标识符“ostream"是未定义的错误

i need to implement a number class that support operator << for output. i have an error: "identifier "ostream" is undefined" from some reason eventhough i included and try also

这里是头文件:

数字.h

#ifndef NUMBER_H
#define NUMBER_H
#include <iostream>
class Number{
public:
//an output method (for all type inheritance from number):
virtual void show()=0;

//an output operator:
friend ostream& operator << (ostream &os, const Number &f);


};

#endif

为什么编译器不识别友元函数中的ostream?

    std::ostream
//  ^^^^^

friend std::ostream& operator << (std::ostream &os, const Number &f);
//     ^^^^^                      ^^^^^

using std::ostream;