5.10 Qualifiers

As you've seen, C++ allows you to specify a number of qualifiers for variable declarations. Qualifiers may be thought of as adjectives that describe the type that follows. Table 5-2 summarizes the various qualifiers; they are explained in detail in the following sections.

Table 5-2. Qualifiers and simple types

Special	Constant	Storage class	Size	Sign	Type
volatile	const	register	long	signed	int
<blank>	<blank>	static	short	unsigned	float
		extern	double	<blank>	char
		auto	<blank>		wchar_t
		<blank>			<blank>

5.10.1 Special

The volatile keyword is used for specialized programming such as I/O drivers and shared memory applications. It is an advanced modifier whose use is far beyond the scope of this book.

volatile

Indicates a special variable whose value may change at any time

<blank>

Normal variable

5.10.2 Constant

The const keyword indicates a value that cannot be changed.

const

Indicates that this is a declaration of constant data

<blank>

Normal variable

5.10.3 Storage Class

The class of a variable is discussed in detail in Chapter 9. A brief description of the various classes follows:

register

This indicates a frequently used variable that should be kept in a machine register. See Chapter 17.

static

The meaning of this word depends on the context. This keyword is described in Chapter 9 and Chapter 23.

extern

The variable is defined in another file. See Chapter 23 for more information.

auto

A variable allocated from the stack. This keyword is hardly ever used.

<blank>

Indicates that the default storage class is selected. For variables declared outside a function, this makes the variable global. Variables inside a function are declared auto.

5.10.4 Size

The size qualifier allows you to select the most efficient size for the variable.

long

Indicates a larger than normal number.

short

Indicates a smaller than normal integer.

double

Indicates a double-size floating-point number.

<blank>

Indicates a normal-size number.

5.10.5 Sign

Numbers can be signed or unsigned. This qualifier applies only to char and int types. Floating-point numbers are always signed. The default is signed for int and undefined for characters.

5.10.6 Type

This specifies the type of the variable. Simple types include:

int

Integer.

float

Floating-point number.

char

Single character, but can also be used for a very short integer.

wchar_t

Single wide character. Can also be used for a short integer, but most people don't because the other integer types are more appropriate to use.