命名空间
变体
操作

std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::cbegin

来自 cppreference.com
< cpp‎ | 容器‎ | 无序映射
 
 
 
 
iterator begin() noexcept;
(1) (自 C++11 起)
const_iterator begin() const noexcept;
(2) (自 C++11 起)
const_iterator cbegin() const noexcept;
(3) (自 C++11 起)

返回指向 unordered_map 的第一个元素的迭代器。

如果 unordered_map 为空,则返回的迭代器将等于 end().

range-begin-end.svg

内容

[编辑] 参数

(无)

[编辑] 返回值

指向第一个元素的迭代器。

[编辑] 复杂度

常数。

[编辑] 示例

#include <cmath>
#include <iostream>
#include <unordered_map>
 
struct Node { double x, y; };
 
int main()
{
    Node nodes[3] = {{1, 0}, {2, 0}, {3, 0}};
 
    // mag is a map mapping the address of a Node to its magnitude in the plane
    std::unordered_map<Node*, double> mag =
    {
        { nodes + 0, 1 },
        { nodes + 1, 2 },
        { nodes + 2, 3 }
    };
 
    // Change each y-coordinate from 0 to the magnitude
    for (auto iter = mag.begin(); iter != mag.end(); ++iter)
    {
        auto cur = iter->first; // pointer to Node
        cur->y = mag[cur]; // could also have used cur->y = iter->second;
    }
 
    // Update and print the magnitude of each node
    for (auto iter = mag.begin(); iter != mag.end(); ++iter)
    {
        auto cur = iter->first;
        mag[cur] = std::hypot(cur->x, cur->y);
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
        std::cout << iter->second << '\n';
    }
 
    // Repeat the above with the range-based for loop
    for (auto i : mag)
    {
        auto cur = i.first;
        cur->y = i.second;
        mag[cur] = std::hypot(cur->x, cur->y);
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
        std::cout << mag[cur] << '\n';
        // Note that in contrast to std::cout << iter->second << '\n'; above, 
        // std::cout << i.second << '\n'; will NOT print the updated magnitude
    }
}

可能的输出

The magnitude of (3, 3) is 4.24264
The magnitude of (1, 1) is 1.41421
The magnitude of (2, 2) is 2.82843
The magnitude of (3, 4.24264) is 5.19615
The magnitude of (1, 1.41421) is 1.73205
The magnitude of (2, 2.82843) is 3.4641

[编辑] 另请参阅

返回指向末尾的迭代器
(公有成员函数) [编辑]
(C++11)(C++14)
返回指向容器或数组开头的迭代器
(函数模板) [编辑]