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Showing content from https://cplusplus.com/unordered_multimap::begin below:

public member type

<unordered_map>

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

Return iterator to beginning

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

Notice that an unordered_multimap object makes no guarantees on which specific element is considered its first element. But, in any case, the range that goes from its begin to its end covers all the elements in the container (or the bucket), until invalidated.



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 An iterator to the first element in the container (1) or the bucket (2).

All return types (iterator, const_iterator, local_iterator and const_local_iterator) are member types. In the unordered_multimap class template, these are forward iterator types.


Local iterators are of the same category as non-local iterators. 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::begin/end example
#include <iostream>
#include <string>
#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.begin(); it != myumm.end(); ++it )
    std::cout << " " << it->first << ":" << it->second;
  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.begin(i); local_it!= myumm.end(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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