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Basics of C++
Classes of C++
C++ Programming and Object-Oriented Design
1.5 Basic Input/Output
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:
||standard input stream|
||standard output stream|
||standard error (output) stream|
||standard logging (output) stream|
We are going to see in more detail only
cin(the standard output and input streams);
clogare 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
For formatted output operations,
coutis used together with the insertion operator, which is written as
<<(i.e., two "less than" signs).
<<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
xinto 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,
xis 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:
Multiple insertion operations (<<) may be chained in a single statement:
This last statement would print the text
This is a single C++ statement. Chaining insertions is especially useful to mix literals and variables in a single statement:
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
What cout does not do automatically is add line breaks at the end, unless instructed to do so. For example, take the following two statements inserting into
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:
This is a sentence.This is another sentence.
To insert a line break, a new-line character shall be inserted at the exact position the line should be broken. In C++, a new-line character can be specified as
\n(i.e., a backslash character followed by a lowercase
n). For example:
This produces the following output:
endlmanipulator can also be used to break lines. For example:
This would print:
endlmanipulator 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
coutis (generally) not a fully buffered stream. Still, it is generally a good idea to use
endlonly 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 (开销), 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
For formatted input operations,
cinis 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. For example:
The first statement declares a variable of type
age, and the second extracts from
cina 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
cinis reached, the program will wait for as long as needed until some input is introduced.
The extraction operation on
cinuses the type of the variable after the
>>operator to determine how it interprets the characters read from the input; if it is an integer, the format expected is a series of digits, if a string a sequence of characters, etc.
Please enter an integer value: 702 The value you entered is 702 and its double is 1404.
As you can see, extracting from
cinseems to make the task of getting input from the standard input pretty simple and straightforward. But this method also has a big drawback. What happens in the example above if the user enters something else that cannot be interpreted as an integer? Well, in this case, the extraction operation fails. And this, by default, lets the program continue without setting a value for variable
i, producing undetermined results if the value of
iis used later.
This is very poor program behavior. Most programs are expected to behave in an expected manner no matter what the user types, handling invalid values appropriately. Only very simple programs should rely on values extracted directly from
cinwithout further checking. A little later we will see how stringstreams can be used to have better control over user input.
cincan also be chained to request more than one datum in a single statement:
This is equivalent to:
In both cases, the user is expected to introduce two values, one for variable
a, and another for variable
b. Any kind of space is used to separate two consecutive input operations; this may either be a space, a tab, or a new-line character.
cin and stringsThe extraction operator can be used on
cinto get strings of characters in the same way as with fundamental data types:
cinextraction always considers spaces (whitespaces, tabs, new-line...) as terminating the value being extracted, and thus extracting a string means to always extract a single word, not a phrase or an entire sentence.
To get an entire line from
cin, there exists a function, called
getline, that takes the stream (
cin) as first argument, and the string variable as second. For example:
What's your name? Homer Simpson Hello Homer Simpson. What is your favorite team? The Isotopes I like The Isotopes too!
Notice how in both calls to
getline, we used the same string identifier (
mystr). What the program does in the second call is simply replace the previous content with the new one that is introduced.
The standard behavior that most users expect from a console program is that each time the program queries the user for input, the user introduces the field, and then presses ENTER (or RETURN). That is to say, input is generally expected to happen in terms of lines on console programs, and this can be achieved by using
getlineto obtain input from the user. Therefore, unless you have a strong reason not to, you should always use
getlineto get input in your console programs instead of extracting from
stringstreamThe standard header
<sstream>defines a type called
stringstreamthat allows a string to be treated as a stream, and thus allowing extraction or insertion operations from/to strings in the same way as they are performed on
cout. This feature is most useful to convert strings to numerical values and vice versa. For example, in order to extract an integer from a string we can write:
This declares a
stringwith initialized to a value of
"1204", and a variable of type
int. Then, the third line uses this variable to extract from a
stringstreamconstructed from the string. This piece of code stores the numerical value
1204in the variable called
Enter price: 22.25 Enter quantity: 7 Total price: 155.75
In this example, we acquire numeric values from the standard input indirectly: Instead of extracting numeric values directly from
cin, we get lines from it into a string object (
mystr), and then we extract the values from this string into the variables
quantity. Once these are numerical values, arithmetic operations can be performed on them, such as multiplying them to obtain a total price.
With this approach of getting entire lines and extracting their contents, we separate the process of getting user input from its interpretation as data, allowing the input process to be what the user expects, and at the same time gaining more control over the transformation of its content into useful data by the program.