# Input and Output

C++ uses a convenient abstraction called streams to perform input and output operations in sequential media such as the screen, the keyboard or a file. A stream is an entity where a program can either insert or extract characters to/from. There is no need to know details about the media associated to the stream or any of its internal specifications. All the programmer needs to know is that streams are a source/destination of characters, and that these characters are provided/accepted sequentially.

The standard library defines a handful of stream objects that can be used to access what are considered the standard sources and destinations of characters by the environment where the program runs.

Stream	Description
`cin`	Standard Input Stream
`cout`	Standard Output Stream
`cerr`	Standard Error Stream
`clog`	Standard Logging Stream

The 'c' in the name refers to "character", because iostreams map values to and from byte (char) representations.

Typically input comes from the keyboard through cin, and "normal" output is sent to the console through cout. Error messages or diagnostics are also sent to the screen but through cerr. C++ uses two output streams so that the user can, when desired, separate them with command line operators and send the two outputs to different destinations.

All output streams (cout, cerr and clog) work exactly the same way. Only their actual destination may be different.

cin is an instance of a class named istream, while cout and cerr are instances of of a class named ostream. All three instances or objects are stored in the standard library, which the linker searches while creating the final executable program.

# Standard Output Stream

In C++, output to the display console is done via cout and the stream insertion operator <<. The << operator inserts the data that follows it into the stream that precedes it.

cout << "Hello World" << endl;

Output

Hello World

The previous example makes use of the endl manipulator which inserts an end-of-line and flushes the output buffer.

Multiple items can be inserted into the coutstream one after another by chaining them with the insertion << operator.

cout << "Hello World" << endl
  << "My name is Nico De Witte" << endl
  << "I am " << 34 << " years of age at this moment" << endl;

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Output

Hello World
My name is Nico De Witte
I am 34 years of age at this moment

While the previous examples demonstrated insertion of literals, its also perfectly possible to print variabel values to the terminal by inserting them into the stream.

int myAge = 34;
double height = 1.86;

cout << "Hello, my name is " << "Nico De Witte" << endl;
cout << "I am " << myAge << " years of age at this moment" << endl;
cout << "BTW. I am about " << height << "m tall." << endl;

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Output

Hello, my name is Nico De Witte
I am 34 years of age at this moment
BTW. I am about 1.86m tall.

Notice that the string literals are enclosed in double quotes ", while the variables and integral literals are not. The double quoting is what makes the difference; when the text is enclosed between them, the text is printed literally; when they are not, the text is interpreted as the identifier of a variable, and its value is printed instead.

Also notice that each line here is constructed using a separate cout statement in comparison to the previous example where the three lines were constructed using a single statement. There is no real difference in the end result and no preference towards one construct.

# Escape Characters

Escape characters are special characters than can be used inside a string. They always are prefixed with a backslash \. Most programming languages have them. The code below shows examples of all these techniques.

cout << "There\tis\ta\ttab\tbetween\teach\tword\n" << endl;
cout << "You can also use quotes here but they need to be escaped:\n" << endl;
cout << "\t\"C makes it easy to shoot yourself in the foot;" << endl;
cout << "\tC++ makes it harder,\n"
        << "\tbut when you do it blows your whole leg off.\"" << endl;
cout << "\t\t\t\t- by Bjarne Stroustrup" << endl;

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Output

There   is      a       tab     between each    word

You can also use quotes here but they need to be escaped:

        "C makes it easy to shoot yourself in the foot;
        C++ makes it harder,
        but when you do it blows your whole leg off."
                                - by Bjarne Stroustrup

The table below lists the most used escape sequences.

Escape Sequence	Description
`\t`	Insert a tab in the text at this point.
`\b`	Insert a backspace in the text at this point.
`\n`	Insert a newline in the text at this point.
`\'`	Insert a single quote character in the text at this point.
`\"`	Insert a double quote character in the text at this point.
`\\`	Insert a backslash character in the text at this point.

# Standard Input Stream

The cin object in C++ is an object of class istream. It is used to accept the input from the standard input device. In most program environments, the standard input is by default the keyboard.

The cin object is used together with the extraction operator >>. This operator is then followed by the variable where the extracted data is to be stored.

