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Showing content from https://cplusplus.com/reference/map/map/begin/ below:

public member function

<map>

std::map::begin 
      iterator begin();const_iterator begin() const;
      iterator begin() noexcept;const_iterator begin() const noexcept;

Return iterator to beginning

Returns an iterator referring to the first element in the map container.

Because map containers keep their elements ordered at all times, begin points to the element that goes first following the container's sorting criterion.

If the container is empty, the returned iterator value shall not be dereferenced.



Parameters none

Return Value An iterator to the first element in the container.

If the map object is const-qualified, the function returns a const_iterator. Otherwise, it returns an iterator.

Member types iterator and const_iterator are bidirectional iterator types pointing to elements (of type value_type).


Notice that value_type in map containers is an alias of pair<const key_type, mapped_type>.

Example 
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// map::begin/end
#include <iostream>
#include <map>

int main ()
{
  std::map<char,int> mymap;

  mymap['b'] = 100;
  mymap['a'] = 200;
  mymap['c'] = 300;

  // show content:
  for (std::map<char,int>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
    std::cout << it->first << " => " << it->second << '\n';

  return 0;
}

Output: 
a => 200
b => 100
c => 300


Complexity Constant.

Iterator validity No changes.

Data races The container is accessed (neither the const nor the non-const versions modify the container).
No contained elements are accessed by the call, but the iterator returned can be used to access or modify elements. Concurrently accessing or modifying different elements is safe.

Exception safetyNo-throw guarantee: this member function never throws exceptions.
The copy construction or assignment of the returned iterator is also guaranteed to never throw.

See also
map::end
Return iterator to end (public member function)
map::rbegin
Return reverse iterator to reverse beginning (public member function)
map::rend
Return reverse iterator to reverse end (public member function)

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