C++ Primer: Dealing with Data

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C++ Primer Plus, 6th Edition

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From the author of

From the author of 

C++ Primer Plus, 6th Edition

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In this chapter you'll learn about the following:

• Rules for naming C++ variables
• C++'s built-in integer types: unsigned long, long, unsigned int, int, unsigned short, short, char, unsigned char, signed char, and bool
• The climits file, which represents system limits for various integer types
• Numeric constants of various integer types
• Using the const qualifier to create symbolic constants
• C++'s built-in floating-point types: float, double, and long double
• The cfloat file, which represents system limits for various floating-point types
• Numeric constants of various floating-point types
• C++'s arithmetic operators
• Automatic type conversions
• Forced type conversions (type casts)

The essence of object-oriented programming (OOP) is designing and extending your own data types. Designing your own data types represents an effort to make a type match the data. If you do this properly, you'll find it much simpler to work with the data later. But before you can create your own types, you must know and understand the types that are built in to C++ because those types will be your building blocks.

The built-in C++ types come in two groups: fundamental types and compound types. In this chapter you'll meet the fundamental types, which represent integers and floating-point numbers. That might sound like just two types; however, C++ recognizes that no one integer type and no one floating-point type match all programming requirements, so it offers several variants on these two data themes. Chapter 4, "Compound Types," follows up by covering several types that are built on the basic types; these additional compound types include arrays, strings, pointers, and structures.

Of course, a program also needs a means to identify stored data. In this chapter you'll examine one method for doing so—using variables. Then, you'll look at how to do arithmetic in C++. Finally, you'll see how C++ converts values from one type to another.

Simple Variables

Programs typically need to store information—perhaps the current price of IBM stock, the average humidity in New York City in August, the most common letter in the U.S. Constitution and its relative frequency, or the number of available Elvis impersonators. To store an item of information in a computer, the program must keep track of three fundamental properties:

• Where the information is stored

• What value is kept there

• What kind of information is stored

The strategy the examples in this book have used so far is to declare a variable. The type used in the declaration describes the kind of information, and the variable name represents the value symbolically. For example, suppose Chief Lab Assistant Igor uses the following statements:

int braincount;
braincount = 5;

These statements tell the program that it is storing an integer and that the name braincount represents the integer's value, 5 in this case. In essence, the program locates a chunk of memory large enough to hold an integer, notes the location, assigns the label braincount to the location, and copies the value 5 into the location. These statements don't tell you (or Igor) where in memory the value is stored, but the program does keep track of that information, too. Indeed, you can use the & operator to retrieve braincount's address in memory. You'll learn about that operator in the next chapter, when you investigate a second strategy for identifying data—using pointers.

Names for Variables

C++ encourages you to use meaningful names for variables. If a variable represents the cost of a trip, you should call it cost_of_trip or costOfTrip, not just x or cot. You do have to follow a few simple C++ naming rules:

• The only characters you can use in names are alphabetic characters, numeric digits, and the underscore (_) character.

• The first character in a name cannot be a numeric digit.

• Uppercase characters are considered distinct from lowercase characters.

• You can't use a C++ keyword for a name.

• Names beginning with two underscore characters or with an underscore character followed by an uppercase letter are reserved for use by the implementation—that is, the compiler and the resources it uses. Names beginning with a single underscore character are reserved for use as global identifiers by the implementation.

• C++ places no limits on the length of a name, and all characters in a name are significant.

The next-to-last point is a bit different from the preceding points because using a name such as __time_stop or _Donut doesn't produce a compiler error; instead, it leads to undefined behavior. In other words, there's no telling what the result will be. The reason there is no compiler error is that the names are not illegal but rather are reserved for the implementation to use. The bit about global names refers to where the names are declared; Chapter 4 touches on that topic.

The final point differentiates C++ from ANSI C (C99), which guarantees only that the first 63 characters in a name are significant. (In ANSI C, two names that have the same first 63 characters are considered identical, even if the 64th characters differ.)

