Copyright © 2016
Robert W. Hasker

Note 11: Streams

Ch. 10

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 == "Charles"

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;               // why bad before c++11?
        }

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

Can overload >>, <<:

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

        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;

ofstream: output file stream, derived from ostream

        ofstream image("results.dat");  // opens file
        if ( !image.good() )
           cerr << "Could not write to results.dat" << endl;

sstream: 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

Copyright © 2016 Robert W. Hasker