• Allow different elements having equivalent values
• Keys are immutable, cannot be modified, but can be inserted and removed
• Store elements in no particular order
• Elements are organized into buckets depending on their hash values
• Implemented as hash table

#include <iostream>
#include <unordered_set>

int main(int argc, char *argv[])
{
	std::unordered_multiset<int> s = {1, 2, 3, 4, 4};

	for(int i = 0; i < s.bucket_count(); i++)
	{
		std::cout<<"Bucket "<<i<<": ";
		for(auto it = s.begin(i); it != s.end(i); ++it)
			std::cout<<*it<<" ";
		std::cout<<std::endl;
	}

	return 0;
}
			

#include <iostream>
#include <unordered_set>

int main(int argc, char *argv[])
{
	std::unordered_multiset<int> s = {1, 2, 3, 4, 4};

	std::cout<<"Size: "<<s.size()<<std::endl;//5

	s.insert(3);// 1 2 3 3 4 4

	auto it = s.find(3);
	s.erase(it);// 1 2 3 4 4
	s.erase(4);//1 2 3

	for(int i = 0; i < s.bucket_count(); i++)
	{
		std::cout<<"Bucket "<<i<<": ";
		for(auto it = s.begin(i); it != s.end(i); ++it)
			std::cout<<*it<<" ";
		std::cout<<std::endl;
	}

	return 0;
}
			

//Rectangle.h
#ifndef RECTANGLE_H
#define RECTANGLE_H
class Rectangle
{
	private:
		double width;
		double length;
	public:
		//constructor
		Rectangle(){}
		Rectangle(double w, double l):width(w), length(l){}

		//accessor
		double getWidth() const;
		double getLength() const;
		double getArea() const;

		//mutator
		void setWidth(double w);
		void setLength(double l);
};
#endif

//Rectangle.cpp
#include "Rectangle.h"

double Rectangle::getWidth() const
{
	return width;
}

double Rectangle::getLength() const
{
	return length;
}

double Rectangle::getArea() const
{
	return width*length;
}

void Rectangle::setWidth(double w)
{
	width = w;
}

void Rectangle::setLength(double l)
{
	length = l;
}

//main.cpp
#include <iostream>
#include <functional>
#include <unordered_set>
#include "Rectangle.h"

struct comp{
	bool operator()(const Rectangle &a, const Rectangle &b) const
	{
		return (a.getArea() == b.getArea());
	}
};

struct RectangleHash
{
	size_t operator()(const Rectangle &r) const
	{
		std::hash<double> h;
		return (h(r.getWidth()) ^ h(r.getLength()));
	}
};

template <class T>
void display(T it, T end)
{
	while(it != end)
	{
		std::cout<<it->getArea()<<" ";
		std::advance(it, 1);
	}
	std::cout<<std::endl;
}

int main(int argc, char *argv[])
{
	std::unordered_multiset<Rectangle, RectangleHash, comp> s;
	s.insert(Rectangle(1, 2));
	s.insert(Rectangle(2, 3));
	s.insert(Rectangle(3, 4));
	s.insert(Rectangle(4, 5));
	s.insert(Rectangle(1, 2));

	std::cout<<"Size: "<<s.size()<<std::endl;

	for(int i = 0; i < s.bucket_count(); i++)
	{
		std::cout<<"Bucket "<<i<<": ";
		for(auto it = s.begin(i); it != s.end(i); ++it)
			std::cout<<it->getArea()<<" ";
		std::cout<<std::endl;
	}

	display(s.begin(), s.end());

	return 0;
}
			

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