C++ Primer Plus

You can also use this variant:

int main(void)     // very explicit style

Using the keyword void in the parentheses is an explicit way of saying that the function takes no arguments. Under C++ (but not C), leaving the parentheses empty is the same as using void in the parentheses. (In C, leaving the parentheses empty means you are remaining silent about whether there are arguments.)

Some programmers use this header and omit the return statement:

void main()

This is logically consistent because a void return type means the function doesn’t return a value. However, although this variant works on some systems, it’s not part of the C++ Standard. Thus, on other systems it fails. So you should avoid this form and use the C++ Standard form; it doesn’t require that much more effort to do it right.

Finally, the ISO C++ Standard makes a concession to those who complain about the tiresome necessity of having to place a return statement at the end of main(). If the compiler reaches the end of main() without encountering a return statement, the effect will be the same as if you ended main() with this statement:

return 0;

This implicit return is provided only for main() and not for any other function.

Why main() by Any Other Name Is Not the Same

There’s an extremely compelling reason to name the function in the myfirst.cpp program main(): You must do so. Ordinarily, a C++ program requires a function called main(). (And not, by the way, Main() or MAIN() or mane(). Remember, case and spelling count.) Because the myfirst.cpp program has only one function, that function must bear the responsibility of being main(). When you run a C++ program, execution always begins at the beginning of the main() function. Therefore, if you don’t have main(), you don’t have a complete program, and the compiler points out that you haven’t defined a main() function.

There are exceptions. For example, in Windows programming you can write a dynamic link library (DLL) module. This is code that other Windows programs can use. Because a DLL module is not a standalone program, it doesn’t need a main(). Programs for specialized environments, such as for a controller chip in a robot, might not need a main(). Some programming environments provide a skeleton program calling some nonstandard function, such as _tmain(); in that case there is a hidden main() that calls _tmain(). But your ordinary standalone program does need a main(); this books discusses that sort of program.

C++ Comments

The double slash (//) introduces a C++ comment. A comment is a remark from the programmer to the reader that usually identifies a section of a program or explains some aspect of the code. The compiler ignores comments. After all, it knows C++ at least as well as you do, and, in any case, it’s incapable of understanding comments. As far as the compiler is concerned, Listing 2.1 looks as if it were written without comments, like this:

#include

int main()

{

    using namespace std;

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

    cout << endl;

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

    return 0;

}

C++ comments run from the // to the end of the line. A comment can be on its own line, or it can be on the same line as code. Incidentally, note the first line in Listing 2.1:

// myfirst.cpp -- displays a message

In this book all programs begin with a comment that gives the filename for the source code and a brief program summary. As mentioned in Chapter 1, the filename extension for source code depends on your C++ system. Other systems might use myfirst.C or myfirst.cxx for names.

Tip

You should use comments to document your programs. The more complex the program, the more valuable comments are. Not only do they help others to understand what you have done, but also they help you understand what you’ve done, especially if you haven’t looked at the program for a while.

C-Style Comments

C++ also recognizes C comments, which are enclosed between /* and */ symbols:

#include /* a C-style comment */

Because the C-style comment is terminated by */ rather than by the end of a line, you can spread it over more than one line. You can use either or both styles in your programs. However, try sticking to the C++ style. Because it doesn’t involve remembering to correctly pair an end symbol with a begin symbol, it’s less likely to cause problems. Indeed, C99 has added the // comment to the C language.

The C++ Preprocessor and the iostream File

Here’s the short version of what you need to know. If your program is to use the usual C++ input or output facilities, you provide these two lines:

#include

using namespace std;