Copyright © 2016
Robert W. Hasker

Note 13: Streams

zyBook Ch. 9

I/O Streams

• Generally obvious - read textbook
• Reading a std::string (or C string) reads one word:

          int main() {
             string first;
             char second[10];
             cin >> first >> second;
             ...
          }
  

  • If user enters "My name is Charles" we get

            first == "My"
            second == "name"
    

  • What would happen if enter "King Charlemagne"?
• Use getline if want whitespace/multiple words:

          std::string full_name_with_titles;
          getline(cin, full_name_with_titles);
  

  • Reads through next newline, but the newline not placed in the destination
  • To read full text file (up to maximum size of std::string):

            string file_contents;
            getline(cin, file_contents, '\0');
    

• If pass istream, ostream as a function parameter, need to pass it by reference:

          vector<int> read_numbers(istream& is) {
             vector<int> result;
             int number;
             while ( is >> number )
                result.push_back(number);
             return result;
          }
  

I/O State

• Consider above loop reading in a vector of ints:
  • stops when something that's not an integer is entered
• Stream state:
  • goodbit: no errors
  • eofbit: end-of-file reached
  • failbit: logical error
  • badbit: read/write error on i/o operation
• Tests on stream state:
  • .eof(): have reached end-of-file (eofbit set)
  • .fail(): failbit is set
  • .bad(): badbit or failbit is set
  • .good(): only goodbit is set - covers eof, all other failures
• .good(): true if all stream operations succeeded - this is more robust than checking .eof
• better way to write above loop:

          is >> number;
          while ( is.good() ) {
             result.push_back(number);
             is >> number;
          }
    

  • Why not write

            while ( is.good() ) {
               is >> number;
               result.push_back(number);
            }
    

  • Key: if cin >> x; fails, just sets stream state and leaves x unchanged
• .clear(): clear failure state for recovery
• When cast stream to a boolean, this checks !fail()
  • In particular, does not cover eof in all states
  • Avoid while ( cin )

Defining your own read, write operations

• Suppose we have

          class Student { public: std::string& name(); std::string& id();... };
  

  and you want to support

          Student a_student;
          cin >> a_student;
          ... update
          cout << a_student;
  

• Overload >>, <<:

          istream& operator>>(istream& input, Student& stu) {
             getline(input, stu.name);
             input >> stu.id;
             return input;
          }
  
          ostream& operator<<(ostream& output, const Student& stu) {
             output << stu.name << endl;
             output << stu.id;
             return output;
          }
  

Formatting

• Generally, include iomanip, though some formatting is already in iostream
• Print integer as hexadecimal and decimal:

          long num;
          ...
          cout << hex << num << ", decimal " << num;
  

  • hex applies to next item
• scientific, fixed, other formats for floating-point numbers:

          cout << fixed << 1.5e10;  // prints 1500000000
  

• Controlling precision:

          cout.setprecision(4); // all floats printed with up to 4 digits after decimal
  

• Controlling width:

          cout << 'x' << setw(4) << 5; // prints: x   5
          cout << 'y' << setw(4) << '5' << 'z'; // prints: x5   z
  

Other streams

• ifstream: input file stream, derived from istream:

          ifstream mydata("config.txt");  // opens file
          if ( !mydata.good() )
             cerr << "Could not open config.txt" << endl;
          int x; string y;
          mydata >> x >> y;
          if ( mydata.eof() ) cerr << "Unexpected end of file." << endl;
  

• ofstream: output file stream, derived from ostream

          ofstream image("results.dat");  // opens file
          if ( !image.good() )
             cerr << "Could not write to results.dat" << endl;
          image << "Image data: ";
          for(int i = 0; i < 10000; ++i) image << stuff[i];
  

• sstream library: contains classes to convert data into a string:

          #include <sstream>
          using namespace std;
  
          ...
          ostringstream buf;
          buf << fixed << 6.0221109e23
          string converted = buf.str();
  
          istringstream namebuf("Tom Riddle");
          string first, last;
          namebuf >> first >> last;
    

Review

• streams, files, formatting