Command Line Arguments in C++
The main fuction in a C++ program can either be defined as:
int main () OR int main ( int argc, char** argv );
The second variation allows you to specify command line arguments to be passed
to your main function. You may have as many command line arguments as you
like. The number of arguments is stored in argc and argv is a array of
points to strings which represent the parameters that you passed in to main.
Argc is always at least equal to 1 because argv[0] is the name of your
program. So if your program is called sort and you typed "sort file1 file2",
the following parameters to main are set: argc = 3, argv[0] = sort, argv[1]
= file1, and argv[2] = file2.
To demonstrate, write the following code:
int main ( int argc, char** argv ) {
for ( int x = 0; x < argc; x ++ ) {
cout << argv[x] << endl;
}
}
Compile this a run "a.out param1 param2 param3" and the arguments will be
printed.
Argc tells you how many command line parameters were passed in. You use this
parameter to error check the usage of your program to make sure the exact
number of expected parameters were passed to the main program. You should
always error check the number of command line parameters and if the user does
not enter the EXACT number of expected command line parameters, you should
print a message stating the expected usage of the program and then exit.
I always like to make my usage message match that of a typical unix usage
message. To see what I mean, type the command grep at the command prompt
in a Unix shell with no command line parameters. You get an expected usage
message. It then tells you to type "grep --help" for info on the actual
parameters. I don't expect you to do all of this for your assignments but
you should at the very least error check argc and print out a usage
message. The help menu is something that you should be doing
but is not required.