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 是一个映射，将 Node 的地址映射到其在平面中的模长
    std::unordered_map<Node*, double> mag =
    {
        { nodes + 0, 1 },
        { nodes + 1, 2 },
        { nodes + 2, 3 }
    };
    // 将每个 y 坐标从 0 更改为模长
    for (auto iter = mag.begin(); iter != mag.end(); ++iter)
    {
        auto cur = iter->first; // 指向 Node 的指针
        cur->y = mag[cur]; // 也可以使用 cur->y = iter->second;
    }
    // 更新并打印每个节点的模长
    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';
    }
    // 使用基于范围的 for 循环重复上述操作
    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：与上面的 std::cout << iter->second << '\n'; 不同，
        // std::cout << i.second << '\n'; 将不会打印更新后的模长
    }
}