std::map<Key,T,Compare,Allocator>::begin, std::map<Key,T,Compare,Allocator>::cbegin

#include <iostream>
#include <map>
 
int main()
{
    std::map<int, float> num_map;
    num_map[4] = 4.13;
    num_map[9] = 9.24;
    num_map[1] = 1.09;
    // Calls num_map.begin() and num_map.end()
    for (auto it = num_map.begin(); it != num_map.end(); ++it)
        std::cout << it->first << ", " << it->second << '\n';
}

#include <cmath>
#include <iostream>
#include <map>
 
struct Point { double x, y; };
 
// Compare the x-coordinates of two Point pointers.
struct PointCmp
{
    bool operator()(const Point* lhs, const Point* rhs) const
    {
        return lhs->x < rhs->x; 
    }
};
 
int main()
{
    // Note that although the x-coordinates are out of order, the
    // map will be iterated through by increasing x-coordinates.
    Point points[3] = {{2, 0}, {1, 0}, {3, 0}};
 
    // mag is a map sending the address of node to its magnitude in the x-y plane.
    // Although the keys are pointers-to-Point, we want to order the map by the
    // x-coordinates of the points and NOT by the addresses of the Points. This
    // is done by using the PointCmp class's comparison method.
    std::map<Point*, double, PointCmp> mag(
        {{points, 2}, {points + 1, 1}, {points + 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.
        // If auto &i : mag was used instead, it will print the updated magnitude.
    }
}