C++入门指南：针对Java程序员

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"C++-for-Java 是一本面向有Java背景的程序员的入门指南，由 Scott D. Anderson 撰写，旨在帮助Java程序员理解并学习C和C++这两种语言。尽管C语言比Java简单且历史悠久，但其简洁性并不意味着对Java程序员来说容易上手。同样，由于C++是从C语言发展而来，它保留了C的一些设计原则，这也可能给Java程序员带来困惑。文档目前处于草案阶段，特别是关于C++的部分还在完善中，作者欢迎读者提出改进建议。" 本文档分为多个章节，逐步介绍C和C++的基础知识。首先，" Beginning "章节介绍了该文档的意图和目标，以及其他可参考的学习资源。" C Design Philosophy "部分讨论了C语言的设计哲学，强调其简洁性和效率，但同时也指出这种简洁可能会导致错误处理等方面的挑战。此外，文档还提到了错误处理和一些警告与建议，提醒读者在编程时需要注意的问题。接着，" Overview of C "章节对C语言进行了概述，为读者提供了一个整体的理解。在" The C Language "部分，详细阐述了C语言的语法，包括注释、常量、函数语法（函数调用和定义）、局部变量等基础知识。对于函数，文档不仅解释了如何调用和定义函数，还提到了局部变量的作用域。 " Printing "章节则专注于输出操作，讲解了如何打印变量以及提供了相关的示例。这部分还涉及到了缓冲区的概念，这是C语言中处理输入/输出时的一个重要特性。通过这些内容，Java程序员可以逐步了解如何在C语言中进行基本的输入输出操作。然而，由于文档尚未完成，关于C++的部分没有详细展开。对于Java程序员来说，C++的学习将涉及到类、对象、模板、异常处理等更复杂的话题，这些都是C++相对于C的重要扩展，也是C++面向对象编程的关键特性。总体而言，"C++-for-Java"是一个为Java程序员提供过渡到C和C++学习的宝贵资源，尽管它目前仍处于开发阶段，但已经包含了足够的信息来引导读者开始这段旅程。随着文档的进一步完善，它将成为一个全面而实用的学习工具，帮助Java程序员更好地理解和掌握这两种强大的系统级编程语言。

1.5 Warnings and Advice

Here are some quick general warnings. Some of these will make more sense after you’ve read through

the text, but they’re up front so you can easily remind yourself.

• C and C

do not do bounds-checking for arrays, so you can take the 15th or 150th element of

an array of 10 elements. What happens? All bets are off. Often, your program just crashes

(segmentation fault). Other times, it computes mysterious things. Just be really careful.

• C and C

do not check pointers before following them. So, all the same warnings from the

previous item apply. (In fact, in C/C

an array is just a pointer to its ﬁrst element, so these are

actually the same thing.) You won’t get a NullPointerException; you’ll get a segmentation

fault.

• C and C

do not automatically initialize variables. If you say:

int x;

int* y; /* pointer to an integer */

x could have any value at all, and y could point anywhere in memory. Pointers aren’t initialized

to NULL, and integers aren’t initialized to zero. You should always initialize your variables and

you should always check a pointer before following it; the cost in efﬁciency is simply not worth

admitting this source of error.

• Don’t name a program test, even though it’s a test program. That’s because there’s a built-in

command in Unix called “test,” which will take precedence over your program name. You can call

it pretty much anything else. If you’re ever worried that the program that is running is not the

one you wrote, just add a print statement in the ﬁrst line of main.

• Print statements in C and C

use buffered I/O, which means they aren’t printed to the screen the

moment they are executed, but instead are printed to an internal buffer to be printed later. This

can be confusing for debugging statements. Therefore, your debugging statements should always

end with a newline character, like this:

printf("Got to this line.\n");

• There is no “new” operator in C, as there is in Java, so you have to think about variables in the C

way.

• Until you feel more comfortable with the language, avoid dynamic memory allocation in C if you

can. It won’t hurt to pre-allocate strings and arrays while you’re still a novice.

• Error messages from the C or C

compiler can be cryptic. One that often trips people up is “bad

lvalue.” This means that the compiler came across something that it thinks is an assignment, but

the thing on the left (the lvalue) isn’t a storage location. For example, you might get that error

message with the following code:

if( x+y = z ) { ...

You probably meant to write x+y==z, but accidentally left out an equals sign — a common error

for people using C, C

or Java. The error message just means that you can’t assign a value to

x+y, since that’s not a storage location.

Chapter 2

Overview of C

In this opening chapter, I’ll use a small example to introduce some general mechanics of programming

in C. The next chapter will survey the C language, comparing it to Java when that’s helpful. I’ll defer

all the details of the code until those sections. For now, let’s just dive in and get our hands dirty.

As with most programming, we start in an editor. Start with your favorite editor, which should be

Emacs.

Write the following code. This is the nauseatingly famous “hello world” program.

#include <stdio.h>

int main() {

printf("Hello World!\n");

}

Save that as hello.c, and then switch to a “shell.” (I’m assuming you’re programing in Unix/Linux).

We now need to compile and run the program.

The usual C compiler on a Linux machine is the GNU C Compiler, called gcc. You can run it on

your program, putting the output in hello as follows:

% gcc hello.c -o hello

Notice that if it succeeds, the command just ﬁnishes without any noise or warning. Don’t look for

any congratulations from Unix.

You could put your output in any ﬁlename, but it’s traditional to name it the same as the main

source ﬁle, but without an “extension” (anything after the dot).

You now need to run the program. This is done by giving its name on the command line, just as if

it were a command.

% hello

Hello World!

(On some computers, you need to precede the command name with ./, because you don’t have “.”

(dot) on your PATH. Consult a local guru or system administrator if you don’t know what that last

sentence means.)

Returning to the code, you’ll see that there is a function named main, and it calls a function named

printf to print a string (the stuff in quotation marks). Don’t worry about the #include line for now.

Let’s do something slightly more interesting. Let’s write a program that will tell us the successor

of any number we give it. Type the following code into an editor:

Forgive me this burst of bigotry; I’ll try to be more even-handed in the language comparisons.

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lvsc1987

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