Urgent:fstream error

[mohit.choudhary]

void returnDirSize(int& size)
{
	string s1,s2,s3;
	ifstream inp;
	ofstream output;
                inp.open("dir.txt");
                
                if(!inp)
	{	
                     cout<<"unable to open"<<endl;
	     exit(1);
	}
	while(inp >> s1 >> s2 >> s3)
               {
	      size++;
	}
	inp.close();
}

It shows an error:
binary '>>' : no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion).
It works fine elsewhere
Thank you

[mohit.choudhary]

Sorry got it. forgot to include string.h