c++ - C++中的评估顺序初始化数组

Question

我喜欢 c++11 可变参数模板，所以我经常用它写一些小代码。

看这个例子:

#include <cstdio>
#include <type_traits>
#include <vector>

template< typename ... T >
auto make_vector(T ... t ) -> std::vector< typename std::common_type<T...>::type >
{
    std::vector< typename  std::common_type<T...>::type > v;
    v.reserve( sizeof...(T) );

    using list = int[];
    (void)list{ 0, ( (void)v.push_back(std::move(t)) ,0)... };
    //                |/ / / /
    //                --------
    //                 \-- How are evaluated v.push_back()s, sequentially or arbitrary ?
    return v;
}

int main()
{
    auto v = make_vector(2, 3.0, 'a', 7UL );

    for(auto e : v )
      printf("%.2lf ", e);

    printf("\n");

}

问:数组初始化的求值顺序是顺序的还是任意的(或者实现定义的，未定义的行为)？

如果 make_vector 错误，我该如何解决？

Answer 1

它们是按顺序计算的。 C++11 § 8.5.4 [dcl.init.list] 第 4 段:

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.

鉴于 vector 有一个 initializer_list 构造函数，您可以将您的函数简化为:

template <typename ... T>
auto make_vector(T ... t) ->
  std::vector< typename std::common_type<T...>::type >
{
  return { static_cast<typename std::common_type<T...>::type>(t)... };
}

而且不必担心神秘的初始化语义；)