评估顺序初始化数组在c ++ [英] evaluation order initialization array in c++

问题描述

我喜欢c ++ 11可变参数模板，所以我经常写一些小代码。

I like c++11 variadic templates, so I often write some little codes with it.

查看这个例子：

#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");

}

Q：

如果 make_vector 错误，我修复了它？

If make_vector is wrong, how me fix its?

推荐答案

C ++ 11§8.5.4 [dcl.init.list]第4段：

They are evaluated sequentially. C++11 § 8.5.4 [dcl.init.list] paragraph 4:

时， initializer-clause （包括pack
扩展（14.5.3）的任何结果）都在 braced-init- 。

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.

由于向量有 initializer_list 构造函数，您可以将函数简化为：

Given that vector has an initializer_list constructor, you could simplify your function to:

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)... };
}

，而不必担心奥术初始化语义;）

and not have to worry about arcane initialization semantics ;)

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