Here are some valid and invalid C++ names:

int poodle;    // valid
int Poodle;    // valid and distinct from poodle
int POODLE;    // valid and even more distinct
Int terrier;   // invalid -- has to be int, not Int
int my_stars3  // valid
int _Mystars3; // valid but reserved -- starts with underscore
int 4ever;     // invalid because starts with a digit
int double;    // invalid -- double is a C++ keyword
int begin;     // valid -- begin is a Pascal keyword
int __fools;   // valid but reserved – starts with two underscores
int the_very_best_variable_i_can_be_version_112; // valid
int honky-tonk;    // invalid -- no hyphens allowed

If you want to form a name from two or more words, the usual practice is to separate the words with an underscore character, as in my_onions, or to capitalize the initial character of each word after the first, as in myEyeTooth. (C veterans tend to use the underscore method in the C tradition, whereas Pascalians prefer the capitalization approach.) Either form makes it easier to see the individual words and to distinguish between, say, carDrip and cardRip, or boat_sport and boats_port.

Integer Types

Integers are numbers with no fractional part, such as 2, 98, –5286, and 0. There are lots of integers, assuming that you consider an infinite number to be a lot, so no finite amount of computer memory can represent all possible integers. Thus, a language can represent only a subset of all integers. Some languages, such as standard Pascal, offer just one integer type (one type fits all!), but C++ provides several choices. This gives you the option of choosing the integer type that best meets a program's particular requirements. This concern with matching type to data presages the designed data types of OOP.

The various C++ integer types differ in the amount of memory they use to hold an integer. A larger block of memory can represent a larger range in integer values. Also, some types (signed types) can represent both positive and negative values, whereas others (unsigned types) can't represent negative values. The usual term for describing the amount of memory used for an integer is width. The more memory a value uses, the wider it is. C++'s basic integer types, in order of increasing width, are char, short, int, and long. Each comes in both signed and unsigned versions. That gives you a choice of eight different integer types! Let's look at these integer types in more detail. Because the char type has some special properties (it's most often used to represent characters instead of numbers), this chapter covers the other types first.

The short, int, and long Integer Types

Computer memory consists of units called bits. (See the "Bits and Bytes" sidebar, later in this chapter.) By using different numbers of bits to store values, the C++ types short, int, and long can represent up to three different integer widths. It would be convenient if each type were always some particular width for all systems—for example, if short were always 16 bits, int were always 32 bits, and so on. But life is not that simple. However, no one choice is suitable for all computer designs. C++ offers a flexible standard with some guaranteed minimum sizes, which it takes from C. Here's what you get:

• A short integer is at least 16 bits wide.

• An int integer is at least as big as short.

• A long integer is at least 32 bits wide and at least as big as int.

Many systems currently use the minimum guarantee, making short 16 bits and long 32 bits. This still leaves several choices open for int. It could be 16, 24, or 32 bits in width and meet the standard. Typically, int is 16 bits (the same as short) for older IBM PC implementations and 32 bits (the same as long) for Windows 98, Windows NT, Windows XP, Macintosh OS X, VAX, and many other minicomputer implementations. Some implementations give you a choice of how to handle int. (What does your implementation use? The next example shows you how to determine the limits for your system without your having to open a manual.) The differences between implementations for type widths can cause problems when you move a C++ program from one environment to another. But a little care, as discussed later in this chapter, can minimize those problems.

You use these type names to declare variables just as you would use int:

short score;         // creates a type short integer variable
int temperature;     // creates a type int integer variable
long position;       // creates a type long integer variable

Actually, short is short for short int and long is short for long int, but hardly anyone uses the longer forms.

The three types, int, short, and long, are signed types, meaning each splits its range approximately equally between positive and negative values. For example, a 16-bit int might run from –32,768 to +32,767.

If you want to know how your system's integers size up, you can use C++ tools to investigate type sizes with a program. First, the sizeof operator returns the size, in bytes, of a type or a variable. (An operator is a built-in language element that operates on one or more items to produce a value. For example, the addition operator, represented by +, adds two values.) Note that the meaning of byte is implementation dependent, so a 2-byte int could be 16 bits on one system and 32 bits on another. Second, the climits header file (or, for older implementations, the limits.h header file) contains information about integer type limits. In particular, it defines symbolic names to represent different limits. For example, it defines INT_MAX as the largest possible int value and CHAR_BIT as the number of bits in a byte. Listing 3.1 demonstrates how to use these facilities. The program also illustrates initialization, which is the use of a declaration statement to assign a value to a variable.

