C++ Primer Plus

On a system with a 2-byte wchar_t, this code stores each character in a 2-byte unit of memory. This book doesn’t use the wide-character type, but you should be aware of it, particularly if you become involved in international programming or in using Unicode or ISO 10646.

New C++11 Types: char16_t and char32_t

As the programming community gained more experience with Unicode, it became clear that the wchar_t type wasn’t enough. It turns out that encoding characters and strings of characters on a computer system is more complex than just using the Unicode numeric values (called code points). In particular, it’s useful, when encoding strings of characters, to have a type of definite size and signedness. But the sign and size of wchar_t can vary from one implementation to another. So C++11 introduces the types char16_t, which is unsigned and 16 bits, and char32_t, which is unsigned and 32 bits. C++11 uses the u prefix for char16_t character and string constants, as in u'C' and u"be good". Similarly, it uses the U prefix for char32_t constants, as in U'R' and U"dirty rat". The char16_t type is a natural match for universal character names of the form /u00F6, and the char32_t type is a natural match for universal character names of the form /U0000222B. The prefixes u and U are used to indicate character literals of types char16_t and char32_t, respectively:

char16_t ch1 = u'q';           // basic character in 16-bit form

char32_t ch2 = U'/U0000222B';  // universal character name in 32-bit form

Like wchar_t, char16_t and char32_t each have an underlying type, which is one of the built-in integer types. But the underlying type can be different on one system from what it is on another.

The bool Type

The ANSI/ISO C++ Standard has added a new type (new to C++, that is), called bool. It’s named in honor of the English mathematician George Boole, who developed a mathematical representation of the laws of logic. In computing, a Boolean variable is one whose value can be either true or false. In the past, C++, like C, has not had a Boolean type. Instead, as you’ll see in greater detail in Chapters 5 and 6, C++ interprets nonzero values as true and zero values as false. Now, however, you can use the bool type to represent true and false, and the predefined literals true and false represent those values. That is, you can make statements like the following:

bool is_ready = true;

The literals true and false can be converted to type int by promotion, with true converting to 1 and false to 0:

int ans = true;           // ans assigned 1

int promise = false;      // promise assigned 0

Also any numeric or pointer value can be converted implicitly (that is, without an explicit type cast) to a bool value. Any nonzero value converts to true, whereas a zero value converts to false:

bool start = -100;       // start assigned true

bool stop = 0;           // stop assigned false

After the book introduces if statements (in Chapter 6, “Branching Statements and Logical Operators”), the bool type will become a common feature in the examples.

The const Qualifier

Now let’s return to the topic of symbolic names for constants. A symbolic name can suggest what the constant represents. Also if the program uses the constant in several places and you need to change the value, you can just change the single symbol definition. The note about #define statements earlier in this chapter (see the sidebar “Symbolic Constants the Preprocessor Way”) promises that C++ has a better way to handle symbolic constants. That way is to use the const keyword to modify a variable declaration and initialization. Suppose, for example, that you want a symbolic constant for the number of months in a year. Just enter this line in a program:

const int Months = 12;  // Months is symbolic constant for 12

Now you can use Months in a program instead of 12. (A bare 12 in a program might represent the number of inches in a foot or the number of donuts in a dozen, but the name Months tells you what the value 12 represents.) After you initialize a constant such as Months, its value is set. The compiler does not let you subsequently change the value Months. If you try to, for example, g++ gives an error message that the program used an assignment of a read-only variable. The keyword const is termed a qualifier because it qualifies the meaning of a declaration.