c++重载流插入运算符问题?报错

ide为windows下的g++，我重载了流插入运算符<<,在传递参数时出现错误，代码如下：

#include <iostream>

using namespace std;

class number{
 private:
 double a,b,c,d;
 public:
 number(double x1,double x2,double x3,double x4):a(x1),b(x2),c(x3),d(x4){}
 number(){}
 number operator +(number &plu);
 friend number operator *(number &mul,number &mul1);
 number operator --(int);
 friend ostream& operator <<(ostream &,number &);
};

number number::operator +(number &plu)
{
 number num;
 num.a=a+plu.a;
 num.b=b+plu.b;
 num.c=c+plu.c;
 num.d=d+plu.d;
 return num;
}

number operator *(number &mul,number &mul1)
{
 number result;//需要构造函数的重载
 result.a=mul.a*mul1.a;
 result.b=mul.b*mul1.b;
 result.c=mul.c*mul1.c;
 result.d=mul.d*mul1.d;
 return result;
}

number number::operator --(int)
{
 this->a=a--;
 this->b=b--;
 this->c=c--;
 this->d=d--;
 return *this;
}

ostream& operator <<(ostream &output,number &num)
{
 output<<num.a<<endl<<num.b<<endl<<num.c<<endl<<num.d<<endl;
 return output;
}

int main()
{
 number number1(158.1,124.39,332.2,967.03);
 number number2(2.3,32.3,48.4,64.3);
 number number3,number4;
 number3=number1+number2;
 cout<<(number1+number2)<<endl<<(number1*number2)<<endl;

 cout<<number1--<<endl;

 return 0;
}

问题：在int main（）中，cout<<(number1+number2)<<endl<<(number1*number2)<<endl;

 cout<<number1--<<endl;这两句代码有问题，报错是传递参数出错