Listing 3.1 limits.cpp

// limits.cpp -- some integer limits
#include <iostream>
#include <climits>       // use limits.h for older systems
int main()
{
  using namespace std;
  int n_int = INT_MAX;       // initialize n_int to max int value
  short n_short = SHRT_MAX;  // symbols defined in limits.h file
  long n_long = LONG_MAX;

  // sizeof operator yields size of type or of variable
  cout << "int is " << sizeof (int) << " bytes." << endl;
  cout << "short is " << sizeof n_short << " bytes." << endl;
  cout << "long is " << sizeof n_long << " bytes." << endl << endl;

  cout << "Maximum values:" << endl;
  cout << "int: " << n_int << endl;
  cout << "short: " << n_short << endl;
  cout << "long: " << n_long << endl << endl;

  cout << "Minimum int value = " << INT_MIN << endl;
  cout << "Bits per byte = " << CHAR_BIT << endl;
  return 0;
}

Here is the output from the program in Listing 3.1, using Microsoft Visual C++ 7.1:

int is 4 bytes.
short is 2 bytes.
long is 4 bytes.

Maximum values:
int: 2147483647
short: 32767
long: 2147483647

Minimum int value = -2147483648
Bits per byte = 8

Here is the output for a second system, running Borland C++ 3.1 for DOS:

int is 2 bytes.
short is 2 bytes.
long is 4 bytes.

Maximum values:
int: 32767
short: 32767
long: 2147483647

Minimum int value = -32768 
Bits per byte = 8

Program Notes

The following sections look at the chief programming features for this program.

The sizeof Operator and the climits Header File

The sizeof operator reports that int is 4 bytes on the base system, which uses an 8-bit byte. You can apply the sizeof operator to a type name or to a variable name. When you use the sizeof operator with a type name, such as int, you enclose the name in parentheses. But when you use the operator with the name of the variable, such as n_short, parentheses are optional:

cout << "int is " << sizeof (int) << " bytes.\n";
cout << "short is " << sizeof n_short << " bytes.\n";

The climits header file defines symbolic constants (see the sidebar "Symbolic Constants the Preprocessor Way," later in this chapter) to represent type limits. As mentioned previously, INT_MAX represents the largest value type int can hold; this turned out to be 32,767 for our DOS system. The compiler manufacturer provides a climits file that reflects the values appropriate to that compiler. For example, the climits file for Windows XP, which uses a 32-bit int, defines INT_MAX to represent 2,147,483,647. Table 3.1 summarizes the symbolic constants defined in the climits file; some pertain to types you have not yet learned.

Table 3.1 Symbolic Constants from climits

Initialization

Initialization combines assignment with declaration. For example, the statement

int n_int = INT_MAX;

declares the n_int variable and sets it to the largest possible type int value. You can also use regular constants to initialize values. You can initialize a variable to another variable, provided that the other variable has been defined first. You can even initialize a variable to an expression, provided that all the values in the expression are known when program execution reaches the declaration:

int uncles = 5;                    // initialize uncles to 5
int aunts = uncles;                // initialize aunts to 5
int chairs = aunts + uncles + 4;   // initialize chairs to 14

Moving the uncles declaration to the end of this list of statements would invalidate the other two initializations because then the value of uncles wouldn't be known at the time the program tries to initialize the other variables.

The initialization syntax shown previously comes from C; C++ has a second initialization syntax that is not shared with C:

int owls = 101; // traditional C initialization
int wrens(432); // alternative C++ syntax, set wrens to 432

If you know what the initial value of a variable should be, initialize it. True, separating the declaring of a variable from assigning it a value can create momentary suspense:

short year;    // what could it be?
year = 1492;   // oh

But initializing the variable when you declare it protects you from forgetting to assign the value later.

#define INT_MAX 32767

Unsigned Types

Each of the three integer types you just learned about comes in an unsigned variety that can't hold negative values. This has the advantage of increasing the largest value the variable can hold. For example, if short represents the range –32,768 to +32,767, the unsigned version can represent the range 0 to 65,535. Of course, you should use unsigned types only for quantities that are never negative, such as populations, bean counts, and happy face manifestations. To create unsigned versions of the basic integer types, you just use the keyword unsigned to modify the declarations:

unsigned short change;    // unsigned short type
unsigned int rovert;      // unsigned int type
unsigned quarterback;     // also unsigned int
unsigned long gone;       // unsigned long type

Note that unsigned by itself is short for unsigned int.

