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

#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
    }
}