C++ Unordered_map Library - begin() Function

Description

The C++ function std::unordered_map::begin() returns an iterator which refers to the first element of the map.

Declaration

Following is the declaration for std::unordered_map::begin() function form std::unordered_map header.

C++11

iterator begin() noexcept;
const_iterator begin() const noexcept;

Parameters

None

Return value

If object is constant qualified then method returns constant iterator otherwise it returns non-constant reference.

Exceptions

This member function never throws exception.

Time complexity

Constant i.e. O(1)

Example

The following example shows the usage of std::unordered_map::begin() function.

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_map<char, int> um = {
            {'a', 1},
            {'b', 2},
            {'c', 3},
            {'d', 4},
            {'e', 5}
            };

   cout << "Unordered map contains following elements: " << endl;

   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;

   return 0;
}

Let us compile and run the above program, this will produce the following result −

Unordered map contains following elements: 
e = 5
a = 1
b = 2
c = 3
d = 4