Listing 3.2 illustrates the use of unsigned types. It also shows what might happen if your program tries to go beyond the limits for integer types. Finally, it gives you one last look at the preprocessor #define statement.

Listing 3.2 exceed.cpp

// exceed.cpp -- exceeding some integer limits
#include <iostream>
#define ZERO 0      // makes ZERO symbol for 0 value
#include <climits> // defines INT_MAX as largest int value
int main()
{
  using namespace std;
  short sam = SHRT_MAX;   // initialize a variable to max value
  unsigned short sue = sam;// okay if variable sam already defined

  cout << "Sam has " << sam << " dollars and Sue has " << sue;
  cout << " dollars deposited." << endl
     << "Add $1 to each account." << endl << "Now ";
  sam = sam + 1;
  sue = sue + 1; 
  cout << "Sam has " << sam << " dollars and Sue has " << sue;
  cout << " dollars deposited.\nPoor Sam!" << endl;
  sam = ZERO;
  sue = ZERO;
  cout << "Sam has " << sam << " dollars and Sue has " << sue;
  cout << " dollars deposited." << endl;
  cout << "Take $1 from each account." << endl << "Now ";
  sam = sam - 1;
  sue = sue - 1;
  cout << "Sam has " << sam << " dollars and Sue has " << sue;
  cout << " dollars deposited." << endl << "Lucky Sue!" << endl;
  return 0; 
}

Here's the output from the program in Listing 3.2:

Sam has 32767 dollars and Sue has 32767 dollars deposited.
Add $1 to each account.
Now Sam has -32768 dollars and Sue has 32768 dollars deposited.
Poor Sam!
Sam has 0 dollars and Sue has 0 dollars deposited.
Take $1 from each account.
Now Sam has -1 dollars and Sue has 65535 dollars deposited.
Lucky Sue!

The program sets a short variable (sam) and an unsigned short variable (sue) to the largest short value, which is 32,767 on our system. Then, it adds 1 to each value. This causes no problems for sue because the new value is still much less than the maximum value for an unsigned integer. But sam goes from 32,767 to –32,768! Similarly, subtracting 1 from 0 creates no problems for sam, but it makes the unsigned variable sue go from 0 to 65,535. As you can see, these integers behave much like an odometer. If you go past the limit, the values just start over at the other end of the range. (See Figure 3.1.) C++ guarantees that unsigned types behave in this fashion. However, C++ doesn't guarantee that signed integer types can exceed their limits (overflow and underflow) without complaint, but that is the most common behavior on current implementations.

Figure 3.1 Typical overflow behavior for integers.

Choosing an Integer Type

With the richness of C++ integer types, which should you use? Generally, int is set to the most "natural" integer size for the target computer. Natural size refers to the integer form that the computer handles most efficiently. If there is no compelling reason to choose another type, you should use int.

Now look at reasons why you might use another type. If a variable represents something that is never negative, such as the number of words in a document, you can use an unsigned type; that way the variable can represent higher values.

If you know that the variable might have to represent integer values too great for a 16-bit integer, you should use long. This is true even if int is 32 bits on your system. That way, if you transfer your program to a system with a 16-bit int, your program won't embarrass you by suddenly failing to work properly. (See Figure 3.2.)

Figure 3.2 For portability, use long for big integers.

Using short can conserve memory if short is smaller than int. Most typically, this is important only if you have a large array of integers. (An array is a data structure that stores several values of the same type sequentially in memory.) If it is important to conserve space, you should use short instead of int, even if the two are the same size. Suppose, for example, that you move your program from a 16-bit int DOS PC system to a 32-bit int Windows XP system. That doubles the amount of memory needed to hold an int array, but it doesn't affect the requirements for a short array. Remember, a bit saved is a bit earned.

If you need only a single byte, you can use char. We'll examine that possibility soon.

Integer Constants

An integer constant is one you write out explicitly, such as 212 or 1776. C++, like C, lets you write integers in three different number bases: base 10 (the public favorite), base 8 (the old Unix favorite), and base 16 (the hardware hacker's favorite). Appendix A, "Number Bases," describes these bases; here we'll look at the C++ representations. C++ uses the first digit or two to identify the base of a number constant. If the first digit is in the range 1–9, the number is base 10 (decimal); thus 93 is base 10. If the first digit is 0 and the second digit is in the range 1–7, the number is base 8 (octal); thus 042 is octal and equal to 34 decimal. If the first two characters are 0x or 0X, the number is base 16 (hexadecimal); thus 0x42 is hex and equal to 66 decimal. For hexadecimal values, the characters a–f and A–F represent the hexadecimal digits corresponding to the values 10–15. 0xF is 15 and 0xA5 is 165 (10 sixteens plus 5 ones). Listing 3.3 is tailor-made to show the three bases.

