C++ Primer Plus полностью

When you construct a simple home, you begin with the foundation and the framework. If you don’t have a solid structure from the beginning, you’ll have trouble later filling in the details, such as windows, door frames, observatory domes, and parquet ballrooms. Similarly, when you learn a computer language, you should begin by learning the basic structure for a program. Only then can you move on to the details, such as loops and objects. This chapter gives you an overview of the essential structure of a C++ program and previews some topics—notably functions and classes—covered in much greater detail in later chapters. (The idea is to introduce at least some of the basic concepts gradually en route to the great awakenings that come later.)

C++ Initiation

Let’s begin with a simple C++ program that displays a message. Listing 2.1 uses the C++ cout (pronounced “see-out”) facility to produce character output. The source code includes several comments to the reader; these lines begin with //, and the compiler ignores them. C++ is case sensitive; that is, it discriminates between uppercase characters and lowercase characters. This means you must be careful to use the same case as in the examples. For example, this program uses cout, and if you substitute Cout or COUT, the compiler rejects your offering and accuses you of using unknown identifiers. (The compiler is also spelling sensitive, so don’t try kout or coot, either.) The cpp filename extension is a common way to indicate a C++ program; you might need to use a different extension, as described in Chapter 1, “Getting Started with C++.”

Listing 2.1. myfirst.cpp

// myfirst.cpp -- displays a message

#include                            // a PREPROCESSOR directive

int main()                                    // function header

{                                             // start of function body

    using namespace std;                      // make definitions visible

    cout << "Come up and C++ me some time.";  // message

    cout << endl;                             // start a new line

    cout << "You won't regret it!" << endl;   // more output

    return 0;                                 // terminate main()

}                                             // end of function body

Program Adjustments

You might find that you must alter the examples in this book to run on your system. The most common reason is a matter of the programming environment. Some windowing environments run the program in a separate window and then automatically close the window when the program finishes. As discussed in Chapter 1, you can make the window stay open until you strike a key by adding the following line of code before the return statement:

cin.get();

For some programs you must add two of these lines to keep the window open until you press a key. You’ll learn more about cin.get() in Chapter 4, “Compound Types.”

If you have a very old system, it may not support features introduced by the C++98 standard.

Some programs require a compiler with some level of support for the C++11 standard. They will be clearly identified and, if possible, alternative non-C++11 code will be suggested.

After you use your editor of choice to copy this program (or else use the source code files available online from this book’s web page—check the registration link on the back cover for more information), you can use your C++ compiler to create the executable code, as Chapter 1 outlines. Here is the output from running the compiled program in Listing 2.1:

Come up and C++ me some time.

You won't regret it!

C Input and Output

If you’re used to programming in C, seeing cout instead of the printf() function might come as a minor shock. C++ can, in fact, use printf(), scanf(), and all the other standard C input and output functions, provided that you include the usual C stdio.h file. But this is a C++ book, so it uses C++’s input facilities, which improve in many ways upon the C versions.

You construct C++ programs from building blocks called functions. Typically, you organize a program into major tasks and then design separate functions to handle those tasks. The example shown in Listing 2.1 is simple enough to consist of a single function named main(). The myfirst.cpp example has the following elements:

• Comments, indicated by the // prefix

• A preprocessor #include directive

• A function header: int main()

• A using namespace directive

• A function body, delimited by { and }

• Statements that uses the C++ cout facility to display a message

• A return statement to terminate the main() function

Let’s look at these various elements in greater detail. The main() function is a good place to start because some of the features that precede main(), such as the preprocessor directive, are simpler to understand after you see what main() does.

Features of the main() Function