嵌套静态循环因constexpr无法捕获而无法工作 [英] Nested static loop fails to work due to constexpr uncapturable

问题描述

我有这个 static_loop 构造，用于在循环展开时进行类型调度。

I have this static_loop construct that is used for type dispatching over loop unrolling.

template <std::size_t n, typename F> void static_loop(F&& f) {
    static_assert(n <= 8 && "static loop size should <= 8");
    if constexpr (n >= 8)
        f(std::integral_constant<size_t, n - 8>());
    if constexpr (n >= 7)
        f(std::integral_constant<size_t, n - 7>());
    if constexpr (n >= 6)
        f(std::integral_constant<size_t, n - 6>());
    if constexpr (n >= 5)
        f(std::integral_constant<size_t, n - 5>());
    if constexpr (n >= 4)
        f(std::integral_constant<size_t, n - 4>());
    if constexpr (n >= 3)
        f(std::integral_constant<size_t, n - 3>());
    if constexpr (n >= 2)
        f(std::integral_constant<size_t, n - 2>());
    if constexpr (n >= 1)
        f(std::integral_constant<size_t, n - 1>());
}

template <typename T> constexpr size_t tupleSize(T) { return tuple_size_v<T>; }
struct A {
    int a;
    int b;
    void run() {
        auto ab = std::make_tuple(std::ref(a), std::ref(b));
        static_loop<2>([&](auto i) {
            std::get<i>(ab) = i;
            static_loop<2>([&](auto j) { std::get<i * j>(ab) = i; });
            // static_loop<2>([&, i = std::integral_constant<size_t, i>()](auto j) { std::get<i * j>(ab) = i; });
        });
        std::cout << a << " " << b << std::endl;
    }
};

但是在执行嵌套循环时它不会编译。我假设 i 和 j 都是 constexpr 因此 i * j 在 std :: get<> 中有效，但是，编译器似乎不允许这样做。是否可以在内部lambda中将 i 捕获为constexpr？

However it doesn't compile when doing nested loops. I'd assume i and j are both constexpr thus i * j is valid in std::get<>, however, compiler seems not allowing this. Is it possible to capture i as constexpr in the inner lambda?

完整示例位于 godbolt 以及错误消息。

Full example is at godbolt along with the error messages.

推荐答案

static_loop<2>([&](auto i) {
    std::get<i>(ab) = i;
    static_loop<2>([&](auto j) { std::get<i * j>(ab) }
}

IDE会正确地为您强调错误。 i * j 是两个变量的乘积，它不是编译时常量。

The IDE correctly underlines the error for you. i * jis a multiplication of two variables, it is not a compile-time constant.

如果切换到模板参数，则可以嵌套循环而不是函数参数。

You can nest loops if you switch to template parameters rather than function arguments.

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