C++ Unordered_multimap Library - unordered_multimap() Function

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Description

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

Declaration

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

C++11

template <class InputIterator>
unordered_multimap(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.

• 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_multimap::unordered_multimap() function.

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_multimap<char, int> umm1 = {
            {'a', 1},
            {'b', 2},
            {'c', 3},
            {'d', 4},
            {'e', 5}
            };
   unordered_multimap<char, int> umm2(umm1.begin(), umm1.end());

   cout << "Unordered multimap contains following elements" << endl;

   for (auto it = umm2.begin(); it != umm2.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 multimap contains following elements
d = 4
e = 5
a = 1
b = 2
c = 3