Using command-line arguments in C++

By: Lakshmi

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 arguments on the command line. The program name itself is counted, so every program has at least one parameter. The example command line shown previously has four. (The name SomeProgram plus 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 may 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 argc to ensure you've received the expected number of arguments, and to use argv to 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 will need to translate these numbers to strings. On Day 21 you will see how to use the standard library conversions. Listing below illustrates how to use the command-line arguments.

Using command-line arguments.

1:     #include <iostream.h>
2:     int main(int argc, char **argv)
3:     {
4:        cout << "Received " << argc << " arguments...\n";
5:        for (int i=0; i<argc; i++)
6:           cout << "argument " << i << ": " << argv[i] << endl;
7:     return 0;
8: }

Output: TestProgram  Teach Yourself C++ In 21 Days
Received 7 arguments...
argumnet 0: TestProgram.exe
argument 1: Teach
argument 2: Yourself
argument 3: C++
argument 4: In
argument 5: 21
argument 6: Days

Analysis: The function main() declares two arguments: argc is an integer that contains the count of command-line arguments, and argv is a pointer to the array of strings. Each string in the array pointed to by argv is a command-line argument. Note that argv could just as easily have been declared as char *argv[] or char argv[][]. It is a matter of programming style how you declare argv; even though this program declared it as a pointer to a pointer, array offsets were still used to access the individual strings.

On line 4, argc is used to print the number of command-line arguments: seven in all, counting the program name itself.

On lines 5 and 6, each of the command-line arguments is printed, passing the null-terminated strings to cout by indexing into the array of strings.