编译出现error C2678: 二进制“>>”: 没有找到接受“std::istream”类型的左操作数的运算符(或没有可接受的转换)，求指教啊

[sinuos]

写了个运算符重载的函数，编译的时候出现了错误： error C2678: 二进制“>>”: 没有找到接受“std::istream”类型的左操作数的运算符(或没有可接受的转换)，

代码

#ifndef _AAA_
#define _AAA_

#include <iostream>

class plural
{
private:
	int m_nReal;
	int m_nImaginary;
public:
	plural();
	plural(int nReal, int nImaginary);
	plural operator+(const plural &b);
	plural operator-(const plural &b);
	plural operator*(const plural &b);
	friend plural operator*(int n, const plural &b);
	friend std::ostream & operator<<(std::ostream &os, plural &b);
	friend std::istream & operator>>(std::istream &is, const plural &b);
};

#endif

#include <iostream>
#include <string>
#include <cmath>

using namespace std;

#include "mytime.h"

plural::plural()
{
	m_nImaginary = 0;
	m_nReal = 0;
}

plural::plural(int nReal, int nImaginary)
{
	m_nImaginary = nImaginary;
	m_nReal = nReal;
}

plural plural::operator*(const plural &b)
{
	plural temp;
	temp.m_nImaginary = this->m_nImaginary * b.m_nImaginary;
	temp.m_nReal = this->m_nReal * b.m_nImaginary;
	return temp;
}


std::istream & operator>>(std::istream &is, const plural &b)
{
	//std::cout<<"Imaginary:";
	if (is >> b.m_nImaginary)
	{
		cout <<"real:";
		is >> b.m_nReal;
	}

	return is;
}


std::ostream & operator<<(std::ostream &os, plural &b)
{
	os<<'('<<b.m_nReal<<','<<b.m_nImaginary<<"i)"<<endl;

	return os;
}

plural operator*(int n, const plural &b)
{
	plural temp;
	temp.m_nImaginary = b.m_nImaginary * n;
	temp.m_nReal = b.m_nReal * n;
	return temp;
}

int main()
{
	plural a(3, 4);
	plural temp;
	cout<<a;
}

[turing-complete]

 friend std::ostream & operator<<(std::ostream &os, const plural &b);
    friend std::istream & operator>>(std::istream &is, plural &b);

[sinuos]

我晕，多谢了