空括号调用默认构造函数还是调用std :: initializer_list的构造函数？

问题描述

以下是有效现代C ++ （第55页）的报价：

The following is a quote from Effective Modern C++ (page 55):

我尝试过使用std :: array：

I tried this with std::array:

std::array<int, 10> arr{};

，并收到g ++（版本4.8.2）的警告：

and got the warning from g++ (version 4.8.2):

这是当试图从一个空的 std :: initializer_list 构造 std :: array code>（请参阅的讨论这个警告）。

which is the warning one gets when trying to construct an std::array from an empty std::initializer_list (see Why can I initialize a regular array from {}, but not a std::array for a discussion of this warning).

那么，为什么上面的代码不能解释为调用默认构造函数呢？

So, why isn't the above line of code interpreted as calling the default constructor?

推荐答案

这是因为是一个汇总，因此，这在 section 8.5.4 [dcl.init.list] 其中说：

That is because std::array is an aggregate and therefore aggregate initialization is performed this is covered in the draft C++11 standard section 8.5.4 [dcl.init.list] which says:

• 如果初始化器列表没有元素，T是带有默认构造函数的类类型，则对象为
  值初始化。

• If the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

否则，如果T是聚合，则执行聚合初始化（8.5.1）。

Otherwise, if T is an aggregate, aggregate initialization is performed (8.5.1).

double ad[] = { 1, 2.0 }; // OK
int ai[] = { 1, 2.0 }; // error: narrowing

struct S2 {
  int m1;
  double m2, m3;
};

S2 s21 = { 1, 2, 3.0 }; // OK
S2 s22 { 1.0, 2, 3 }; // error: narrowing
S2 s23 { }; // OK: default to 0,0,0




我们可以看到它是不是一个聚合，然后列表继续说：

and we can see if it is not an aggregate then the list goes on and says:

我们可以确认 std :: array 是部分的集合23.3.2.1 [array.overview] ：

array<T, N> a = { initializer-list };

其中initializer-list是最多N个元素的逗号分隔列表
类型可转换为T。

where initializer-list is a comma-separated list of up to N elements whose types are convertible to T.

节 8.5.1 引用 8.5.1 汇总 [dcl.init.aggr] 并说：

，我们回到 4 这是我们开始的地方。

and we come full-circle back to section 8.5.4 which is where we started.

这篇关于空括号调用默认构造函数还是调用std :: initializer_list的构造函数？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！