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This chapter is from the book

This chapter is from the book

1.2 A First Look at Input/Output

C++ does not directly define any statements to do input or output (IO). Instead, IO is provided by the standard library. The IO library provides an extensive set of facilities. However, for many purposes, including the examples in this book, one needs to know only a few basic concepts and operations.

Most of the examples in this book use the iostream library, which handles formatted input and output. Fundamental to the iostream library are two types named istream and ostream , which represent input and output streams, respectively. A stream is a sequence of characters intended to be read from or written to an IO device of some kind. The term "stream" is intended to suggest that the characters are generated, or consumed, sequentially over time.

1.2.1 Standard Input and Output Objects

The library defines four IO objects. To handle input, we use an object of type istream named cin (pronounced "see-in"). This object is also referred to as the standard input. For output, we use an ostream object named cout (pronounced "see-out"). It is often referred to as the standard output. The library also defines two other ostream objects, named cerr and clog (pronounced "see-err" and "see-log," respectively). The cerr object, referred to as the standard error, is typically used to generate warning and error messages to users of our programs. The clog object is used for general information about the execution of the program.

Ordinarily, the system associates each of these objects with the window in which the program is executed. So, when we read from cin, data is read from the window in which the program is executing, and when we write to cout, cerr, or clog, the output is written to the same window. Most operating systems give us a way of redirecting the input or output streams when we run a program. Using redirection we can associate these streams with files of our choosing.

1.2.2 A Program that Uses the IO Library

So far, we have seen how to compile and execute a simple program, although that program did no work. In our overall problem, we'll have several records that refer to the same ISBN. We'll need to consolidate those records into a single total, implying that we'll need to know how to add the quantities of books sold.

To see how to solve part of that problem, let's start by looking at how we might add two numbers. Using the IO library, we can extend our main program to ask the user to give us two numbers and then print their sum:

    #include <iostream>
    int main()
    {
        std::cout << "Enter two numbers:" << std::endl;
        int v1, v2;
        std::cin >> v1 >> v2;
        std::cout << "The sum of " << v1 << " and " << v2
                  << " is " << v1 + v2 << std::endl;
        return 0;
    }

This program starts by printing

      
   Enter two numbers:

on the user's screen and then waits for input from the user. If the user enters

      
   3 7

followed by a newline, then the program produces the following output:

      
   The sum of 3 and 7 is 10

The first line of our program is a preprocessor directive:

      #include <iostream>

which tells the compiler that we want to use the iostream library. The name inside angle brackets is a header. Every program that uses a library facility must include its associated header. The #include directive must be written on a single line—the name of the header and the #include must appear on the same line. In general, #include directives should appear outside any function. Typically, all the #include directives for a program appear at the beginning of the file.

Writing to a Stream

The first statement in the body of main executes an expression. In C++ an expression is composed of one or more operands and (usually) an operator. The expressions in this statement use the output operator (the << operator) to print the prompt on the standard output:

      std::cout << "Enter two numbers:" << std::endl;

This statement uses the output operator twice. Each instance of the output operator takes two operands: The left-hand operand must be an ostream object; the right-hand operand is a value to print. The operator writes its right-hand operand to the ostream that is its left-hand operand.

In C++ every expression produces a result, which typically is the value generated by applying an operator to its operands. In the case of the output operator, the result is the value of its left-hand operand. That is, the value returned by an output operation is the output stream itself.

The fact that the operator returns its left-hand operand allows us to chain together output requests. The statement that prints our prompt is equivalent to

      (std::cout << "Enter two numbers:") << std::endl;

Because (std::cout << "Enter two numbers:") returns its left operand, std::cout, this statement is equivalent to

      std::cout << "Enter two numbers:";
      std::cout << std::endl;

endl is a special value, called a manipulator, that when written to an output stream has the effect of writing a newline to the output and flushing the buffer associated with that device. By flushing the buffer, we ensure that the user will see the output written to the stream immediately.

Using Names from the Standard Library

Careful readers will note that this program uses std::cout and std::endl rather than just cout and endl. The prefix std:: indicates that the names cout and endl are defined inside the namespace named std. Namespaces allow programmers to avoid inadvertent collisions with the same names defined by a library. Because the names that the standard library defines are defined in a namespace, we can use the same names for our own purposes.

One side effect of the library's use of a namespace is that when we use a name from the library, we must say explicitly that we want to use the name from the std namespace. Writing std::cout uses the scope operator (the :: operator) to say that we want to use the name cout that is defined in the namespace std. We'll see in Section 3.1 (p. 78) a way that programs often use to avoid this verbose syntax.

Reading From a Stream

Having written our prompt, we next want to read what the user writes. We start by defining two variables named v1 and v2 to hold the input:

      int v1, v2;

We define these variables as type int, which is the built-in type representing integral values. These variables are uninitialized, meaning that we gave them no initial value. Our first use of these variables will be to read a value into them, so the fact that they have no initial value is okay.

The next statement

      std::cin >> v1 >> v2;

reads the input. The input operator (the >> operator) behaves analogously to the output operator. It takes an istream as its left-hand operand and an object as its right-hand operand. It reads from its istream operand and stores the value it read in its right-hand operand. Like the output operator, the input operator returns its left-hand operand as its result. Because the operator returns its left-hand operand, we can combine a sequence of input requests into a single statement. In other words, this input operation is equivalent to

      std::cin >> v1;
      std::cin >> v2;

The effect of our input operation is to read two values from the standard input, storing the first in v1 and the second in v2.

Completing the Program

What remains is to print our result:

     std::cout << "The sum of " << v1 << " and " << v2
               << " is " << v1 + v2 << std::endl;

This statement, although it is longer than the statement that printed the prompt, is conceptually no different. It prints each of its operands to the standard output. What is interesting is that the operands are not all the same kinds of values. Some operands are string literals, such as

   "The sum of "

and others are various int values, such as v1, v2, and the result of evaluating the arithmetic expression:

   v1 + v2

The iostream library defines versions of the input and output operators that accept all of the built-in types.

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