C++基础教程：变量、表达式、输入/输出和程序风格

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"Absolute C++, 4th Edition.pdf" Absolute C++ 是一本关于 C++ 程序设计语言的教材，旨在帮助读者学习 C++ 语言的基础知识和高级技术。本书共分为多个章节，每章节都涵盖了 C++ 语言的不同方面。 1. C++ 基础知识 C++ 语言是一种面向对象的程序设计语言，由 Bjarne Stroustrup 于 1983 年开发。C++ 语言继承了 C 语言的优点，同时也引入了面向对象的编程理念。C++ 语言的主要特点是高效、灵活、可移植和可扩展。 1.1 C++ 语言的起源 C++ 语言的起源可以追溯到 1983 年，当时 Bjarne Stroustrup 在贝尔实验室工作。他希望创建一种语言，能够继承 C 语言的优点，同时也能满足面向对象编程的需求。C++ 语言的名称来自于 C 语言，表示其是 C 语言的继承者。 1.2 变量、表达式和赋值语句在 C++ 语言中，变量是一种存储数据的单元。变量可以是整数、浮点数、字符、字符串等类型。赋值语句是将值赋给变量的语句。C++ 语言中有多种类型的赋值语句，如基本赋值、复合赋值、自增赋值等。 1.3 控制台输入/输出控制台输入/输出是 C++ 语言中最基本的输入/输出方式。C++ 语言提供了多种输入/输出函数，如 cout、cin 等。cout 函数用于输出数据，cin 函数用于输入数据。 1.4 程序风格程序风格是指编写程序的风格和惯例。好的程序风格可以提高程序的可读性和可维护性。C++ 语言中有多种程序风格，如使用注释、缩进、命名惯例等。 1.5 库和命名空间库是指一组预先编译的函数和变量的集合。C++ 语言中有多种库，如 iostream 库、string 库等。命名空间是指一个命名的集合，用于避免命名冲突。C++ 语言中有多种命名空间，如 std 命名空间等。 Absolute C++ 是一本涵盖 C++ 语言基础知识和高级技术的教材，旨在帮助读者学习 C++ 语言的不同方面。

16 C++ Basics

A more complicated notation for constants of type double is called scientiﬁc nota-

tion or ﬂoating-point notation and is particularly handy for writing very large num-

bers and very small fractions. For instance, 3.67

x 10

, which is the same as

367000000000000000.0

is best expressed in C++ by the constant 3.67e17. The number 5.89 x 10

-6

, which is

the same as

0.00000589, is best expressed in C++ by the constant 5.89e-6. The e

stands for exponent and means “multiply by 10 to the power that follows.” The e may

be either uppercase or lowercase.

Think of the number after the

e as telling you the direction and number of digits to

move the decimal point. For example, to change

3.49e4 to a numeral without an e,

you move the decimal point four places to the right to obtain

34900.0, which is

another way of writing the same number. If the number after the

e is negative, you

move the decimal point the indicated number of spaces to the left, inserting extra zeros

if need be. So,

3.49e-2 is the same as 0.0349.

The number before the

e may contain a decimal point, although it is not required.

However, the exponent after the

e deﬁnitely must not contain a decimal point.

Constants of type

char are expressed by placing the character in single quotes, as

illustrated in what follows:

char symbol = ’Z’;

Note that the left and right single quote symbol are the same symbol.

HAT

OUBLED

Why is the type for numbers with a fractional part called double? Is there a type called “single”

that is half as big

? No, but something like that is true. Many programming languages tradition-

ally used two types for numbers with a fractional part. One type used less storage and was very

imprecise (that is, it did not allow very many significant digits). The second type used double the

amount of storage and so was much more precise; it also allowed numbers that were larger

(although programmers tend to care more about precision than about size). The kind of numbers

that used twice as much storage were called

double-precision

numbers; those that used less stor-

age were called

single precision

. Following this tradition, the type that (more or less) corresponds

to this double-precision type was named

double in C++. The type that corresponds to single pre-

cision in C++ was called

float. C++ also has a third type for numbers with a fractional part, which

is called

long double.

scientific

notation or

floating-

point

notation

01_CH01.fm Page 16 Wednesday, August 20, 2003 2:21 PM

Variables, Expressions, and Assignment Statements 17

Constants for strings of characters are given in double quotes, as illustrated by the

following line taken from Display 1.1:

cout << "How many programming languages have you used? ";

Be sure to notice that string constants are placed inside double quotes, while constants

of type

char are placed inside single quotes. The two kinds of quotes mean different

things. In particular,

’A’ and "A" mean different things. ’A’ is a value of type char and

can be stored in a variable of type

char. "A" is a string of characters. The fact that the

string happens to contain only one character does not make

"A" a value of type char.

