C++编程入门指南：自我教学法

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"C++ Demystified: A Self-Teaching Guide by Jeff Kent 是一本针对C++编程初学者的指南，旨在通过逐步教学的方式帮助读者理解C++程序的工作原理、内存管理、数据类型、变量、运算符、流程控制以及文件输入输出等核心概念。" 在C++编程中，了解一个C++程序如何工作是至关重要的。这通常涉及理解程序的执行顺序、编译过程和链接步骤。在"Chapter 1 - How a C++ Program Works"中，作者可能会介绍从源代码到可执行文件的转换过程，包括预处理、编译、汇编和链接这些阶段。 "Chapter 2 - Memory and Data Types"将涵盖C++中的基本数据类型，如int、float、char等，以及如何管理内存，包括栈和堆的区别。此外，读者还将学习如何声明和初始化变量。 "Chapter 3 - Variables"深入探讨变量的使用，包括变量的作用域、生命周期和存储类别。读者会学习如何声明、定义和操作变量。 "Chapter 4 - Arithmetic Operators"将介绍算术运算符，如加法(+), 减法(-), 乘法(*), 除法(/)和取余(%)，以及它们在表达式中的应用。 "Chapter 5 - Making Decisions: if and switch Statements"和"Chapter 6 - Nested if Statements and Logical Operators"关注条件语句，如if、else、switch，以及逻辑运算符（&&, ||, !）的使用，用于编写基于条件的代码。 "Chapter 7 - The For Loop", "Chapter 8 - While and DoWhile Loops"讲解循环结构，包括for、while和do-while循环，用于重复执行代码块。 "Chapter 9 - Functions"介绍函数的定义、调用和参数传递，是C++中复用代码和组织程序的重要手段。 "Chapter 10 - Arrays"涵盖了数组的概念，包括一维和多维数组的声明、初始化和操作。 "Chapter 11 - What’s the Address? Pointers"讲解指针，这是C++的一个关键特性，让读者能直接操作内存地址。 "Chapter 12 - Character, C-String, and C++ String Class Functions"涉及到字符处理，包括C风格的字符串和C++标准库中的std::string类。 "Chapter 13 - Persistent Data: File Input and Output"教授如何在程序中读写文件，实现数据的持久化存储。 "Chapter 14 - The Road Ahead: Structures and Classes"预告了面向对象编程的基础，包括结构体和类，这是C++中实现封装、继承和多态的关键。书末的"Final Exam"和"Answers to Quizzes and Final Exam"提供了自我评估的机会，而索引和图表列表则方便查找特定主题。这本书是C++初学者的宝贵资源，通过系统性的学习，读者可以建立起扎实的C++编程基础。虽然需要一定的英文阅读能力，但这本书的实用性将使得克服语言障碍变得值得。

What Is a Programming Language?

When you enter a darkened room and want to see what is inside, you turn on a light

switch. When you leave the room, you turn the light switch off.

The first computers were not too different than that light switch. These early computers

consisted of wires and switches in which the electrical current followed a path dependent

on which switches were in the on (one) or off (zero) position. Indeed, I built such a simple

computer when I was a kid (which according to my own children was back when

dinosaurs still ruled the earth).

Each switch’s position could be expressed as a number: 1 for the on position, 0 for the off

position. Thus, the instructions given to these first computers, in the form of the switches’

positions, essentially were a series of ones and zeroes.

Today’s computers, of course, are far more powerful and sophisticated than these early

computers. However, the language that computers understand, called machine language,

remains the same, essentially ones and zeroes.

While computers think in ones and zeroes, the humans who write computer programs

usually don’t. Additionally, a complex program may consist of thousands or even millions

of step-by-step machine language instructions, which would require an inordinately long

amount of time to write. This is an important consideration since, due to competitive

market forces, the amount of time within which a program has to be written is becoming

increasingly less and less.

Fortunately, we do not have to write instructions to computers in machine language.

Instead, we can write instructions in a programming language. Programming languages

are far more understandable to programmers than machine language because

programming languages resemble the structure and syntax of human language, not ones

and zeroes. Additionally, code can be written much faster with programming languages

than machine language because programming languages automate instructions; one

programming language instruction can cover many machine language instructions.

C++ is but one of many programming languages. Other popular programming languages

include Java, C#, and Visual Basic. There are many others. Indeed, new languages are

being created all the time. However, all programming languages have essentially the

same purpose, which is to enable a human programmer to give instructions to a computer.

Why learn C++ instead of another programming language? First, it is very widely used,

both in industry and in education. Second, many other programming languages, including

Java and C#, are based on C++. Indeed, the Java programming language was written

using C++. Therefore, knowing C++ makes learning other programming languages easier.

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Anatomy of a C++ Program

It seems to be a tradition in C++ programming books for the first code example to output

to a console window the message “Hello World!” (shown in Figure 1-1).

Figure 1-1: C++ program outputting “Hello World!” to the screen

Note

The term “console” goes back to the days before Windows when the screen did

not have menus and toolbars but just text. If you have typed commands using

DOS or UNIX, you likely did so in a console window. The text “Press any key to

continue” immediately following “Hello World!” is not part of the program, but

instead is a cue for how to close the console window.

