C++ Unordered_map Library - unordered_map() Function

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Description

The C++ function std::unordered_map::unordered_map() constructs an unordered_map with as many elements as in range of first to last.

Declaration

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

C++11

unordered_map(InputIterator first, InputIterator last,
              size_type n = /* Implementation defined */,
              const hasher& hf = hasher(),
              const key_equal& eql = key_equal(),
              const allocator_type& alloc = allocator_type()
             );

Parameters

• first − Input iterator to initial position.

• last − Input iterator to final position.

• n − Maximum number of initial buckets.

• hf − Hash function to use.

• eql − Comparision function object that returns true if provided two container objects are to be considered equal

• alloc − Allocator to use for all memory allocations of this container.

Return value

Constructor never returns value.

Time complexity

Linear i.e. O(n) in average case.

Quadratic i.e. O(n²) in worst case.

Example

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

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_map<char, int> um1 = {
            {'a', 1},
            {'b', 2},
            {'c', 3},
            {'d', 4},
            {'e', 5}
            };

   unordered_map<char, int>um2(um1.begin(), um2.end());

   cout << "Unordered_map contains following elements" << endl;

   for (auto it = um2.begin(); it != um2.end(); ++it)
      cout << it->first << " = " << it->second << endl;

   return 0;
}

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

Unordered_map contains following elements
a = 1
b = 2
c = 3
d = 4
e = 5