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One question I have about the double ampersands when applied to types is: do they represent two different concepts depending on usage? Like I know that they ind

by ziml77 3y ago

One question I have about the double ampersands when applied to types is: do they represent two different concepts depending on usage? Like I know that they indicate an rvalue reference, but they're also used for perfect forwarding. Those two concepts don't seem inherently connected to me, so the double ampersands appear to be overloaded here. Is that the case or is there some relation I'm missing?

gumby 3y ago
I think of the rvalue reference as talking about the object itself rather than how you get to it (i.e. the actual bytes on a chip someplace as opposed to its address or a copy of an object). Hence moving, perfect forwarding, etc are roughly-synonymous ways to think of managing that object, maybe just looking at the same semantics from different angles. This is just a conceptualisation, of course (rvalue definition is a little more subtle), but for me as a Lisp programmer it seems natural and intuitive. If you come from a different background or perspective I understand that it can seem weird — again, most languages are not as expressive. The low level nature that is fundamental to C++, even when you are programming at a very high level, which you now conveniently can, is a consequence of it working hard to be a zero-cost-abstraction systems programming language, leads to issues like this one, or to some odd consequences of move constructors. There are alternative points in the solution space. Compare it to go, which explicitly papers over some of these trade offs, making an opinionated choice instead, and being willing to embrace reduced expressive power (so there are some things you just can’t write in it). I don’t like go myself, but that doesn’t mean people who choose it are fools.
magnio 3y ago
Yes. It normally indicates an rvalue reference, but in the context of deducing types (for example, in template instantiation or auto deduction), it can mean a forwarding reference and has a rather special meaning.[1][2] [1]: https://eli.thegreenplace.net/2014/perfect-forwarding-and-universal-references-in-c#reference-collapsing-and-special-type-deduction-for-rvalues https://eli.thegreenplace.net/2014/perfect-forwarding-and-un... [2]: https://en.cppreference.com/w/cpp/language/reference#Forwarding_references https://en.cppreference.com/w/cpp/language/reference#Forward...

by ziml77 3y ago

gumby 3y ago

I think of the rvalue reference as talking about the object itself rather than how you get to it (i.e. the actual bytes on a chip someplace as opposed to its address or a copy of an object). Hence moving, perfect forwarding, etc are roughly-synonymous ways to think of managing that object, maybe just looking at the same semantics from different angles. This is just a conceptualisation, of course (rvalue definition is a little more subtle), but for me as a Lisp programmer it seems natural and intuitive. If you come from a different background or perspective I understand that it can seem weird — again, most languages are not as expressive. The low level nature that is fundamental to C++, even when you are programming at a very high level, which you now conveniently can, is a consequence of it working hard to be a zero-cost-abstraction systems programming language, leads to issues like this one, or to some odd consequences of move constructors. There are alternative points in the solution space. Compare it to go, which explicitly papers over some of these trade offs, making an opinionated choice instead, and being willing to embrace reduced expressive power (so there are some things you just can’t write in it). I don’t like go myself, but that doesn’t mean people who choose it are fools.

magnio 3y ago

Yes. It normally indicates an rvalue reference, but in the context of deducing types (for example, in template instantiation or auto deduction), it can mean a forwarding reference and has a rather special meaning.[1][2] [1]: https://eli.thegreenplace.net/2014/perfect-forwarding-and-universal-references-in-c#reference-collapsing-and-special-type-deduction-for-rvalues https://eli.thegreenplace.net/2014/perfect-forwarding-and-un... [2]: https://en.cppreference.com/w/cpp/language/reference#Forwarding_references https://en.cppreference.com/w/cpp/language/reference#Forward...