• Multiple elements can have equivalent values
• Elements are always const
• Can be inserted or removed
• Sorted by a specific order indicated by comparison object

#include <iostream>
#include <set>

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

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

	std::multiset<int> s2(s);

	std::multiset<int> s3(s.begin(), s.end());

	int array[] = {1, 1, 2, 2};
	std::multiset<int> s4(array, array+4);

	display(s4.begin(), s4.end());

	return 0;
}
			

//Rectangle.h
#ifndef RECTANGLE_H
#define RECTANGLE_H
class Rectangle
{
	private:
		double width;
		double length;
	public:
		//constructor
		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 <set>
#include <vector>
#include <typeinfo>
#include <algorithm>
#include "Rectangle.h"

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

void display(const std::multiset<Rectangle, comp> &c)
{
	for(auto it = c.begin(); it != c.end(); ++it)
	{
		std::cout<<it->getArea()<<std::endl;
	}
}

int main(int argc, char *argv[])
{
	std::multiset<Rectangle, comp> s;
	Rectangle r1(1, 2);
	Rectangle r2(2, 3);
	Rectangle r3(1, 2);
	s.insert(r1);
	s.insert(r2);
	s.insert(r3);
	display(s);

	Rectangle r4(1, 2);
	auto it = s.find(r4);
	std::cout<<it->getArea()<<std::endl;

	return 0;
}
			

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