#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.
Output:
The magnitude of (1, 1) is 1.41421
The magnitude of (2, 2) is 2.82843
The magnitude of (3, 3) is 4.24264
The magnitude of (1, 1.41421) is 1.73205
The magnitude of (2, 2.82843) is 3.4641
The magnitude of (3, 4.24264) is 5.19615
 
[edit] See also
  
 endcend
(C++11)
  returns an iterator to the end 
 (public member function) [edit]
  
 begincbegin
(C++11)(C++14)
  returns an iterator to the beginning of a container or array 
 (function template) [edit]
            
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