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See this page as a slide show

CS253 I Ostreams

Predefined Streams

There are four predefined streams. Don’t open or close them. Just use them.

• cin: standard input
• cout: standard output (for normal output)
• cerr: standard error (for error output)
• clog: standard error (for error output)

cerr is unbuffered, clog is buffered. Use cerr for error messages, clog if you use standard error for logging purposes.

Formatted output:

The insertion operator, <<, is used for output. You may recognize it as the left shift operator. It’s that, too. Isn’t operator overloading wonderful?

int i = 5<<4;
double d = 4.5;
char c = 'x';
const char *ccs = "My dog";
string s = " has fleas";

cout << i << ' ' << d << " " << c << "\n"
     << ccs << s << '\n';

80 4.5 x
My dog has fleas

How can you tell if << means bit shift or insertion?

endl demonstration

static int zero = 0;
cout << "Alpha" << '\n' << 1/zero << '\n';

SIGFPE: Floating point exception

static int zero = 0;
cout << "Beta" << '\n';
cout << 1/zero << '\n';

SIGFPE: Floating point exception

static int zero = 0;
cout << "Gamma" << endl;
cout << 1/zero << '\n';

Gamma
SIGFPE: Floating point exception

static int zero = 0;
cout << "Delta" << endl << 1/zero << '\n';

Delta
SIGFPE: Floating point exception

endl

cout << "alpha\n";
cout << "beta" << endl;
cout << "gamma" << "\n";
cout << "delta" << '\n';

alpha
beta
gamma
delta

• Some C++ programmers think that endl and \n are synonymous. They are not.
• \n means: Add a newline ('\n') to the output.
• endl means: Add a newline to the output, and flush the output buffer.
• Don’t use endl unless you really want to flush. It’s not free, and it confuses those who know what it does.

Formatted input

The extraction operator, >>, is used for input. You may recognize it as the right shift operator. It’s that, too. Isn’t operator overloading wonderful?

    int i;
    cin >> i;		    // attempt to read an integer
    if (cin)		    // Is the stream in a happy state?
        cout << "Read i=" << i << '\n';

-or-

    if (!(cin >> i))	    // Could we read?
        cerr << "Input failed!\n";

Chaining

Input may be chained, just like output:

    int a, b, c;
    if ((cin>>a) && (cin>>b) && (cin>>c))
        cout << "a=" << a << " b=" << b << " c=" << c << '\n';

-or-

    int a, b, c;
    if (cin >> a >> b >> c)
        cout << "a=" << a << " b=" << b << " c=" << c << '\n';

The && forces left-to-right evaluation, so the numbers are read in the proper order.

Read an entire line

Consider this small file:

$ cat ~cs253/pub/ducks
Huey (red)
Dewey (blue)
Louie (green)

To read an entire line, use getline():

ifstream in("/s/bach/a/class/cs253/pub/ducks");
string line;
while (getline(in, line))
    cout << line << '\n';

Huey (red)
Dewey (blue)
Louie (green)

Read a string

Consider this small file:

$ cat ~cs253/pub/ducks
Huey (red)
Dewey (blue)
Louie (green)

Extracting a string via >> only reads a whitespace-delimited string.

ifstream in("/s/bach/a/class/cs253/pub/ducks");
string s;
while (in >> s)
    cout << s << '\n';

Huey
(red)
Dewey
(blue)
Louie
(green)

Read a character

Consider this small file:

$ cat ~cs253/pub/ducks
Huey (red)
Dewey (blue)
Louie (green)

To read a raw char, without skipping whitespace, use get():

ifstream in("/s/bach/a/class/cs253/pub/ducks");
char c;
while (in.get(c))
    cout << c;

Huey (red)
Dewey (blue)
Louie (green)

Unlike getline in the previous slide, get is a method.

Read a character

Consider this small file:

$ cat ~cs253/pub/ducks
Huey (red)
Dewey (blue)
Louie (green)

Extracting a char via >> only reads a whitespace-delimited character, which is rarely useful. That is, it skips whitespace.

ifstream in("/s/bach/a/class/cs253/pub/ducks");
char c;
while (in >> c)
    cout << c;

Huey(red)Dewey(blue)Louie(green)

Unreading

$ cat ~cs253/pub/ducks
Huey (red)
Dewey (blue)
Louie (green)

To put a character back, use .unget():

ifstream in("/s/bach/a/class/cs253/pub/ducks");
char c;
in >> c;
cout << "First character: " << c << '\n';
in.unget();     // no argument
string s;
in >> s;
cout << "First string: " << s << '\n';

First character: H
First string: Huey

Stream state

• A stream is put into an error state if something, such as reading a number, goes wrong, including EOF (end-of-file).
• Test the state by evaluating the stream as an boolean value: happy=true, error=false.
• To tell the difference between bad input and end-of-file, use cin.eof()

Misuse of eof

while (!cin.eof()) {
    char c = 'X';
    cin.get(c);
    cout << "Read '" << c << "'\n";
}

Read 'X'

while (!cin.eof()) {
    string line = "bogus";
    getline(cin, line);
    cout << "Read '" << line << "'\n";
}

Read ''

while (!cin.eof()) {
    int n = 42;
    cin >> n;
    cout << "Read " << n << "\n";
}

Read 42

Don’t do this:

    int n;
    while (!cin.eof()) {		// BAD CODE!
        cin >> n;
        ...
    }

• cin.eof() does not mean “Will the next read hit end-of-file?”
• Instead, cin.eof() means “Did we already hit end-of-file?”
• That is, it doesn’t predict what the next read will do.
• It tells you what the previous read did.

The better way to do it

Here’s a better way to do it:

    int n;

    while (cin >> n) {
        …
    }

Now with error checking!

    int n;

    while (cin >> n) {
        …
    }

    if (!cin.eof())
        cerr << "Read failed, but not at eof.  Must be non-numeric data.\n";

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