TITLE: One way to process a command line


PROBLEM: terry@uivlsisd.csl.uiuc.edu (Terry Lee), 9 May 94

Is getopt() standardized?  If so, what should be the type of its second
argument?  I've encountered one compiler (IBM xlC) that prototypes the second
argument as const char**.  This is a problem because then I can't pass in
argv because conversion from char** to const char** isn't safe.  Is there no
portable way of using getopt()?  Thanks for any help.


RESPONSE: kanze@us-es.sel.de (James Kanze)

I don't believe that getopt is standard.  I don't remember having seen
it in the ISO C standard.  So if you use it, counting on the system to
provide it, your code is not portable.

But do you really want `getopt'?  I've recently been experimenting
with another approach, which looks very promessing.  Basically, I have
an Option class, with a derived class for each of the different option
types (BooleanOption, NumericOption, etc.).  There is also a class
ProgArgs, which maintains a static list of the options; the
constructor of Option registers itself with ProgArgs.  The constructor
for ProgArgs takes argc and argv as parameters, and iterates over the
args, removing any options from the list and calling the `set'
function (a virtual function of Option) for the corresponding option.
In addition, the derived option classes have conversion operators for
the correct type, e.g.: BooleanOption::operator bool(),
NumericOption::operator int(), etc.

It is easier to show the use than to explain:

	static BooleanOption	bOpt( 'b' ) ;
	static NumericOption	nOpt( 'n' ) ;

	int
	main( int argc , char* argv[] )
	{
	    ProgArgs		    a( argc , argv ) ;
	    if ( bOpt )
	    {
		for ( int i = 0 ; i < nOpt ; i ++ )
	// ...

There's obviously more to it than that, but you get the general idea.
But it makes adding an option *really* easy.  (Maybe too easy for good
interface design:-).

[...]

(The option classes and ProgArgs, above, work together to parse the
 command line. The programmer doesn't write a program, nor even call
 a function.  He just declares a few variables to statically describe
 the possible options; the variables themselves do the rest.)

[...]