Take a look at a small example that requests the age of the user:

cout << "Please enter your age: ";

unsigned int userAge;
cin >> userAge;

cout << "Great. I am also "
  << userAge << " years of age." << endl;

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Output

Please enter your age: 19
Great. I am also 19 years of age.

The extraction operator can actually be used more than once to accept multiple inputs as shown in the next example.

cout << "Please enter two numbers: ";

int numberA;
int numberB;

cin >> numberA >> numberB;

cout << "Sum: " << (numberA + numberB) << endl;

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# For Future Reference

The cin object can also be used with other methods. Some of the commonly used methods are:

cin.get(): Read a single input character
cin.getline(): Reads a stream of characters into the string buffer, It stops when the buffer is filled or when an end-of-line is detected.
cin.read(): Read a number of bytes or until an end-of-line is detected from the stream into a buffer.
cin.ignore(): Ignores a number of characters from the input stream.
cin.eof(): Returns a nonzero value if the end-of-file (eof) is reached.
cin.clear(): Clear error flags on input stream
cin.ignore(): Ignore some characters in the input buffer

More info about these methods can be found at http://www.cplusplus.com/reference/istream/istream/ (opens new window).

# Standard Error Stream

The standard error stream cerr is the default destination for error messages and other diagnostic warnings. Like `cout``, it is usually also directed by default to the text console (generally, on the screen).

Although in many cases both cout and cerr are associated with the same output device (like the console), applications may differentiate between what is sent to cout and what to cerr for the case that one of them is redirected. For example, regular output may be shown in the terminal, while error messages are logged to a file.

cout << "This is normal output" << endl;
cerr << "This is an error" << endl;

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Output

This is normal output
This is an error

Running the previous code does not allow us to differentiate between the output.

However if we run the binary as follows in (works both with PowerShell and bash), the error output can be redirected to a file:

./app 2>error.log
This is normal output

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# Changing the Number System

Sometimes you may require numbers to be inputted or outputted using a different number system such as hexadecimal or octal. This can easily be achieved by setting the correct base using the different manipulators that are available in C++.

Manipulator	Number System	Base Prefix
`dec`	Switch stream to decimal base format	-
`hex`	Switch stream to hexadecimal base format	`0x`
`oct`	Switch stream to octal base format	`0`

Warning - No Binary manipulator

Note that there is no manipulator for the binary system. Almost any type can however be easily converted to binary using the bitset library. More information about bitset can be found here: https://en.cppreference.com/w/cpp/utility/bitset (opens new window).

Consider the following example that outputs an integer value in all three formats.

int value = 15;

cout << "Hexadecimal: " << hex << value << endl;
cout << "Octal: " << oct << value << endl;
cout << "Decimal: " << dec << value << endl << endl;

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Output

Hexadecimal: f
Octal: 17
Decimal: 15

Using setfill and setw, one can for example prefix the number with leading zero's. These manipulators can be used by including the iomanip header.

By default the number format prefix is not shown when outputting values. This can be altered with the showbase and noshowbase manipulators. The showbase manipulator can however give conflicts with other manipulators (such as setfill and setw) so it is often a better option to output this yourself.

# Applied to Standard Input

The number format manipulators can also be applied the standard input stream cin. It will even allow the user to type the number with and without the base prefix.

#include <iostream>

using namespace std;

int main() {
  int value = 0;

  cout << "Please enter a HEX value: ";
  cin >> hex >> value;

  cout << "The value in DEC format: ";
  cout << dec << value << endl;

  return 0;
}

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Output

Please enter a HEX value: 0xAB
The value in DEC format: 171

# Exercises

Try to solve the exercises yourself. Don't go copy pasting other people's solutions.

Mark the exercises using a ✅ once they are finished.

# ❌ Wrong Input

What happens when the user inputs something different than a number. For example a character b or a string Hello? Can you explain what happens? How can we detect this?

cout << "Please enter a number: ";

int number;
cin >> number;

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# ❌ Output Hexadecimal Value

Can you alter the next code snippet so the number is outputted in hexadecimal format, with a leading 0x and a width of 4 hexadecimal digits? Note that this will require the iomanip library to be included (already done for you).

#include <iostream>
#include <iomanip>

using namespace std;

int main() {
  int number = 23;
  cout << "Number in hex format: " << number << endl;

  return 0;
}

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The expected output is:

Output

Number in hex format: 0x0017

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