“折叠” C ++ 17之前的模板参数包设置：惯用方法 [英] "Folding" of template parameter packs in pre C++17: idiomatic approach

问题描述

接受的答案 Q& A的可变参数模板包扩展使用了常见的C ++之前版本17 （在折叠表达式之前）对未扩展的模板参数包进行折叠的方法。

The accepted answer of the Q&A Variadic template pack expansion makes use of a common pre-C++17 (prior to fold expressions) approach to "folding" of an unexpanded template parameter pack.

我见过这种技术的几种不同形式；以上面的Q& A为例：

I've seen a few different variations of this technique; taking the Q&A above as an example:

#include <initializer_list>
#include <iostream>
#include <utility>

template <typename T> static void bar(T) {}

template <typename... Args> static void foo1(Args &&... args) {
  using expander = int[];
  // Left-most void to avoid `expression result unused [-Wunused-value]`
  (void)expander{0, ((void)bar(std::forward<Args>(args)), 0)...};
}

template <typename... Args> static void foo2(Args &&... args) {
  int dummy[] = {0, ((void)bar(std::forward<Args>(args)), 0)...};
  // To avoid `unused variable 'dummy' [-Wunused-variable]`
  (void)dummy;
}

template <typename... Args> static void foo3(Args &&... args) {
  // Left-most void to avoid `expression result unused [-Wunused-value]`
  (void)std::initializer_list<int>{((void)bar(std::forward<Args>(args)), 0)...};
}

template <typename... Args> static void foo4(Args &&... args) {
  auto l = {0, ((void)bar(std::forward<Args>(args)), 0)...};
  // To avoid `unused variable 'l' [-Wunused-variable]`
  (void)l;
}

int main() {
  foo1(1, 2, 3, "3");
  foo1();
  foo2(1, 2, 3, "3");
  foo2();
  foo3(1, 2, 3, "3");
  foo3();
  foo4(1, 2, 3, "3");
  foo4();
  return 0;
}

是否所有这些变体（或其他变体）被视为惯用的 ？

Are any of these variations (or other variations) considered "the idiomatic one"? Are there any subtleties/differences between them that one would need to take care with?

std :: initializer_list ，它们之间是否需要细微的区别？该方法不需要在 braced-init-list 中最难以捉摸的 0 ，因为初始化列表可能为空，而数组不得为零（负数）。可能这可能是 foo3 的一个论据（可以说，复杂度稍有降低，但需要额外的 #include ）。 / p>

The std::initializer_list approach does not require the somewhat elusive left-most 0 in the braced-init-list, as an initializer list may be empty, whereas an array may not be zero(/negative)-sized. Possibly this could be an argument for foo3 (arguably slightly less complexity at the cost of an additional #include).

推荐答案

这些变体（或其他变体）是否被视为惯用的？

Are any of these variations (or other variations) considered "the idiomatic one"?

我会说是。

有

Are there any subtleties/differences between them that one would need to take care with?

大多数情况下，它们是等效的，但是

There are mostly equivalent equivalent, but

foo3 和 foo4 需要 #include< initializer_list>

，而 foo1 和 foo2 则没有。

foo3 and foo4 require #include <initializer_list>
whereas foo1 and foo2 don't.

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