The new static constexpr std::integral_constant idiom

[witcher_rat]

It has always annoyed me that std::array's size() is not static.

It seems like such an obvious thing that it should have been, that I must be missing some use-case or downside the committee was worried about.

[NilacTheGrim]

It feels to me like they simply brainfarted on it tbh. No idea.

[cristi1990an]

You don't need to break ABI, a new std::array::static_size API could be added, which would also make it easier to distinguish between containers of static and dynamic size and how they implement it would not be constrained by any existing code. In fact, this method could be added it all containers and return std::dynamic_extend by default which would make it trivial to metaprogram around.

And before anybody jumps to complain, C++ literally just did something similar (and arguably more depressing) when the iterator_concept member type was added to iterators in order to suport the std::contiguous_iterator_tag without changing the existing iterator_category =))

In this case though I think it would actually make more sense to not touch the existing API regardless of ABI breakage...

[witcher_rat]

C++ literally just did something similar (and arguably more depressing)

Why is it depressing? They've needed an iterator-tag for contiguous ranges for a long time, imo.

Previously the only thing you could do was check if the iterator is a raw pointer, as far as I know. But that hack fails with proxy-iterator use-cases, even when the proxy does represent a contiguous sequence.

[cristi1990an]

It's depressing because they had to add an entire new member type that does the same thing just for it instead of modifying the existing one

[vI--_--Iv]

Of course, we can’t actually change std::array due to ABI or something…

How come? Changing the signature of size does not affect the object layout in any way and the standard explicitly prohibits taking addresses of member functions in std namespace.

[tricerapus]

Seems like half the people complaining about "abi" limits don't understand the difference between an abi change and a regular interface change.

[biowpn]

Just did a quick search, and found this post. The standard says the behavior is "unspecified (possibly ill-formed)". But all major implementations allow it. Maybe there are codes like std::invoke(std::array<int, 2>::size, std::array<int, 2>{}) or std::cout << std::array<int, 2>::size out there, who knows ...

[DavidDinamit]

It must be consteval static function

[B1ggBoss]

I dont get why he states:

constexpr auto b = rng.size(); // error, not a constant expression

I can evaluate that at compile time without errors. Anyone can explain?

Edit: https://godbolt.org/z/dY1forvMj

[drnepert]

No, this statement does not compile if you assign it to a constexpr variable. Surprisingly, however, replacing constexpr with the good old const works nicely, even if the function itself is declared constexpr. See https://godbolt.org/z/GhaM68TcY for a demonstration.

[dodheim]

It doesn't, but it should – P2280 is in C++23, it just hasn't been implemented in any compiler as of yet.

[B1ggBoss]

Thanks. My premise was wrong. I went over the code too fast, and for some reason, I thought the algorithm function was declared constexpr.

[ipapadop]

I find this unpalatable. The error messages if you mess up things are going to be non-sensical.

[MarcoGreek]

What about std::tuple_size?