C++ unordered_map - cbegin() Function

The C++ unordered_map::cbegin function returns the constant iterator (const_iterator) pointing to the first element of the unordered_map container or one of its bucket.

Note: A const_iterator is an iterator that points to constant value. The difference between iterator and const_iterator is that the const_iterator cannot be used to modify the content it points to, even if the map element is not itself constant.

Note: There is no guarantee of a specific element to be considered as the first element of the unordered_map (or the bucket). But the range that goes from begin to end contains all elements of the unordered_map container (or the bucket).

Syntax

• C++11

//container iterator
const_iterator cbegin() const noexcept;

//bucket iterator
const_local_iterator cbegin ( size_type n ) const;

Parameters

Return Value

A const_iterator to the beginning of the unordered_map container or one of its bucket.

Time Complexity

Constant i.e, Θ(1)

Example:

In the example below, the unordered_map::cbegin function returns the const_iterator pointing to the first element of the unordered_map container or one of its bucket. The unordered_map::cbegin function is often used with unordered_map::cend function to specify a range including all elements of the unordered_map container or one of its bucket.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<string, string> uMap;
  uMap = {{"CAN", "Ottawa"}, {"USA", "Washington"}, {"IND", "Delhi"}};

  cout<<"The uMap contains: ";
  for(auto cit = uMap.cbegin(); cit != uMap.cend(); ++cit)
    cout<<cit->first<<":"<<cit->second<<" ";

  cout<<"\n\nThe uMap's bucket contains: ";
  for(unsigned int i = 0; i < uMap.bucket_count(); i++) {
    cout<<"\nBucket #"<<i<<": ";   
    for(auto cit = uMap.cbegin(i); cit != uMap.cend(i); ++cit) {
      cout<<cit->first<<":"<<cit->second<<" ";
    } 
  } 
  return 0;
}

The output of the above code will be:

The uMap contains: IND:Delhi USA:Washington CAN:Ottawa 

The uMap's bucket contains: 
Bucket #0: 
Bucket #1: 
Bucket #2: 
Bucket #3: 
Bucket #4: 
Bucket #5: 
Bucket #6: 
Bucket #7: 
Bucket #8: 
Bucket #9: 
Bucket #10: USA:Washington 
Bucket #11: IND:Delhi 
Bucket #12: CAN:Ottawa