您可以在折叠表达式中使用子表达式吗？ [英] Can you use a subexpression within fold expressions?

问题描述

以下合法的折叠表达式吗？

Is the following a legal fold expression?

template <std::size_t N, std::size_t... Ix>
bool in_range(std::index_sequence<Ix...>) {
  return ((Ix < N) && ...);
}

它使用 c声，但不是 gcc

推荐答案

Clang在做正确的事情，即折叠表达式提案如下：

Clang is doing the correct thing, the grammar from the Folding expressions Proposal is as follows:

fold-expression:
      ( cast-expression fold-operator ... )
      ( ... fold-operator cast-expression )
      ( cast-expression fold-operator ... fold-operator cast-expression )

并包含以下适用的措辞这种情况（强调我的）：

and contains the following wording that applies to this case (emphasis mine):

以下形式的表达式（... op e），其中op是一个折叠运算符，称为一元左折。形式为（e op ...）的表达式（其中op是折叠运算符）被称为一元右折叠。一元左折叠和一元右折叠统称为一元折叠。 在一元折叠中，强制转换表达式应包含未扩展的参数包。

An expression of the form (... op e) where op is a fold-operator is called a unary left fold. An expression of the form (e op ...) where op is a fold-operator is called a unary right fold. Unary left folds and unary right folds are collectively called unary folds. In a unary fold, the cast-expression shall contain an unexpanded parameter pack.

和（Ix< N）确实是强制转换表达式，因此这看起来是有效的。我们可以看到到达这里的链，如下节 5 中的语法所示：

and (Ix < N) is indeed a cast-expression, so this looks valid. We can see the chain that gets us there, as follows from the grammar in section 5:

cast-expression -> unary-expression -> postfix-expression -> 
 primary-expression -> (expression)

T.C。指出了以下gcc错误报告 [c ++ 1z]折叠表达式时出现折叠表达式错误，报告了类似的问题，但仍未得到确认。

T.C. pointed out the following gcc bug report [c++1z] "binary expression in operand of fold-expression" error when folding an expression which reports a similar issue but it is still unconfirmed.

在gcc中，对于二进制文件，它似乎也被破坏了左折和右折，例如：

It looks like this is also broken in gcc for binary left and right folds, for example:

return ( (Ix < N) && ... &&  (N < 10) );

和：

return ( (N < 10) && ... &&  (Ix < N) );

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