c++ - for 语句中的 constexpr

Question

c++17提供 if constexpr ，其中:

the value of condition must be a contextually converted constant expression of type bool. If the value is true, then statement-false is discarded (if present), otherwise, statement-true is discarded

有没有办法在 for 语句中也使用它？要在编译时展开循环？我希望能够做这样的事情:

template <int T>
void foo() {
   for constexpr (auto i = 0; i < T; ++i) cout << i << endl;
}

Answer 1

Is there a way to use this in a for-statement as well? To unroll a loop at compile time? Id like to be able to do something like this

我不这么想。

但如果你能负担得起辅助函数，使用 std::integer_sequence 和未使用的 C 风格整数数组的初始化，从 C++14 开始，你可以执行以下操作

#include <utility>
#include <iostream>

template <int ... Is>
void foo_helper (std::integer_sequence<int, Is...> const &)
 {
   using unused = int[];

   (void)unused { 0, (std::cout << Is << std::endl, 0)... };
 }

template <int T>
void foo ()
 { foo_helper(std::make_integer_sequence<int, T>{}); }

int main ()
 {
   foo<42>();
 }

如果可以使用C++17，就可以避免unused数组，使用折叠，foo_helper()可以简单的写成如下

template <int ... Is>
void foo_helper (std::integer_sequence<int, Is...> const &)
 { ((std::cout << Is << std::endl), ...); }