors as an industry IMO. The most promising possible direction I've seen is in some of the possible future extensions to C++ - it's widely recognized as an area for improvement.

Template code bloat is another case of not imposing more cost than if you implemented it yourself and you have pretty good mechanisms in C++ for managing the tradeoffs.

[MichaelSalib]

> When it comes to exceptions, it's generally true on x64 that you don't pay for what you don't use (there's no performance penalty to exception handling if you don't throw) although that hasn't always been true for all platforms and implementations. It's also generally true that you couldn't implement that kind of non local flow control more efficiently yourself, although the value of that guarantee is a little questionable with exception handling.

I'm inclined to agree with you but just about everyone at Google says the opposite and most C++ shops I've seen agree with them. I've made this argument and lost repeatedly. So, it seems like the community can't even agree on which abstractions are zero cost (or maybe whether some zero cost abstractions are actually zero cost?). To the extent that the community itself has no consensus about these things, maybe they're not a marketing slogan that's helpful to use.

[steveklabnik]

1. Thank you!

2. The other reply is right. Zero cost compared to the best possible implementation.

[mattnewport]

Lots of things come with tradeoffs but relative to a certain set of goals and priorities there are design decisions that are net better than others. Not everything is a zero sum game.

In this context 'zero-cost abstractions' refers to zero runtime performance cost and C++ comes closer to achieving that than most other languages. It doesn't mean zero compile time cost or zero implementation complexity cost but both of those things can end up better or worse due to design decisions and quality of implementation. Given that zero cost refers to runtime performance however, the committee is not 'pretending' they have zero cost abstractions.

It is true that simplicity and comprehensibility are not C++'s highest goals / values but they are not ignored or seen as having no value. Indeed they are major topics of discussion when new features are being considered. Sometimes they are in tension with or even in direct conflict with other goals but not always.