c++ - 增加初始化列表中使用两次的变量 - 未定义的行为？

Question

编辑:尚未回答 - 链接的问题是关于普通 r 值的，如果相关的话，初始化列表是一个单独的概念。

这个语句是否定义良好，或者在初始化列表中使用前缀递增运算符，在列表中出现两次的变量上，未定义的行为？

struct T t = { i, ++i };

我对 ANSI C 最感兴趣，但了解其他版本的 C 和/或 C++ 是否不同也会很有用。如果像下面这样的类似结构是合法的:

struct T t = { i, i++ };

struct T t = { ++i, ++i };

struct T t = { i++, ++i };

struct T t = { i++, i++ };

Answer 1

C++11 及更高版本

list initialization 的行为是明确定义的.根据sequenced-before rules (C++11 起):

10) In list-initialization, every value computation and side effect of a given initializer clause is sequenced before every value computation and side effect associated with any initializer clause that follows it in the brace-enclosed comma-separated list of initalizers.

所以对于 struct T t = { i,++i };, i 将首先被评估，然后 ++i，顺序明确。所有其他样本也可以。

引用 C++ 标准，$8.6.4/4 List-initialization[dcl.init.list] :

(强调我的)

Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions ([temp.variadic]), are evaluated in the order in which they appear. That is, every value computation and side effect associated with a given initializer-clause is sequenced before every value computation and side effect associated with any initializer-clause that follows it in the comma-separated list of the initializer-list. [ Note: This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of the initializer-list are interpreted as arguments of a constructor call, even though ordinarily there are no sequencing constraints on the arguments of a call. — end note ]