CS253 I/O Streams

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?

If the left operand is an output stream (or the result of another insertion) then it’s 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.
istream::get(), 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 istream::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, istream::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 istream::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 istream::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 istream::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
►

istream::eof() doesn’t ask “Will the next read hit end-of-file?”
Instead, istream::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
1755102.26 20761612.30

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

► 1.7551e+06
► 2.07616e+07

Now with error checking!

% cat /proc/uptime
1755102.27 20761612.37

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";

► 1.7551e+06
► 2.07616e+07

Now with error checking!

% cat /proc/uptime
1755102.28 20761612.43

ifstream in("/proc/uptime");
if (!in) {  // Is the stream in a bad state?
    cerr << "Can’t open /proc/uptime\n";
    return 1;
}

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 << "File not full of ints.  😠\n";

► 1755102
File not full of ints.  😠

CS253: Software Development with C++

Spring 2021

I/O Streams

CS253 I/O Streams

Predefined Streams

Formatted output:

endl demonstration

endl

Formatted input

Chaining

Read an entire line

Beware

Read a string

Read a character

Read a character

Unreading

Peeking

Stream state

Don’t do this:

The better way to do it

Same for numbers

Now with error checking!

Now with error checking!