C++ Unordered_map Library - size() Function

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Description

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

Declaration

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

C++11

size_type size() const noexcept;

Parameters

None

Return value

Returns the actual objects present in unordered_map.

Exceptions

This member function never throws exception.

Time complexity

Constant i.e. O(1)

Example

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

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_map<char, int> um;

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

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

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

   return 0;
}

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

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