C++ Primer Plus

As in function naming, capitalization is a key issue in variable naming (see the sidebar “Naming Conventions” in Chapter 2, “Setting Out to C++”), but many programmers may insert an additional level of information in a variable name—a prefix that describes the variable’s type or contents. For instance, the integer myWeight might be named nMyWeight; here, the n prefix is used to represent an integer value, which is useful when you are reading code and the definition of the variable isn’t immediately at hand. Alternatively, this variable might be named intMyWeight, which is more precise and legible, although it does include a couple extra letters (anathema to many programmers). Other prefixes are commonly used in like fashion: str or sz might be used to represent a null-terminated string of characters, b might represent a Boolean value, p a pointer, c a single character.

As you progress into the world of C++, you will find many examples of the prefix naming style (including the handsome m_lpctstr prefix—a class member value that contains a long pointer to a constant, null-terminated string of characters), as well as other, more bizarre and possibly counterintuitive styles that you may or may not adopt as your own. As in all the stylistic, subjective parts of C++, consistency and precision are best. You should use variable names to fit your own needs, preferences, and personal style. (Or, if required, choose names that fit the needs, preferences, and personal style of your employer.)

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 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, long, and, with C++11, long long. Each comes in both signed and unsigned versions. That gives you a choice of ten 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, long, and long 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, long, and long long can represent up to four 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. 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.

• A long long integer is at least 64 bits wide and at least as big as long.

Bits and Bytes

The fundamental unit of computer memory is the bit. Think of a bit as an electronic switch that you can set to either off or on. Off represents the value 0, and on represents the value 1. An 8-bit chunk of memory can be set to 256 different combinations. The number 256 comes from the fact that each bit has two possible settings, making the total number of combinations for 8 bits 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2, or 256. Thus, an 8-bit unit can represent, say, the values 0 through 255 or the values –128 through 127. Each additional bit doubles the number of combinations. This means you can set a 16-bit unit to 65,536 different values, a 32-bit unit to 4,294,672,296 different values, and a 64-bit unit to 18,446,744,073,709,551,616 different values. As a point of comparison, unsigned long can’t hold the Earth’s current population or the number of stars in our galaxy, but long long can.