In file I/O, sometimes we want to input the name of a file from the keyboard. This can be done using a string (object).

1. A string is an object containing a sequence of characters.
   
   
2. We need to declare a variable to hold a string. This can be done by the declaration:
   
   string filename;
   
   which can hold a string containing any number of characters.
   
   
3. Strings will be dealt with in greater depth in ch. 8.
   
   
4. There is one complication when a string is used to store the name of a file which will be attached to a file for input or output. This is due to the fact that the argument of the member function open must be a cstring and not a string object.
   
   
5. Fortunately a string object has a member function c_str( ) which can be used to convert the string object into a cstring.
   
   
6. An example is shown as follows:
   //Read three numbers from the file specified by the user, add up the
   //numbers, and write the sum to another file specified by the user.
   #include <fstream>
   #include <iostream>
   #include <string>
   #include <cstdlib>
   using namespace std;
   
   int main( )
   {
       int first, second, third;
       // declare 2 character arrays for the input and output file names
       string in_file_name, out_file_name;
       ifstream in_stream;
       ofstream out_stream;
   
       cout << "I will sum three numbers taken from an input\n"
             << "file and write the sum to an output file.\n";
       cout << "Enter the input file name (with no spaces):\n";
       cin >> in_file_name;
       cout << "Enter the output file name (with no spaces):\n";
       cin >> out_file_name;
       cout << "I will read numbers from the file "
             << in_file_name << " and\n"
             << "place the sum in the file "
             << out_file_name << endl;
   
       in_stream.open(in_file_name.c_str( ));
       if (in_stream.fail( ))
          {
             cout << "Input file opening failed.\n";
             exit(1);
          }
   
       out_stream.open(out_file_name.c_str( ));
       if (out_stream.fail( ))
          {
             cout << "Output file opening failed.\n";
             in_stream.close( );        // otherwise you exit the program without closing
                                                    // the input file!
             exit(1);
          }
   
       in_stream >> first >> second >> third;
       out_stream << "The sum of the first 3\n"
             << "numbers in " << in_file_name << endl
             << "is " << (first + second + third)
             << endl;
   
       in_stream.close( );
       out_stream.close( );
   
       cout << "End of Program.\n";
       return 0;
   } //end main