Also notice that for both strings and characters, the left and right quotes are the same.

Strings in double quotes, like

"Hello", are often called C-strings. In Chapter 9 we

will see that C++ has more than one kind of string, and this particular kind happens to

be called C-strings.

The type

bool has two constants, true and false. These two constants may be

assigned to a variable of type

bool or used anyplace else an expression of type bool is

allowed. They must be spelled with all lowercase letters.

■

ESCAPE SEQUENCES

A backslash, \ , preceding a character tells the compiler that the sequence following the

backslash

does not have the same meaning as the character appearing by itself. Such a

sequence is called an escape sequence. The sequence is typed in as two characters with

no space between the symbols. Several escape sequences are deﬁned in C++.

If you want to put a backslash,

\, or a quote symbol, ", into a string constant, you

must escape the ability of the

" to terminate a string constant by using \", or the ability

of the

\ to escape, by using \\. The \\ tells the compiler you mean a real backslash, \,

not an escape sequence; the

\" tells it you mean a real quote, not the end of a string

constant.

A stray

\, say \z, in a string constant will have different effects on different compil-

ers. One compiler may simply give back a

z; another might produce an error. The

ANSI/ISO standard states that unspeciﬁed escape sequences have undeﬁned behavior.

This means a compiler can do anything its author ﬁnds convenient. The consequence

is that code that uses undeﬁned escape sequences is not portable. You should not use

any escape sequences other than those provided by the C++ standard. These C++ con-

trol characters are listed in Display 1.3.

■

NAMING CONSTANTS

Numbers in a computer program pose two problems. The ﬁrst is that they carry no

mnemonic value. For example, when the number

10 is encountered in a program, it

gives no hint of its signiﬁcance. If the program is a banking program, it might be the

number of branch ofﬁces or the number of teller windows at the main ofﬁce. To

quotes

C-string

escape

sequence

01_CH01.fm Page 17 Wednesday, August 20, 2003 2:21 PM

Variables, Expressions, and Assignment Statements 19

There is, however, one problem with this method of naming number constants: You

might inadvertently change the value of one of these variables. C++ provides a way of

marking an initialized variable so that it cannot be changed. If your program tries to

change one of these variables, it produces an error condition. To mark a variable decla-

ration so that the value of the variable cannot be changed, precede the declaration with

the word

const (which is an abbreviation of constant). For example:

const int BRANCH_COUNT = 10;

const int WINDOW_COUNT = 10;

If the variables are of the same type, it is possible to combine the above lines into one

declaration, as follows:

const int BRANCH_COUNT = 10, WINDOW_COUNT = 10;

However, most programmers ﬁnd that placing each name deﬁnition on a separate line

is clearer. The word

const is often called a modiﬁer, because it modiﬁes (restricts) the

variables being declared.

A variable declared using the

const modiﬁer is often called a declared constant.

Writing declared constants in all uppercase letters is not required by the C++ language,

but it is standard practice among C++ programmers.

Once a number has been named in this way, the name can then be used anywhere

the number is allowed, and it will have exactly the same meaning as the number it

names. To change a named constant, you need only change the initializing value in the

const variable declaration. The meaning of all occurrences of BRANCH_COUNT, for

instance, can be changed from

10 to 11 simply by changing the initializing value of 10

in the declaration of BRANCH_COUNT.

Display 1.4 contains a simple program that illustrates the use of the declaration

modiﬁer

const.

■

ARITHMETIC OPERATORS AND EXPRESSIONS

As in most other languages, C++ allows you to form expressions using variables, con-

stants, and the arithmetic operators:

+ (addition), - (subtraction), * (multiplication),

/ (division), and % (modulo, remainder). These expressions can be used anyplace it is

legal to use a value of the type produced by the expression.

All the arithmetic operators can be used with numbers of type

int, numbers of type

double, and even with one number of each type. However, the type of the value pro-

duced and the exact value of the result depend on the types of the numbers being com-

bined. If both operands (that is, both numbers) are of type

int, then the result of

combining them with an arithmetic operator is of type

int. If one or both of the oper-

ands are of type

double, then the result is of type double. For example, if the variables

baseAmount and increase are of type int, then the number produced by the following

expression is of type

int:

baseAmount + increase

const

modifier

declared

constant

mixing

types

01_CH01.fm Page 19 Wednesday, August 20, 2003 2:21 PM

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