But the "size of the container" is constexpr, like the condition in if
constexpr, and unlike a regular for loop
template for is codegen based on sizeof...(xs), if constexpr is codegen
based on the condition (roughly)

On Thu, Aug 14, 2025 at 7:06 AM Oliver Hunt via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
>
> On Aug 13, 2025, at 9:50 PM, Oliver Hunt <oliver_at_[hidden]> wrote:
>
>
>
> On Aug 13, 2025, at 9:41 PM, 叶易安 via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
> C++26 `static reflection` proposal wanna to bring a syntax called
> `template for`. We know that C++17 introduced one similar thing called `if
> constexpr`, So why not make the style unified and call it `for constexpr` ?
>
>
> template for is not a constexpr, so this would be incorrect.
>
> `if constexpr` is an if that is guaranteed to be evaluated at compile
> time, template for is just a for loop where each “iteration” may have a
> different type. The type unrolling occurs at compile time, but the body of
> the for loop is not evaluated at compile time, which is what constexpr
> would be saying.
>
>
> Sorry was written super unclearly.
>
> In `If constexpr` the condition is a constexpr, whereas in `template for`
> the object being iterated is not required to be a constexpr, and because
> the body is not a constant it could in principle have runtime controlled
> continue, break, return, throw, (goto? :-O), etc impacting the number of
> iterations (I’m not sure if the current proposal permits that but if not
> it’s an obvious future step).
>
> For example template for permits (directly citing p1306)
>
> template <class... Ts> <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-2>void print_all(Ts... xs) { <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-3> template for (auto elem : {xs...}) { <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-4> std::println("{}", elem); <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-5> } <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-6>}
>
>
> Which is clearly not a constexpr. The equivalent constexpr if - as if we
> were to make `template for` become `for constexpr` - would be
>
> template <class T> <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-2>void print_x(T x) { <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-3> if constexpr (x) { <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-4> std::println("{}", x); <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-5> } <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html#cb4-6>}
>
>
> As you can see the analogous syntax would not be a valid constexpr if.
>
> —Oliver
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