std::map::begin,std::map::cbegin(3)

NAME
std::map::begin,std::map::cbegin - std::map::begin,std::map::cbegin

Synopsis
   iterator begin(); (1) (noexcept since C++11)
   const_iterator begin() const; (2) (noexcept since C++11)
   const_iterator cbegin() const noexcept; (3) (since C++11)
   Returns an iterator to the first element of the map.
   If the map is empty, the returned iterator will be equal to end().
   range-begin-end.svg

Parameters
   (none)

Return value
   Iterator to the first element.

Complexity
   Constant.

Notes
   libc++ backports cbegin() to C++98 mode.

Example
// Run this code
 #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';
 }

Output:
 1, 1.09
 4, 4.13
 9, 9.24
     Example using a custom comparison function // Run this code
 #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

See also
   end returns an iterator to the end
   cend (public member function)
   (C++11)
   begin
   cbegin returns an iterator to the beginning of a container or array
   (C++11) (function template)
   (C++14)