Listing 3.3 hexoct1.cpp

// hexoct1.cpp -- shows hex and octal constants
#include <iostream>
int main()
{
  using namespace std;
  int chest = 42;    // decimal integer constant
  int waist = 0x42;  // hexadecimal integer constant
  int inseam = 042;  // octal integer constant

  cout << "Monsieur cuts a striking figure!\n";
  cout << "chest = " << chest << "\n";
  cout << "waist = " << waist << "\n";
  cout << "inseam = " << inseam << "\n";
  return 0; 
}

By default, cout displays integers in decimal form, regardless of how they are written in a program, as the following output shows:

Monsieur cuts a striking figure!
chest = 42 (42 in decimal)
waist = 66 (0x42 in hex)
inseam = 34 (042 in octal)

Keep in mind that these notations are merely notational conveniences. For example, if you read that the CGA video memory segment is B000 in hexadecimal, you don't have to convert the value to base 10 45,056 before using it in your program. Instead, you can simply use 0xB000. But whether you write the value ten as 10, 012, or 0xA, it's stored the same way in the computer—as a binary (base 2) value.

By the way, if you want to display a value in hexadecimal or octal form, you can use some special features of cout. Recall that the iostream header file provides the endl manipulator to give cout the message to start a new line. Similarly, it provides the dec, hex, and oct manipulators to give cout the messages to display integers in decimal, hexadecimal, and octal formats, respectively. Listing 3.4 uses hex and oct to display the decimal value 42 in three formats. (Decimal is the default format, and each format stays in effect until you change it.)

Listing 3.4 hexoct2.cpp

// hexoct2.cpp -- display values in hex and octal
#include <iostream>
using namespace std;
int main()
{
  using namespace std;
  int chest = 42;
  int waist = 42; 
  int inseam = 42;

  cout << "Monsieur cuts a striking figure!" << endl;
  cout << "chest = " << chest << " (decimal)" << endl;
  cout << hex;   // manipulator for changing number base
  cout << "waist = " << waist << " hexadecimal" << endl;
  cout << oct;   // manipulator for changing number base
  cout << "inseam = " << inseam << " (octal)" << endl;
  return 0; 
}

Here's the program output for Listing 3.4:

Monsieur cuts a striking figure!
chest = 42 (decimal)
waist = 2a hexadecimal
inseam = 52 (octal)

Note that code like

cout << hex;

doesn't display anything onscreen. Instead, it changes the way cout displays integers. Thus, the manipulator hex is really a message to cout that tells it how to behave. Also note that because the identifier hex is part of the std namespace and the program uses that namespace, this program can't use hex as the name of a variable. However, if you omitted the using directive and instead used std::cout, std::endl, std::hex, and std::oct, you could still use plain hex as the name for a variable.

How C++ Decides What Type a Constant Is

A program's declarations tell the C++ compiler the type of a particular integer variable. But what about constants? That is, suppose you represent a number with a constant in a program:

cout << "Year = " << 1492 << "\n";

Does the program store 1492 as an int, a long, or some other integer type? The answer is that C++ stores integer constants as type int unless there is a reason to do otherwise. Two such reasons are if you use a special suffix to indicate a particular type or if a value is too large to be an int.

First, look at the suffixes. These are letters placed at the end of a numeric constant to indicate the type. An l or L suffix on an integer means the integer is a type long constant, a u or U suffix indicates an unsigned int constant, and ul (in any combination of orders and uppercase and lowercase) indicates a type unsigned long constant. (Because a lowercase l can look much like the digit 1, you should use the uppercase L for suffixes.) For example, on a system using a 16-bit int and a 32-bit long, the number 22022 is stored in 16 bits as an int, and the number 22022L is stored in 32 bits as a long. Similarly, 22022LU and 22022UL are unsigned long.

