The example programs of the previous sections provided little interaction with the user, if any at all. They simply printed simple values on screen, but the standard library provides many additional ways to interact with the user via its input/output features. This section will present a short introduction to some of the most useful.
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 we need to know is that streams are a source/destination of characters, and that these characters are provided/accepted sequentially (i.e., one after another).
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 (output) stream
clog standard logging (output) stream
We are going to see in more detail only cout and cin (the standard output and input streams); cerr and clog are also output streams, so they essentially work like cout, with the only difference being that they identify streams for specific purposes: error messages and logging; which, in many cases, in most environment setups, they actually do the exact same thing: they print on screen, although they can also be individually redirected.
Standard output (cout)
On most program environments, the standard output by default is the screen, and the C++ stream object defined to access it is cout.
For formatted output operations, cout is used together with the insertion operator, which is written as << (i.e., two “less than” signs).
cout << “Output sentence”; // prints Output sentence on screen
cout << 120; // prints number 120 on screen
cout << x; // prints the value of x on screen
The << operator inserts the data that follows it into the stream that precedes it. In the examples above, it inserted the literal string Output sentence, the number 120, and the value of variable x into the standard output stream cout. Notice that the sentence in the first statement is enclosed in double quotes (“) because it is a string literal, while in the last one, x is 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. For example, these two sentences have very different results:
cout << “Hello”; // prints Hello
cout << Hello; // prints the content of variable Hello
Multiple insertion operations (<<) may be chained in a single statement:
cout << “This ” << ” is a ” << “single C++ statement”;
cout << “I am ” << age << ” years old and my zipcode is ” << zipcode;
Assuming the age variable contains the value 24 and the zipcode variable contains 90064, the output of the previous statement would be:
I am 24 years old and my zipcode is 90064
cout << “This is a sentence.”;
cout << “This is another sentence.”;
The output would be in a single line, without any line breaks in between. Something like:
cout << “First sentence.\n”;
cout << “Second sentence.\nThird sentence.”;
This produces the following output:
First sentence.
Second sentence.
Third sentence.
Alternatively, the endl manipulator can also be used to break lines. For example:
cout << “First sentence.” << endl;
cout << “Second sentence.” << endl;
This would print:
First sentence.
Second sentence.
The endl manipulator produces a newline character, exactly as the insertion of ‘\n’ does; but it also has an additional behavior: the stream’s buffer (if any) is flushed, which means that the output is requested to be physically written to the device, if it wasn’t already. This affects mainly fully buffered streams, and cout is (generally) not a fully buffered stream. Still, it is generally a good idea to use endl only when flushing the stream would be a feature and ‘\n’ when it would not. Bear in mind that a flushing operation incurs a certain overhead, and on some devices it may produce a delay.
Standard input (cin)
In most program environments, the standard input by default is the keyboard, and the C++ stream object defined to access it is cin.
For formatted input operations, cin is used together with the extraction operator, which is written as >> (i.e., two “greater than” signs). This operator is then followed by the variable where the extracted data is stored.
int age;
cin >> age;
The first statement declares a variable of type int called age, and the second extracts from cin a value to be stored in it. This operation makes the program wait for input from cin; generally, this means that the program will wait for the user to enter some sequence with the keyboard. In this case, note that the characters introduced using the keyboard are only transmitted to the program when the ENTER (or RETURN) key is pressed. Once the statement with the extraction operation on cin is reached, the program will wait for as long as needed until some input is introduced.
