std::unordered_map::begin,(3)

NAME
std::unordered_map::begin, - std::unordered_map::begin,

Synopsis
   iterator begin() noexcept; (1) (since C++11)
   const_iterator begin() const noexcept; (2) (since C++11)
   const_iterator cbegin() const noexcept; (3) (since C++11)
   Returns an iterator to the first element of the unordered_map.
   If the unordered_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.

Example
// Run this code
 #include <cmath>
 #include <iostream>
 #include <unordered_map>
 struct Node { double x, y; };
 int main()
 {
     Node nodes[3] = {{1, 0}, {2, 0}, {3, 0}};
     // mag is a map mapping the address of a Node to its magnitude in the plane
     std::unordered_map<Node*, double> mag =
     {
         { nodes + 0, 1 },
         { nodes + 1, 2 },
         { nodes + 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
     }
 }

Possible output:
 The magnitude of (3, 3) is 4.24264
 The magnitude of (1, 1) is 1.41421
 The magnitude of (2, 2) is 2.82843
 The magnitude of (3, 4.24264) is 5.19615
 The magnitude of (1, 1.41421) is 1.73205
 The magnitude of (2, 2.82843) is 3.4641

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