原文地址:http://www.cplusplus.com/reference/array/array/at/

std::array::at

      reference at ( size_type n );
const_reference at ( size_type n ) const;

Returns a reference to the element at position n in the array.
返回一个指向位置为n的元素的引用。

The function automatically checks whether n is within the bounds of valid elements in the container, throwing an out_of_range exception
if it is not (i.e., if n is greater or equal than its size). This is in contrast with member operator[],
that does not check against bounds.
该方法会自动检测n在该array中是否是一个有效的值，当n大于或者等于它的size的时候，将会抛出out_of_range异常，这和成员方法operato[]是一个对比，因为operator[]不会检测n的范围。例子：

<span style="font-size:18px;">#include <array>
#include <iostream>
using namespace std;
void at(){
    array<int,5> ai{1,2,3,4,5};
    cout<<"ai is ";
    for(int &i:ai)
        cout<<i<<" ";
    cout<<endl;
    cout<<"ai.at(3)="<<ai.at(3)<<endl;
    cout<<"ai.at(10)="<<ai.at(10)<<endl;
}
</span>

运行截图:

Parameters

n

Position of an element in the array.
If this is greater than or equal to the array size, an exception of type out_of_range is
thrown.
Notice that the first element has a position of 0 (not 1).
Member type size_type is an alias of the unsigned integral type size_t.

n为元素在array中的位置。

如果n大于或者等于array的大小，将抛出out_of_range异常。

注意第一个元素的位置是0.

size_type是一个无符号整型。

Return value

The element at the specified position in the array.
返回array中一个在特定位置的元素。

If the array object is const-qualified, the function returns a const_reference. Otherwise, it returns a reference.如果array对象是const属性的，那么返回一个const_reference,否则，返回一个reference。

Member types reference and const_reference are the reference types to the elements of the array (see array
member types).

reference以及constreference都是指向元素类型的引用。

Example

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

// array::at
#include <iostream>
#include <array>

int main ()
{
  std::array<int,10> myarray;

  // assign some values:
  for (int i=0; i<10; i++) myarray.at(i) = i+1;

  // print content:
  std::cout << "myarray contains:";
  for (int i=0; i<10; i++)
    std::cout << ' ' << myarray.at(i);
  std::cout << '\n';

  return 0;
}

Output:

myarray contains: 1 2 3 4 5 6 7 8 9 10

Complexity

Constant.

Iterator validity

No changes.

Data races

The reference returned can be used to access or modify elements. Concurrently accessing or modifying different elements is safe.

返回的引用可以用来访问以及修改元素，并且这些操作都是安全的。

例子：

<span style="font-size:18px;"><span style="color:#cc33cc;">void at2(){
    array<int,5> ai{1,2,3,4,5};
    cout<<"ai is ";
    for(int &i:ai)
        cout<<i<<" ";
    cout<<endl;
    int &r=ai.at(3);
    cout<<"ai.at(3)="<<r<<endl;
    </span><span style="color:#ff0000;">r=15;</span><span style="color:#cc33cc;">
     cout<<"after alter ai is ";
    for(int &i:ai)
        cout<<i<<" ";
    cout<<endl;

}</span></span>

运行截图:

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the container.
It throws out_of_range if n is out of bounds.

<span style="font-size:18px;">#include <array>
#include <iostream>
using namespace std;
void at(){
    array<int,5> ai{1,2,3,4,5};
    cout<<"ai is ";
    for(int &i:ai)
        cout<<i<<" ";
    cout<<endl;
    cout<<"ai.at(3)="<<ai.at(3)<<endl;
    try{
    cout<<"ai.at(10)="<<ai.at(10)<<endl;
    }catch(out_of_range)
    {
        cout<<"catch a out_of_range exceptions"<<endl;
         cout<<"ai is ";
        for(int &i:ai)
            cout<<i<<" ";
        cout<<endl;
    }

}</span>