头文件

#ifndef STONEWT_H_INCLUDED
#define STONEWT_H_INCLUDED
class Stonewt
{
private:
    enum {Lbs_per_Sto=14};
    double stone;
    double pounds;
    double pds_left;
public:
    Stonewt();
    Stonewt(double pds);
    Stonewt(double st,double p_left);
    ~Stonewt();
    void show();
    operator int() const;
    operator double() const;
    Stonewt operator+(double pds) const;
    friend std::ostream & operator<<(std::ostream &io,Stonewt &t);
    friend Stonewt operator+(double pds, const Stonewt &T)
    {
        std::cout<<"调用友元函数"<<std::endl;
        return T+pds;
    }
};

#endif // STONEWT_H_INCLUDED

#include <iostream>
#include "stonewt.h"
using namespace std;

Stonewt::~Stonewt()
{
    
}

Stonewt::Stonewt()
{
    stone=pounds=pds_left=0;
}

Stonewt::Stonewt(double pds)
{
    cout<<"调用了一个参数的构造函数"<<endl;
    stone=int(pds)/Lbs_per_Sto;
    pds_left=int(pds)%Lbs_per_Sto+pds-int(pds);
    pounds=pds;
}

Stonewt::Stonewt(double st, double p_left)
{
    cout<<"调用了2个参数的构造函数"<<endl;
    stone=st;
    pds_left=p_left;
    pounds=st*Lbs_per_Sto+p_left;
}

void Stonewt::show()
{
    cout<<pounds<<endl;
}

Stonewt::operator int() const
{
    return int(pounds+0.5);
}

Stonewt::operator double() const
{
    return double(pounds);
}

Stonewt Stonewt::operator+(double pds) const
{
    Stonewt temp;
    temp=pounds+pds;
    return temp;
}
/*
Stonewt operator+(double pds, const Stonewt &p)
{
    return p+pds;
}
*/
ostream & operator<<(ostream &io, Stonewt &p)
{
    cout<<"调用<<重载"<<endl;
    io<<p.pounds<<endl;
    return io;

}

#include <iostream>
#include "stonewt.h"

using namespace std;

int main()
{
    Stonewt poppins(9,2.8);
    double p_w = poppins;
    cout<<"p_w="<<p_w<<endl;
    int p_i= poppins;
    cout<<"p_i="<<p_i<<endl;
    Stonewt ny=19.8;
    ny.show();
    Stonewt nm=20.1+ny;
    nm.show();
    cout<<nm<<endl;
    system("pause");
    return 0;
}

分成以上三个文件，VS中是可以运行的，不知道为什么G++编译就出错了
stonewt.h:19:9: error: ‘ostream’ in namespace ‘std’ does not name a type
  friend std::ostream & operator<<(std::ostream &io,Stonewt &t);
         ^
stonewt.h: In function ‘Stonewt operator+(double, const Stonewt&)’:
stonewt.h:22:3: error: ‘cout’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
   ^
stonewt.h:22:36: error: ‘endl’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
                                    ^
main.cpp: In function ‘int main()’:
main.cpp:18:16: error: ‘system’ was not declared in this scope
  system("pause");
                ^
zyf@ubuntu:~/Desktop/test$ g++ stonewt.h stone1.cpp main.cpp
stonewt.h:19:9: error: ‘ostream’ in namespace ‘std’ does not name a type
  friend std::ostream & operator<<(std::ostream &io,Stonewt &t);
         ^
stonewt.h: In function ‘Stonewt operator+(double, const Stonewt&)’:
stonewt.h:22:3: error: ‘cout’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
   ^
stonewt.h:22:36: error: ‘endl’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
                                    ^
zyf@ubuntu:~/Desktop/test$ g++ stonewt.h
stonewt.h:19:9: error: ‘ostream’ in namespace ‘std’ does not name a type
  friend std::ostream & operator<<(std::ostream &io,Stonewt &t);
         ^
stonewt.h: In function ‘Stonewt operator+(double, const Stonewt&)’:
stonewt.h:22:3: error: ‘cout’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
   ^
stonewt.h:22:36: error: ‘endl’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
                                    ^
zyf@ubuntu:~/Desktop/test$ clear

zyf@ubuntu:~/Desktop/test$ g++ stonewt.h
stonewt.h:19:9: error: ‘ostream’ in namespace ‘std’ does not name a type
  friend std::ostream & operator<<(std::ostream &io,Stonewt &t);
         ^
stonewt.h: In function ‘Stonewt operator+(double, const Stonewt&)’:
stonewt.h:22:3: error: ‘cout’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
   ^
stonewt.h:22:36: error: ‘endl’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
                                    ^
zyf@ubuntu:~/Desktop/test$ g++ -i stonewt.h
g++: error: unrecognized command line option ‘-i’
zyf@ubuntu:~/Desktop/test$ g++ stonewt.h stone1.cpp main.cpp
stonewt.h:19:9: error: ‘ostream’ in namespace ‘std’ does not name a type
  friend std::ostream & operator<<(std::ostream &io,Stonewt &t);
         ^
stonewt.h: In function ‘Stonewt operator+(double, const Stonewt&)’:
stonewt.h:22:3: error: ‘cout’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;
   ^
stonewt.h:22:36: error: ‘endl’ is not a member of ‘std’
   std::cout<<"调用友元函数"<<std::endl;

求大神解惑，谢谢

没人吗？

我刚刚在头文件中加了#include<iostream> 程序就不报错了，可是为什么呢？使用ostream时， 我加了实std::的