const 질문입니다

안녕하세요

#include "Cents.h"

using namespace std;

template<typename T>
T getMax(T x, T y)
{
	return (x > y) ? x : y;
}



int main()
{
	cout << getMax(1, 2) << endl;
	cout << getMax(3.14, 1.592) << endl;
	cout << getMax(1.0f, 3.4f) << endl;
	cout << getMax('a','c') << endl;

	std::cout << getMax(Cents(5), Cents(9)) << std::endl;
	return 0;
}

#pragma once

#include <iostream>	

class Cents
{
private:
	int m_cents;

public:
	Cents(int cents)
		:m_cents(cents)
	{}

	friend bool operator > (const Cents &c1, const Cents &c2)
	{
		return (c1.m_cents > c2.m_cents);
	}

	friend std::ostream &operator << (std::ostream &out,  const Cents &cents)
	{
		out << cents.m_cents << " cents";
		return out;
	}

};

전체 코드입니다.

Cents 클래스 내에서 operator > 와 operator <<  함수에서

operator > 함수 파라미터에선 const 없이

friend bool operator > (Cents &c1, Cents &c2)

{

return ( c1.m_cents > c2.m_cents);

}

가 가능 한데

operator << 함수 파라미터에서 const 없이

friend std::ostream &operator << (std::ostream &out, Cents &cents)

{

out << cents.m_cents << " cents";

return out;

}

이렇게 할 경우 에러가 나는데 

const 붙여서 수정 못하게 할 때 붙이는줄 알았는데

어떤 차이 인가요..?

계속 질문드려서 죄송합니다 ㅜ 

template<typename T>
T getMax(T x, T y)
{
	return (x > y) ? x : y;
}