Next, look at size. C++ has slightly different rules for decimal integers than it has for hexadecimal and octal integers. (Here decimal means base 10, just as hexadecimal means base 16; the term decimal does not necessarily imply a decimal point.) A decimal integer without a suffix is represented by the smallest of the following types that can hold it: int, long, or unsigned long. On a computer system using a 16-bit int and a 32-bit long, 20000 is represented as type int, 40000 is represented as long, and 3000000000 is represented as unsigned long. A hexadecimal or octal integer without a suffix is represented by the smallest of the following types that can hold it: int, unsigned int, long, or unsigned long. The same computer system that represents 40000 as long represents the hexadecimal equivalent 0x9C40 as an unsigned int. That's because hexadecimal is frequently used to express memory addresses, which intrinsically are unsigned. So unsigned int is more appropriate than long for a 16-bit address.

The char Type: Characters and Small Integers

It's time to turn to the final integer type: char. As you probably suspect from its name, the char type is designed to store characters, such as letters and numeric digits. Now, whereas storing numbers is no big deal for computers, storing letters is another matter. Programming languages take the easy way out by using number codes for letters. Thus, the char type is another integer type. It's guaranteed to be large enough to represent the entire range of basic symbols—all the letters, digits, punctuation, and the like—for the target computer system. In practice, most systems support fewer than 256 kinds of characters, so a single byte can represent the whole range. Therefore, although char is most often used to handle characters, you can also use it as an integer type that is typically smaller than short.

The most common symbol set in the United States is the ASCII character set, described in Appendix C, "The ASCII Character Set." A numeric code (the ASCII code) represents each character in the set. For example, 65 is the code for the character A, and 77 is the code for the character M. For convenience, this book assumes ASCII code in its examples. However, a C++ implementation uses whatever code is native to its host system—for example, EBCDIC (pronounced "eb-se-dik") on an IBM mainframe. Neither ASCII nor EBCDIC serve international needs that well, and C++ supports a wide-character type that can hold a larger range of values, such as are used by the international Unicode character set. You'll learn about this wchar_t type later in this chapter.

Try the char type in Listing 3.5.

Listing 3.5 chartype.cpp

// chartype.cpp -- the char type
#include <iostream>
int main( )
{
  using namespace std;
  char ch;    // declare a char variable

  cout << "Enter a character: " << endl;
  cin >> ch;
  cout << "Holla! ";
  cout << "Thank you for the " << ch << " character." << endl;
  return 0;
}

Here's the output from the program in Listing 3.5:

Enter a character:
M
Holla! Thank you for the M character.

The interesting thing is that you type an M, not the corresponding character code, 77. Also, the program prints an M, not 77. Yet if you peer into memory, you find that 77 is the value stored in the ch variable. The magic, such as it is, lies not in the char type but in cin and cout. These worthy facilities make conversions on your behalf. On input, cin converts the keystroke input M to the value 77. On output, cout converts the value 77 to the displayed character M; cin and cout are guided by the type of variable. If you place the same value 77 into an int variable, cout displays it as 77. (That is, cout displays two 7 characters.) Listing 3.6 illustrates this point. It also shows how to write a character constant in C++: Enclose the character within two single quotation marks, as in 'M'. (Note that the example doesn't use double quotation marks. C++ uses single quotation marks for a character and double quotation marks for a string. The cout object can handle either, but, as Chapter 4 discusses, the two are quite different from one another.) Finally, the program introduces a cout feature, the cout.put() function, which displays a single character.

Listing 3.6 morechar.cpp

// morechar.cpp -- the char type and int type contrasted
#include <iostream>
int main()
{
  using namespace std;
  char ch = 'M';    // assign ASCII code for M to c
  int i = ch;       // store same code in an int
  cout << "The ASCII code for " << ch << " is " << i << endl;

  cout << "Add one to the character code:" << endl;
  ch = ch + 1;     // change character code in c
  i = ch;          // save new character code in i
  cout << "The ASCII code for " << ch << " is " << i << endl;

  // using the cout.put() member function to display a char
  cout << "Displaying char ch using cout.put(ch): ";
  cout.put(ch);

  // using cout.put() to display a char constant
  cout.put('!');

  cout << endl << "Done" << endl;
  return 0;
}

Here is the output from the program in Listing 3.6:

The ASCII code for M is 77
Add one to the character code:
The ASCII code for N is 78
Displaying char ch using cout.put(ch): N! 
Done

Program Notes

In the program in Listing 3.6, the notation 'M' represents the numeric code for the M character, so initializing the char variable c to 'M' sets c to the value 77. The program then assigns the identical value to the int variable i, so both c and i have the value 77. Next, cout displays c as M and i as 77. As previously stated, a value's type guides cout as it chooses how to display that value—just another example of smart objects.

