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C++20 added `[[no_unique_address]]`, which lets a `std::is_empty` field alias another field, so long as there is only 1 field of that `is_empty` type. https://g
by celrod 2y ago
C++20 added `[[no_unique_address]]`, which lets a `std::is_empty` field alias another field, so long as there is only 1 field of that `is_empty` type.
https://godbolt.org/z/soczz4c76 https://godbolt.org/z/soczz4c76
That is, example 0 shows 8 bytes, for an `int` plus an empty field.
Example 1 shows two empty fields with the `int`, but only 4 bytes thanks to `[[no_unique_address]]`.
Example 2 unfortunately is back up to 8 bytes because we have two empty fields of the same type...
`[[no_unique_address]]` is far from perfect, and inherited the same limitations that inheriting from an empty base class had (which was the trick you had to use prior to C++20).
The "no more than 1 of the same type" limitation actually forced me to keep using CRTP instead of making use of "deducing this" after adopting c++23: a `static_assert` on object size failed, because an object grew larger once an inherited instance, plus an instance inherited by a field, no longer had different template types.
So, I agree that it is annoying and seems totally unnecessary, and has wasted my time; a heavy cost for a "feature" (empty objects having addresses) I have never wanted.
But, I still make a lot of use of empty objects in C++ without increasing the size of any of my non-empty objects.
C++20 concepts are nice for writing generic code, but (from what I have seen, not experienced) Rust traits look nice, too.
- tialaramex 2y agoIt's probably mean for me to say "empty type" to C++ people because of course just as std::move doesn't move likewise std::is_empty doesn't detect empty types. It can't because C++ doesn't have any. You may need to sit down. An empty type has no values. Not one value, like the unit type which C++ makes a poor job of as you explain, but no values. None at all. Because it has no values we will never be called upon to store one, we can't call functions which take one as a parameter, operations whose result is an empty type must diverge (ie control flow escapes, we never get to use the value because there isn't one). Code paths which are predicated on the value of an empty type are dead and can be pruned. And so on. Rust uses this all over the place. C++ can't express it.
- plasticeagle 2y agoHelp me out here. What is this empty type for? Could you provide an old man with a nice concrete example of this in action? I've used empty types in C++ to mark the end of recursive templates - which I used implement typelists before variadic templates were available. But then you mention being unable to call functions which take an empty type as a parameter. At which point I cease to understand the purpose.
- deleted 2y ago[deleted]
- tialaramex 2y agoI don't know that I'll be able to convince you but I'll give a couple of examples. What is the type of the expression "return x" ? Rust says that's ! pronounced Never, an empty type. This expression never had a value, control flow diverges. So this means we can just use simple type arithmetic to decide that a branch which returns contributed nothing to the type of the expression - it has no possible value. This wasn't a special case, it's just type arithmetic. Ok, lets introduce another. Rust has a suite of conversion traits. From, Into, TryFrom and TryInto. They're chained, so if I implement From<Goose> for Doodad, everybody gets the three other implied conversions. But the Try conversions are potentially fallible, hence the word Try. So they have an error type. Generic Code handling the Error type of potentially failing conversion will thus be written, even if in some cases the conversion undertaken chained back to my From<Goose> code. But wait, that conversation can't fail! Sure enough the chained TryFrom and TryInto produced will have the error type Infallible, which is an Empty Type. So the compiler can trim all the error handling code, it depends upon this value which we know can't exist, therefore it never executes.
- plasticeagle 2y agoGot it. Which of course is equivalent to the statement "I have begun the process of understanding, but do not yet know what I do not know". My old High School teacher used to complain that I claimed understanding long before I actually reached it. Anyway, thank you, and that seems a clever concept. I can't help but think that it's solving a problem that the language itself created - though that it doubtless an artifact of my as-yet limited understanding. So "From" has to return something that might be an error, in some way. Just so that the Try... variants can be generated. And generic callers have to write something to handle that error - though presumably concrete callers do not because of the empty type.
- plorkyeran 2y agovoid is an empty type in C++. It's less useful than it could be, but it does exist.
- tialaramex 2y agovoid isn't a type. If you try to use it as a type you'll be told "incomplete type". People who want void to be a type in C++ (proponents of "regular void") mostly want it to be a unit type. If they're really ambitious they want it to have zero size. Generally a few committee meetings will knock that out of them.
- celrod 2y agoThanks for the clarification.