Unfortunately, all too often the “Hello World!” example is followed quickly by many other

program examples without the book or teacher first stopping to explain how the “Hello

World!” program works. The result soon is a confused reader or student who’s ready to

say “Goodbye, Cruel World.”

While the “Hello World!” program looks simple, there actually is a lot going on behind the

scenes of this program. Accordingly, we are going to go through the following code for the

“Hello World!” program line by line, though not in top-to-bottom order.

#include <iostream>

using namespace std;

int main(void)

{

cout << "Hello World!";

return 0;

}

Note

The code a programmer writes is referred to as source code, which is saved in a

file that usually has a .cpp extension, standing for C++.

The main Function

As discussed in the “What Is a Programming Language?” section, the purpose of C++, or

any programming language, is to enable a programmer to write instructions for a

computer. Often, a task is too complex for just one instruction. Instead, several related

instructions are required.

A function is a group of related instructions, also called statements, which together

perform a particular task. The name of the function is how you refer to these related

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Anatomy of a C++ Program

statements. In the “Hello World!” program, main is the name of a function. A program may

have many functions, and in Chapter 9 I will show you how to create and use functions.

However, a program must have one main function, and only one main function. The

reason is that the main function is the starting point for every C++ program. If there was

no main function, the computer would not know where to start the program. If there was

more than one main function, the program would not know whether to start at one or the

other.

Note

The main function is preceded by int and followed by void in parentheses. We

will cover the meaning of both in

Chapter 9.

The Function Body

Each of the related instructions, or statements, which belong to the main function are

contained within the body of that function. A function body starts with a left curly brace, {,

and ends with a right curly brace, }.

Each statement usually ends with a semicolon. The main function has two statements:

cout << "Hello World!";

return 0;

Statements are executed in order, from top to bottom. Don’t worry, the term “executed”

doesn’t mean the statement is put to death. Rather, it means that the statement is carried

out, or executed, by the computer.

cout

The first statement is

cout << "Hello World!";

cout is pronounced “C-out.” The “out” refers to the direction in which cout sends a stream

of data.

A data stream may flow in one of two directions. One direction is input—into your program

from an outside source such as a file or user keyboard input. The other direction is output

—out from your program to an outside source such as a monitor, printer, or file.

cout concerns the output stream. It sends information to the standard output device. The

standard output device usually is your monitor, though it can be something else, such as

a printer or a file on your hard drive.

The << following cout is an operator. You likely have used operators before, such as the

arithmetic operators +, –, *, and /, for addition, subtraction, multiplication, and division,

respectively.

The << operator is known as the stream insertion operator. It inserts the information

immediately to its right—in this example, the text “Hello World!” into the data stream. The

cout object then sends that information to the standard output device—in this case, the

monitor.

Note

In Chapter 3, you will learn about the counterparts to the cout object and the <<

operator, the cin object, which concerns the input stream, and the >> operator

used with the cin object.

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Anatomy of a C++ Program

The return 0 Statement

The second and final statement returns a value of zero to the computer’s operating

system, whether Windows, UNIX, or another. This tells the operating system that the

program ended normally. Sometimes programs do not end normally, but instead crash,

such as if you run out of memory during the running of the program. The operating

system may need to handle this abnormal program termination differently than normal

termination. That is why the program tells the operating system that this time it ended

normally.

The #include Directive

Your C++ program “knows” to start at the main function because the main function is part

of the core of the C++ language. We certainly did not write any code that told the C++

program to start at main.

Similarly, your C++ program seems to know that the cout object, in conjunction with the

stream insertion operator <<, outputs information to the monitor. We did not write any

code to have the cout object and the << operator achieve this result.

However, the cout object is not part of the C++ core language. Rather, it is defined

elsewhere, in a standard library file. C++ has a number of standard library files, each

defining commonly used objects. Outputting information to the monitor certainly is a

common task. While you could go to the trouble of writing your own function that outputs

information to the screen, a standard library file’s implementation of cout saves you the

trouble of “reinventing the wheel.”

While C++ already has implemented the cout object for you in a standard library file, you

still have to tell the program to include that standard library file in your application. You do

so with the #include directive, followed by the name of the library file. If the library file is a

standard library file, as opposed to one you wrote (yes, you can create your own), then

the file name is enclosed in angle brackets, < and >.

The cout object is defined in the standard library file iostream. The “io” in iostream refers

to input and output—“stream” to a stream of data. To use the cout object, we need to

include the iostream standard library file in our application. We do so with the following

include directive:

#include <iostream>

The include directive is called a preprocessor directive. The preprocessor, together with

the compiler and linker, are discussed later in this chapter in the section “Translating the

Code for the Computer.” The preprocessor directive, unlike statements, is not ended by a

semicolon.

Namespace

The final statement to be discussed in the Hello World! example is

using namespace std;

C++ uses namespaces to organize different names used in programs. Every name used

in the iostream standard library file is part of a namespace called std. Consequently, the

cout object is really called std::cout. The using namespace std statement avoids the need

for putting std:: before every reference to cout, so we can just use cout in our code.

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