C++ Unordered_map Library - unordered_map() Function

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Description

The C++ function std::unordered_map::unordered_map() constructs an empty unordered_map with zero elements.

Declaration

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

C++11

explicit unordered_map(size_type n = /* implementation defined */,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& alloc = allocator_type()
                      );

Parameters

• 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

Constant i.e. O(1)

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> um;

   cout << "Size of unordered_map = " << um.size() << endl;

   return 0;
}

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

Size of unordered_map = 0