how do i write an overload operator>> function of class string?

Question

class string 
{ 
  public: 
        friend istream& operator >> ( istream& is, string& str); 
  private: 
        char *m_data; 
}; 

int main() 
{ 
  string str; 
  freopen("in.txt","r",stdin); 
  while( cin >> str) 
  { 
      cout < < str < < endl; 
  } 
  return 0; 
}

the content of in.txt are:

asdfsfgfdgdfg

in the overload function, i use is.get() to read those charaters one by one,but program jump out the circle when cin finish,that means cout will not run. on the other way,i try getchar() instead, but it can not jump out the circle.

question: is there any wrong in my idea towards this function? or there is another better way to fulfill. thx :)

========================================================================================= new edit: here my code: @Artem Barger code detail

#include <iostream>

namespace Str

{
    class string
    {
    public:
     string():k(0){}

 friend bool operator >> ( std::istream& is, string& str)
 {
  int size = 100;
  char m;

  if( (m = getchar()) && m == -1)
   return false;

  str.m_data = new char[size];

  do
  {
   if( str.k == size)
   {
    size *= 2;
    char *temp = new char[size];
    for( int j = 0; j < str.k; ++j)
    {
     char *del = str.m_data;
     temp[j] = *str.m_data++;
     delete del;
    }
    str.m_data = temp;
    temp = NULL;
   }

   str.m_data[str.k++] = m;
  }while( (m = getchar()) && m != -1);

  return true;
 }

 friend void operator << ( std::ostream& os, string& str)
 {
  os << str.m_data;
  str.k = 0;
  delete []str.m_data;
 }

private:
 char *m_data;
 int k;
};

}

using namespace Str; int main() { string str;

while( std::cin >> str)
{
 std::cout << str;
}

return 0;

}

still have some problem in the content of do { }while();

Answer 1

Perhaps you could rewrite your code like this, which should fix your problem:

bool success = true;

while (sucess) {
  success = cin >> str;
  cout << str;
}

However, I don't understand why you still want the cout to go ahead - if the cin call didn't succeed, you will only be printing the old contents of the string - you do not clear it anywhere (unless you do so in other code which you haven't posted here).