r/cpp_questions • u/Expert_Sheepherder24 • 1d ago

OPEN C++26 template for loop std::span<T,N>

Hi, C++26 added "template for" operator , https://cppreference.com/cpp/language/template_for .
GCC 16.2 it supported. It works with std::tuple, std::array, but can't compile with std::span<T,N> fixed size.

How to achieve that, std::span<T,N> also worked with "template for" ?

https://godbolt.org/z/6s9MfzTeb

It be very useful for hand-by unroll loop.

I try specialize std::tuple_size, std::tuple_element, std::get for std::span<T,N> , but it don't helped.

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3

u/SyntheticDuckFlavour 1d ago

As far as I can tell, this is only possible if the span itself is a constant expression. It needs to view a static storage of constexpr data.

3

u/IyeOnline 1d ago

If you mark sp as constexpr static it compiles.

To me, that suggests that this is a compiler bug/limitation, but I have not read the wording on template for.

1

u/TotaIIyHuman 1d ago 
constexpr std::span<const int, 3> sp{arr};
static constexpr auto seq{std::make_index_sequence<3>{}};
template for (auto x : seq)
    std::println("{}", sp[x]);