英文:
assign all values (with the same value) a std:array: brace vs .fill, what's the differences?
问题
如果我需要将相同的值分配给数组中的所有值，大括号赋值和.fill之间有什么区别？
#include <iostream>
#include <array>
int main()
{
std::array<int, 4> array;
array[3] = 2;
array = {0};
std::cout << array[3] << std::endl;
array[3] = 2;
array.fill(0);
std::cout << array[3] << std::endl;
}
它们看起来一样...
英文:
If I need to assign to an array the same values on all values, what's the differences between brace assign and .fill?
#include <iostream>
#include <array>
int main()
{
std::array<int, 4> array;
array[3] = 2;
array = {0};
std::cout << array[3] << std::endl;
array[3] = 2;
array.fill(0);
std::cout << array[3] << std::endl;
}
They looks the same...
答案1
得分: 4
对于std::array<int, 4> array：
array = {0}->0,0,0,0array = {2}->2,0,0,0array = {2,4}->2,4,0,0array = {2,4,27,-3}->2,4,27,-3array = {2,4,27,-3,1}-> 编译错误
编辑：最终找到了一个合适的参考资料（也许我眼瞎，但我在cppreference.com上找不到它）：
来自C++26工作草案：
9.4.2 聚合体[dcl.init.aggr]
3 当聚合体由初始化器列表初始化，如9.4.5所规定，初始化器列表的元素被视为聚合体元素的初始化器。聚合体的显式初始化元素如下确定：
—(3.1) 如果初始化器列表是花括号括起的指定初始化器列表，那么聚合体必须是类类型，每个指示符中的标识符必须命名类的直接非静态数据成员，并且聚合体的显式初始化元素是那些是或包含这些成员的元素。
—(3.2) 如果初始化器列表是花括号括起的初始化器列表，则聚合体的显式初始化元素是初始化器列表中元素的前n个元素，其中n是初始化器列表中的元素数。
—(3.3) 否则，初始化器列表必须为{}，并且没有显式初始化元素。
5 对于非联合聚合体，未显式初始化的每个元素如下初始化：
—(5.1) 如果元素具有默认成员初始化器（11.4），则从该初始化器初始化元素。
—(5.2) 否则，如果元素不是引用，则从空初始化器列表（9.4.5）进行复制初始化。
—(5.3) 否则，程序非法。
9.4.5 列表初始化[dcl.init.list]
[...]
(3.11) 否则，如果初始化器列表没有元素，则对象被值初始化。
我认为这是与此问题相关的所有部分（但我不习惯直接阅读标准，所以欢迎更正）。
英文:
For std::array<int, 4> array:
array = {0}->0,0,0,0array = {2}->2,0,0,0array = {2,4}->2,4,0,0array = {2,4,27,-3}->2,4,27,-3array = {2,4,27,-3,1}-> compile error
EDIT: Finally found a proper reference (perhaps I'm blind, but I cannot find it on cppreference.com):
From the C++26 working draft:
> ### 9.4.2 Aggregates [dcl.init.aggr]
>
> 3 When an aggregate is initialized by an initializer list as specified
> in 9.4.5, the elements of the initializer list are taken as
> initializers for the elements of the aggregate. The explicitly
> initialized elements of the aggregate are determined as follows:
>
> —(3.1) If the initializer list is a brace-enclosed
> designated-initializer-list, the aggregate shall be of class type, the
> identifier in each designator shall name a direct non-static data
> member of the class, and the explicitly initialized elements of the
> aggregate are the elements that are, or contain, those members.
>
> —(3.2) If the initializer list is a brace-enclosed initializer-list,
> the explicitly initialized elements of the aggregate are the first n
> elements of the aggregate, where n is the number of elements in the
> initializer list.
>
> —(3.3) Otherwise, the initializer list must be {}, and there are no
> explicitly initialized elements.
> 5 For a non-union aggregate, each element that is not an explicitly
> initialized element is initialized as follows:
>
> —(5.1) If the element has a default member initializer (11.4), the
> element is initialized from that initializer.
>
> —(5.2) Otherwise, if the element is not a reference, the element is
> copy-initialized from an empty initializer list (9.4.5).
>
> —(5.3) Otherwise, the program is ill-formed.
> ### 9.4.5 List-initialization [dcl.init.list]
>
> [...]
>
> (3.11) Otherwise, if the initializer list has no elements, the object
> is value-initialized.
I think that is all the relevant parts for this question (but I am not used to reading the standard directly so corrections are welcome).
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