Because c is really an integer, you can apply integer operations to it, such as adding 1. This changes the value of c to 78. The program then resets i to the new value. (Equivalently, you can simply add 1 to i.) Again, cout displays the char version of that value as a character and the int version as a number.

The fact that C++ represents characters as integers is a genuine convenience that makes it easy to manipulate character values. You don't have to use awkward conversion functions to convert characters to ASCII and back.

Finally, the program uses the cout.put() function to display both c and a character constant.

A Member Function: cout.put()

Just what is cout.put(), and why does it have a period in its name? The cout.put() function is your first example of an important C++ OOP concept, the member function. Remember that a class defines how to represent data and how to manipulate it. A member function belongs to a class and describes a method for manipulating class data. The ostream class, for example, has a put() member function that is designed to output characters. You can use a member function only with a particular object of that class, such as the cout object, in this case. To use a class member function with an object such as cout, you use a period to combine the object name (cout) with the function name (put()). The period is called the membership operator. The notation cout.put() means to use the class member function put() with the class object cout. You'll learn about this in greater detail when you reach classes in Chapter 10, "Objects and Classes." Now, the only classes you have are the istream and ostream classes, and you can experiment with their member functions to get more comfortable with the concept.

The cout.put() member function provides an alternative to using the << operator to display a character. At this point you might wonder why there is any need for cout.put(). Much of the answer is historical. Before Release 2.0 of C++, cout would display character variables as characters but display character constants, such as 'M' and 'N', as numbers. The problem was that earlier versions of C++, like C, stored character constants as type int. That is, the code 77 for 'M' would be stored in a 16-bit or 32-bit unit. Meanwhile, char variables typically occupied 8 bits. A statement like

char c = 'M';

copied 8 bits (the important 8 bits) from the constant 'M' to the variable c. Unfortunately, this meant that, to cout, 'M' and c looked quite different from one another, even though both held the same value. So a statement like

cout << '$';

would print the ASCII code for the $ character rather than simply display $. But

cout.put('$');

would print the character, as desired. Now, after Release 2.0, C++ stores single-character constants as type char, not type int. Therefore, cout now correctly handles character constants.

The cin object has a couple different ways of reading characters from input. You can explore these by using a program that uses a loop to read several characters, so we'll return to this topic when we cover loops in Chapter 5, "Loops and Relational Expressions."

char Constants

You have several options for writing character constants in C++. The simplest choice for ordinary characters, such as letters, punctuation, and digits, is to enclose the character in single quotation marks. This notation stands for the numeric code for the character. For example, an ASCII system has the following correspondences:

'A' is 65, the ASCII code for A

'a' is 97, the ASCII code for a

'5' is 53, the ASCII code for the digit 5

' ' is 32, the ASCII code for the space character

'!' is 33, the ASCII code for the exclamation point

Using this notation is better than using the numeric codes explicitly. It's clearer, and it doesn't assume a particular code. If a system uses EBCDIC, then 65 is not the code for A, but 'A' still represents the character.

There are some characters that you can't enter into a program directly from the keyboard. For example, you can't make the newline character part of a string by pressing the Enter key; instead, the program editor interprets that keystroke as a request for it to start a new line in your source code file. Other characters have difficulties because the C++ language imbues them with special significance. For example, the double quotation mark character delimits strings, so you can't just stick one in the middle of a string. C++ has special notations, called escape sequences, for several of these characters, as shown in Table 3.2. For example, \a represents the alert character, which beeps your terminal's speaker or rings its bell. The escape sequence \n represents a newline. And \" represents the double quotation mark as an ordinary character instead of a string delimiter. You can use these notations in strings or in character constants, as in the following examples:

char alarm = '\a';
cout << alarm << "Don't do that again!\a\n";
cout << "Ben \"Buggsie\" Hacker\nwas here!\n";

Table 3.2 C++ Escape Sequence Codes

The last line produces the following output:

Ben "Buggsie" Hacker
was here!

