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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_map_size.htm below:

C++ Map Library - size() Function

The C++ function std::map::size() returns the number of elements present in the map.

Following is the declaration for std::map::size() function form std::map header.

size_type size() const;

size_type size() const noexcept;

None

Returns the actual objects present in map.

This member function never throws exception.

Constant i.e. O(1)

The following example shows the usage of std::map::size() function.

#include <iostream>
#include <map>

using namespace std;

int main(void) {
   map<char, int> m;

   cout << "Initial size of map = " << m.size() << endl;

   m = {
      {'a', 1},
      {'b', 2},
      {'c', 3},
      {'d', 4},
      {'e', 5},
      };

     cout << "Size of map after inserting elements = " << m.size() << endl;

   return 0;
}

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

Initial size of map = 0
Size of map after inserting elements = 5

map.htm

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