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Showing content from https://cplusplus.com/reference/unordered_map/unordered_multimap/cbegin/ below:

public member function

<unordered_map>

std::unordered_multimap::cbegin container iterator (1) 
const_iterator cbegin() const noexcept;
bucket iterator (2) 
const_local_iterator cbegin ( size_type n ) const;

Return const_iterator to beginning

Returns a const_iterator pointing to the first element in the unordered_multimap container (1) or in one of its buckets (2).

A const_iterator is an iterator that points to const content. This iterator can be increased and decreased (unless it is itself also const), just like the iterator returned by unordered_multimap::begin, but it cannot be used to modify the contents it points to.



Parameters
n
Bucket number. This shall be lower than bucket_count.
It is an optional parameter that changes the behavior of this member function: if set, the iterator retrieved points to the first element of a bucket, otherwise it points to the first element of the container.
Member type size_type is an unsigned integral type.

Return Value A const_iterator to the first element in the container (1) or the bucket (2).

Both const_iterator and const_local_iterator are member types. In the unordered_multimap class template, these are forward iterator types.


const_local_iterator is an interator of the same category as const_iterator. Their value_type, difference_type, pointer and reference member types are also the same. But the iterators themselves are not necessarily of the same type.

Example 
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// unordered_multimap::cbegin/cend example
#include <iostream>
#include <unordered_map>

int main ()
{
  std::unordered_multimap<std::string,std::string> myumm = {
    {"apple","red"},
    {"apple","green"},
    {"orange","orange"},
    {"strawberry","red"}
  };

  std::cout << "myumm contains:";
  for ( auto it = myumm.cbegin(); it != myumm.cend(); ++it )
    std::cout << " " << it->first << ":" << it->second;  // cannot modify *it
  std::cout << std::endl;

  std::cout << "myumm's buckets contain:\n";
  for ( unsigned i = 0; i < myumm.bucket_count(); ++i) {
    std::cout << "bucket #" << i << " contains:";
    for ( auto local_it = myumm.cbegin(i); local_it!= myumm.cend(i); ++local_it )
      std::cout << " " << local_it->first << ":" << local_it->second;
    std::cout << std::endl;
  }

  return 0;
}

Possible output: 
myumm contains: apple:red apple:green orange:orange strawberry:red
myumm's buckets contain:
bucket #0 contains:
bucket #1 contains:
bucket #2 contains: apple:red apple:green
bucket #3 contains: orange:orange
bucket #4 contains: strawberry:red


Complexity Constant.

Iterator validity No changes.

See also
unordered_multimap::end
Return iterator to end (public member type)
unordered_multimap::cbegin
Return const_iterator to beginning (public member function)
unordered_multimap::find
Get iterator to element (public member function)

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