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Use variable from function outside a class in a method inside a class

Hi, I have trouble making my program to use a string from a function outside of the class in a method inside the class. How do I solve this?
When I type a text in the main and then call the method 'int calcAbs ()' I can make the method work. But when I try to give the file name using the function 'void file_name (string in_file) I can't make it work. The two functions are also working as they should. So what have I forgotten to do?

#include <string>
#include <cctype>
#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
 
const int NUMBER_OF_LETTERS = 26;  //A-Z
const int NUMBER_OF_LANGUAGES = 4;
 
class Text
{
private:
  string text ;
  int letters ;
  int other ;
  int n[NUMBER_OF_LETTERS] ;
public:
  Text() ;
  void setText ( string nyText ) ; 
  int calcAbs () ;
  void writeAbs () ;
  void absToRel ( int numb[], double histRel[] ) ;
  void plotRel ( double histRel[] ) ;
  string interpret ( double histRel[] ) ;
} ;
 
void file_name (string &in_file) ;
void read (string &in_file, string &out_string) ;
 
int main()
{
  string text ;
  int numb ;
  Text myText ; 
  
  int number[NUMBER_OF_LETTERS] = { 0 };
  double histRel[NUMBER_OF_LETTERS] = { 0 };   
  
  string in_file ;
  string out_string = "" ; 
  
  file_name (in_file) ;
  read (in_file, out_string) ;
 
  myText.setText ( text ) ;
 
  numb = myText.calcAbs () ;
  myText.writeAbs () ;
  myText.absToRel ( number, histRel ) ;
  myText.plotRel ( histRel ) ;
 
  return 0;
}
// -----------------------------------------------------
Text::Text ()
{
}
// -------------------------------------------------------
void Text::setText ( string newText ) 
{
  text = newText ;
}
// -------------------------------------------------------
int Text::calcAbs ()
{  
  letters = other = 0 ;
 
  for (int i=0; i<(int) text.length(); i++)
  {
    if (isalpha(text.at(i))) letters++ ;
    else other++ ;
  }
  
  cout << "Number of letters in the text is: " << letters << endl ;
 
  for (int i = 0; i < NUMBER_OF_LETTERS; i++ )
       n[i] = 0 ;
 
  for (int i = 0; i < (int) text.length(); i++)
  {
    int index ;
    
    if (text.at(i) >= 'a' && text.at(i) <= 'z')
    {
      index = text.at(i) - 'a'; n[index]++ ;
    }
    if (text.at(i) >= 'A' && text.at(i) <= 'Z')
    {
	    index = text.at(i) - 'A'; n[index]++ ;
    }
  }
  return letters ;
} 
//-----------------------------------------------------------
// The rest of the methods are here...
// Then comes the functions:
//------------------------------------------------------------
void file_name (string &in_file)
{
  string file_ending = ".txt" ;
  
  cout << "Type the name of the file: " << endl ;
  getline ( cin, in_file ) ;
 
  if (in_file.rfind(file_ending) != in_file.length() - 4)
    in_file.append (file_ending) ;
}
// --------------------------------------------------------
void read (string &in_file, string &out_string)
{
  string in_row ;
 
  ifstream fin ( in_file.c_str() ) ;
   
  if ( !fin ) 
  {  
    cout << "The file doesn't exist" << endl ;
    exit ( EXIT_FAILURE ) ;
  } 
 
  while ( getline (fin, in_row) )
  {
    out_string.append(in_row) ;
    out_string.append("\n") ; 
  } 
  text = out_string ;
}

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