Sams Teach Yourself C++ in 21 Days

Working with Streams 631

17

On lines 5–20, a stripped-down Animalclass is declared. On lines 24–34, a file is

created and opened for output in binary mode. An animal whose weight is 50 and

who is 100 days old is created on line 36, and its data is written to the file on line 37.

The file is closed on line 39 and reopened for reading in binary mode on line 41. A sec-

ond animal is created, on line 48, whose weight is 1 and who is only one day old. The

data from the file is read into the new animal object on line 53, wiping out the existing

data and replacing it with the data from the file. The output confirms this.

Command-line Processing ..................................................................................

Many operating systems, such as DOS and Unix, enable the user to pass parameters to

your program when the program starts. These are called command-line options and are

typically separated by spaces on the command line. For example,

SomeProgram Param1 Param2 Param3

These parameters are not passed to main()directly. Instead, every program’s main()

function is passed two parameters. The first is an integer count of the number of argu-

ments on the command line. The program name itself is counted, so every program has

at least one parameter. The sample command line shown previously has four. (The name

SomeProgramplus the three parameters make a total of four command-line arguments.)

The second parameter passed to main()is an array of pointers to character strings.

Because an array name is a constant pointer to the first element of the array, you can

declare this argument to be a pointer to a pointer to char, a pointer to an array of char,

or an array of arrays of char.

Typically, the first argument is called argc(argument count), but you can call it anything

you like. The second argument is often called argv(argument vector), but again this is

just a convention.

It is common to test argcto ensure you’ve received the expected number of arguments

and to use argvto access the strings themselves. Note that argv[0]is the name of the

program, and argv[1]is the first parameter to the program, represented as a string. If

your program takes two numbers as arguments, you need to translate these numbers to

strings. On Day 21, you will see how to use the Standard Library conversions. Listing

17.19 illustrates how to use the command-line arguments.

ANALYSIS