Note that you treat an escape sequence, such as \n, just as a regular character, such as Q. That is, you enclose it in single quotes to create a character constant and don't use single quotes when including it as part of a string.

The newline character provides an alternative to endl for inserting new lines into output. You can use the newline character in character constant notation ('\n') or as character in a string ("\n"). All three of the following move the screen cursor to the beginning of the next line:

cout << endl;  // using the endl manipulator
cout << '\n';  // using a character constant 
cout << "\n";  // using a string

You can embed the newline character in a longer string; this is often more convenient than using endl. For example, the following two cout statements produce the same output:

cout << endl << endl << "What next?" << endl << "Enter a number:" << endl;
cout << "\n\nWhat next?\nEnter a number:\n";

When you're displaying a number, endl is a bit easier to type than "\n" or '\n', but, when you're displaying a string, ending the string with a newline character requires less typing:

cout << x << endl;   // easier than cout << x << "\n";
cout << "Dr. X.\n";  // easier than cout << "Dr. X." << endl;

Finally, you can use escape sequences based on the octal or hexadecimal codes for a character. For example, Ctrl+Z has an ASCII code of 26, which is 032 in octal and 0x1a in hexadecimal. You can represent this character with either of the following escape sequences: \032 or \x1a. You can make character constants out of these by enclosing them in single quotes, as in '\032', and you can use them as parts of a string, as in "hi\x1a there".

Listing 3.7 demonstrates a few escape sequences. It uses the alert character to get your attention, the newline character to advance the cursor (one small step for a cursor, one giant step for cursorkind), and the backspace character to back the cursor one space to the left. (Houdini once painted a picture of the Hudson River using only escape sequences; he was, of course, a great escape artist.)

Listing 3.7 bondini.cpp

// bondini.cpp -- using escape sequences
#include <iostream>
int main()
{
  using namespace std;
  cout << "\aOperation \"HyperHype\" is now activated!\n";
  cout << "Enter your agent code:________\b\b\b\b\b\b\b\b";
  long code;
  cin >> code;
  cout << "\aYou entered " << code << "...\n";
  cout << "\aCode verified! Proceed with Plan Z3!\n";
  return 0; 
}

When you start the program in Listing 3.7, it puts the following text onscreen:

Operation "HyperHype" is now activated!
Enter your agent code:________

After printing the underscore characters, the program uses the backspace character to back up the cursor to the first underscore. You can then enter your secret code and continue. Here's a complete run:

Operation "HyperHype" is now activated!
Enter your agent code:42007007
You entered 42007007...
Code verified! Proceed with Plan Z3! 

Universal Character Names

C++ implementations support a basic source character set—that is, the set of characters you can use to write source code. It consists of the letters (uppercase and lowercase) and digits found on a standard U.S. keyboard, the symbols, such as { and =, used in the C language, and a scattering of other characters, such as newline and space characters. Then there is a basic execution character set (that is, characters that can be produced by the execution of a program), which adds a few more characters, such as backspace and alert. The C++ Standard also allows an implementation to offer extended source character sets and extended execution character sets. Furthermore, those additional characters that qualify as letters can be used as part of the name of an identifier. Thus, a German implementation might allow you to use umlauted vowels and a French implementation might allow accented vowels. C++ has a mechanism for representing such international characters that is independent of any particular keyboard: the use of universal character names.

Using universal character names is similar to using escape sequences. A universal character name begins either with \u or \U. The \u form is followed by 8 hexadecimal digits, and the \U form by 16 hexadecimal digits. These digits represent the ISO 10646 code for the character. (ISO 10646 is an international standard under development that provides numeric codes for a wide range of characters. See "Unicode and ISO 10646," later in this chapter.)

If your implementation supports extended characters, you can use universal character names in identifiers, as character constants, and in strings. For example, consider the following code:

int k\u00F6rper;
cout << "Let them eat g\u00E2teau.\n";

The ISO 10646 code for ö is 00F6, and the code for is 00E2. Thus, this C++ code would set the variable name to körper and display the following output:

Let them eat gteau.

If your system doesn't support ISO 10646, it might display some other character for or perhaps simply display the word gu00E2teau.

The International Organization for Standardization (ISO) established a working group to develop ISO 10646, also a standard for coding multilingual text. The ISO 10646 group and the Unicode group have worked together since 1991 to keep their standards synchronized with one another.