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

来自cppreference.com

 
 
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(C++11)
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(C++20)
 
 
iterator begin() noexcept;
(C++11 起)
const_iterator begin() const noexcept;
(C++11 起)
const_iterator cbegin() const noexcept;
(C++11 起)

返回指向 unordered_map 首元素的迭代器。

unordered_map 为空,则返回的迭代器将等于 end()

range-begin-end.svg

参数

(无)

返回值

指向首元素的迭代器。

复杂度

常数。


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 是映射 Node 地址到其平面中长度的 map
    std::unordered_map<Node *, double> mag = {
        { nodes,     1 },
        { nodes + 1, 2 },
        { nodes + 2, 3 }
    };
 
    // 从 0 到长度更改每个 y 坐标
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
        auto cur = iter->first; // 指向 Node 的指针
        cur->y = mag[cur]; // 亦能使用 cur->y = iter->second;
    }
 
    // 更新并打印每个结点的长度
    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';
    }
 
    // 以基于范围的 for 循环重复上述内容
    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';
        // 注意与上述 std::cout << iter->second << '\n'; 相反,
        // std::cout << i.second << '\n'; 将不会打印更新的长度
    }
}

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

参阅

(C++11)
返回指向末尾的迭代器
(公开成员函数)