Back in the DOS days, the IDE I was using provided a cool options that converted tabs to spaces once you saved the file.

Also, in the same years I wrote a small and unoptimized program that converted tabs to a specified number of spaces in text files. Here it is. And yes, it's an exercise to learn how to use fstream classes.

//
// TABEXP.CPP
//

#include 
#include 

// self-explanatory
const unsigned short DEFAULT_TABSIZE = 4;

int main (int NArg, char *const arg[]) {

  // help if invalid syntax is used
  if (NArg < 3) {
    cout << "\nTABEXP - Tab Expander"
         << "\nUsage:"
         << "\n"
         << "\n " << arg[0] << "  infile  outfile  [tabsize]"
         << "\n"
         << "\nTab Expander expands all TABs in infile to  spaces in outfile"
         << "\ninfile must be a text file, default tabsize is " << DEFAULT_TABSIZE
         << "\nWARNING: data loss MAY occur; if you lose data, I'm sorry :)"
         << "\n";
  } else {
    // correct syntax used

    // open input file
    ifstream inFile(arg[1]);

    // error
    if (!inFile.good()) {
      cout << "\nSorry cannot open input file " << arg[1] << " - Exiting...\n";
      return 1;
    }

    // open output file, cannot be overwritten (only at the moment...)
    ofstream outFile(arg[2], ios::out|ios::noreplace);

    // error
    if (!outFile.good()) {
      cout << "\nSorry cannot open output file " << arg[2] << " - Exiting...\n";
      return 1;
    }

    // obvious
    unsigned short tabSize = 0;

    // tabSize is non-zero if third parameter exists and is valid
    if (NArg > 3) tabSize = atoi (arg[3]);

    // decide wether to use default value
    if (tabSize == 0) tabSize = DEFAULT_TABSIZE;

    // print some info on the console
    cout << "\nUsing tabsize = " << tabSize;
    cout << "\nWorking... ";

    // ch is used for input and output; slow but it works well
    char ch;

    // a counter
    int i;

    // read char as long as it is not at EOF
    // if it's a tab, then write 'tabSize' spaces in output
    // if it's not, then copy in in output
    while (!inFile.get(ch).eof()) {
      if (ch == '\t') {
        for (i=0; i < tabSize; i++) {
          outFile.put(' ');
        }
      } else {
        outFile.put(ch);
      }
    }

    // politely close input file and *safely* close output file
    inFile. close();
    outFile.close();

    // if you don't see this, start worrying...
    cout << "operation completed.\n";

  }

  // return 'no-error' errorlevel
  return 0;

}