std :: initializer_list和元素的求值顺序 [英] std::initializer_list and order of evaluation of the elements

问题描述

逗号（，）是 std :: initializer_list 吗？

例如：是这个UB：

#include <vector>

int main() 
{
    auto nums = []
    {
        static unsigned x = 2;
        return ( x++ % 2 ) + 1;
    };

    std::vector< int > v{ nums(), nums(), nums(), nums(), nums() };
    // not sure if this is different: (note the additional brackets)
    // std::vector< int > v({ nums(), nums(), nums(), nums(), nums() });
    for( auto i : v )
    {
        std::cout << i;
    }

    return 0;
}

推荐答案

根据C ++ 11第8.5.4节[dcl.init.list]第4段：

According to C++11 § 8.5.4 [dcl.init.list] paragraph 4:

4在br​​aced-init-list的initializer-
initializer-clause，包括从包展开
（14.5.3）的任何结果，按它们出现的顺序被评估。也就是说，
每个值计算和与给定的
initializer子句相关联的副作用在每个值计算之前被排序，并且
副作用与在$ b $中的任何初始化子句相关联b初始化列表的逗号分隔列表。

4 Within the initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack expansions (14.5.3), 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.

据我所知，GCC 4.8.1有一个相对于评估的错误的初始化器。我在这里描述它

As far as I know GCC 4.8.1 has a bug relative to evaluation of initializers. I described it here

http://cpp.forum24.ru/?1-3-0-00000063-000-0-0-1378892425

尽管文本是用Russion编写的，但是可以使用例如google translate来简单地翻译成英语。

Though the text is written in Russion but it can be simply translated in English by using for example google translate.

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