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CS253 IOStreams

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

Division by zero is undefined behavior, so none of this is guaranteed.

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():

const string home = getpwnam("cs253")->pw_dir;
ifstream in(home+"/pub/ducks");
for (string line; getline(in, line); )
    cout << "► " << line << '\n';

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

Beware

There are two versions of getline:

• A free function, discussed above, that takes an input stream and a std::string.
• A method of istream that deals with C-style strings.

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.

const string home = getpwnam("cs253")->pw_dir;
ifstream in(home+"/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():

const string home = getpwnam("cs253")->pw_dir;
ifstream in(home+"/pub/ducks");
char c;
while (in.get(c))
    cout << "►" << c;

►H►u►e►y► ►(►r►e►d►)►
►D►e►w►e►y► ►(►b►l►u►e►)►
►L►o►u►i►e► ►(►g►r►e►e►n►)►

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.

const string home = getpwnam("cs253")->pw_dir;
ifstream in(home+"/pub/ducks");
char c;
while (in >> c)
    cout << "►" << c;

►H►u►e►y►(►r►e►d►)►D►e►w►e►y►(►b►l►u►e►)►L►o►u►i►e►(►g►r►e►e►n►)

Unreading

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

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

const string home = getpwnam("cs253")->pw_dir;
ifstream in(home+"/pub/ducks");
char c;
in.get(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

Peeking

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

To look at the next char without consuming it, use .peek():

const string home = getpwnam("cs253")->pw_dir;
ifstream in(home+"/pub/ducks");
char c = in.peek();
cout << "First character: " << c << '\n';
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()

Don’t do this:

% # This file is only one line.
% cat /etc/hostname
beethoven
% wc -l /etc/hostname
1 /etc/hostname

ifstream in("/etc/hostname");
while (!in.eof()) { // BAD! 😖🙀
    string s;
    getline(in, s);
    cout << "► " << s << '\n';
}

► beethoven
► 

• .eof() doesn’t ask “Will the next read hit end-of-file?”
• Instead, .eof() asks “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

% cat /etc/hostname
beethoven

ifstream in("/etc/hostname");
string s;
while (getline(in, s))
    cout << "► " << s << '\n';

► beethoven

Don’t try to predict the future. Just ask for a line, and stop when the answer is “no”.

Same for numbers

% cat /proc/uptime
1776519.63 21014883.58

ifstream in("/proc/uptime");
double d;
while (in >> d)
    cout << "► " << d << '\n';

► 1.77652e+06
► 2.10149e+07

Now with error checking!

% cat /proc/uptime
1776519.63 21014883.65

ifstream in("/proc/uptime");
double d;
while (in >> d)
    cout << "► " << d << '\n';

// Did we stop due to EOF, or bad data?
if (!in.eof())
    cerr << "😠\n";

► 1.77652e+06
► 2.10149e+07

Now with error checking!

% cat /proc/uptime
1776519.64 21014883.70

ifstream in("/proc/uptime");
int n;  // OOPS——should be double
while (in >> n)
    cout << "► " << n << '\n';

// Did we stop due to EOF, or bad data?
if (!in.eof())
    cerr << "😠\n";

► 1776519
😠