C++ Primer Plus

Figure 2.2. Using cout to display a string.

A First Look at Operator Overloading

If you’re coming to C++ from C, you probably noticed that the insertion operator (<<) looks just like the bitwise left-shift operator (<<). This is an example of operator overloading, by which the same operator symbol can have different meanings. The compiler uses the context to figure out which meaning is intended. C itself has some operator overloading. For example, the & symbol represents both the address operator and the bitwise AND operator. The * symbol represents both multiplication and dereferencing a pointer. The important point here is not the exact function of these operators but that the same symbol can have more than one meaning, with the compiler determining the proper meaning from the context. (You do much the same when you determine the meaning of “sound” in “sound card” versus “sound financial basis.”) C++ extends the operator overloading concept by letting you redefine operator meanings for the user-defined types called classes.

The Manipulator endl

Now let’s examine an odd-looking notation that appears in the second output statement in Listing 2.1:

cout << endl;

endl is a special C++ notation that represents the important concept of beginning a new line. Inserting endl into the output stream causes the screen cursor to move to the beginning of the next line. Special notations like endl that have particular meanings to cout are dubbed manipulators. Like cout, endl is defined in the iostream header file and is part of the std namespace.

Note that the cout facility does not move automatically to the next line when it prints a string, so the first cout statement in Listing 2.1 leaves the cursor positioned just after the period at the end of the output string. The output for each cout statement begins where the last output ended, so omitting endl would result in this output for Listing 2.1:

Come up and C++ me some time.You won't regret it!

Note that the Y immediately follows the period. Let’s look at another example. Suppose you try this code:

cout << "The Good, the";

cout << "Bad, ";

cout << "and the Ukulele";

cout << endl;

It produces the following output:

The Good, theBad, and the Ukulele

Again, note that the beginning of one string comes immediately after the end of the preceding string. If you want a space where two strings join, you must include it in one of the strings. (Remember that to try out these output examples, you have to place them in a complete program, with a main() function header and opening and closing braces.)

The Newline Character

C++ has another, more ancient, way to indicate a new line in output—the C notation \n:

cout << "What's next?\n";    // \n means start a new line

The \n combination is considered to be a single character called the newline character.

If you are displaying a string, you need less typing to include the newline as part of the string than to tag an endl onto the end:

cout << "Pluto is a dwarf planet.\n";         // show text, go to next line

cout << "Pluto is a dwarf planet." << endl;   // show text, go to next line

On the other hand, if you want to generate a newline by itself, both approaches take the same amount of typing, but most people find the keystrokes for endl to be more comfortable:

cout << "\n";   // start a new line

cout << endl;   // start a new line

Typically, this book uses an embedded newline character (\n) when displaying quoted strings and the endl manipulator otherwise. One difference is that endl guarantees the output will be flushed (in, this case, immediately displayed onscreen) before the program moves on. You don’t get that guarantee with "\n", which means that it is possible on some systems in some circumstances a prompt might not be displayed until after you enter the information being prompted for.

The newline character is one example of special keystroke combinations termed “escape sequences”; they are further discussed in Chapter 3, “Dealing with Data.”

C++ Source Code Formatting

Some languages, such as FORTRAN, are line-oriented, with one statement to a line. For these languages, the carriage return (generated by pressing the Enter key or the Return key) serves to separate statements. In C++, however, the semicolon marks the end of each statement. This leaves C++ free to treat the carriage return in the same way as a space or a tab. That is, in C++ you normally can use a space where you would use a carriage return and vice versa. This means you can spread a single statement over several lines or place several statements on one line. For example, you could reformat myfirst.cpp as follows:

#include

     int

main

() {   using

    namespace

         std; cout

            <<

"Come up and C++ me some time."

;    cout <<

endl; cout <<

"You won't regret it!" <<

endl;return 0; }