main.cpp

/*	A modest getopt tutorial - example really.
	Perry Kivolowitz
	Carthage College
*/

#include <iostream>
#include <getopt.h>
#include <string>

using namespace std;

/*	This program is a modest tutorial on the use of getopt.

	getopt is a very old C library that facilitates the parsing of command line arguments.
	This tutorial (more like an example) is not complete. You can find documentation on getopt
	online.
*/


int main(int argc, char * argv[]) {
	/*	getopt is typically isolated in a function or method specifically intended to process
		command line options. Below, I have defined some variables of different types and will
		pass these to the getopt managing function. Notice I have initialized these variables with
		default values.

		That is, if the corresponding option is not specified on the command line, the default
		value is retained. 

		One of these variables will be *required* - you'll see how this can be implemented.
	*/

	/*	Forward references to supporting functions. Of course, 
		the forward references would not be needed using a c++ class or by reordering these
		functions.
	*/
	void PrintUsage();
	bool HandleOptions(int argc, char ** argv, int &, bool &, char **, string &, float &);

	int an_integer = 17;
	float a_float = 12.345;
	bool a_bool = false;
	char * a_c_string = nullptr;
	string a_cpp_string_this_will_be_required;

	if (!HandleOptions(argc, argv, an_integer, a_bool, &a_c_string, a_cpp_string_this_will_be_required, a_float)) {
		PrintUsage();
		return 1;
	}
	cout << "An integer: " << an_integer << endl;
	cout << "A bool:     " << a_bool << endl;
	cout << "A c-string: " << ((a_c_string == nullptr) ? "" : a_c_string) << endl;
	cout << "A string:   " << a_cpp_string_this_will_be_required << endl;
	cout << "A float:    " << a_float << endl;
	return 0;
}

/*	This is not the tradional usage format but is fairly informative
*/
void PrintUsage() {
	cerr << "Usage:" << endl;
	cerr << "-s string       this is requred." << endl;
	cerr << "-i integer      optional - an integer" << endl;
	cerr << "-b              optional - if present a boolean is set to true" << endl;
	cerr << "-c string       optional - a c string" << endl;
	cerr << "-s string       optional - a c++ string" << endl;
	cerr << "-f float        optional - a float" << endl;
	cerr << "Note that without care, a string is just one \"word\"" << endl;
}

bool HandleOptions(int argc, char ** argv, int & an_int, bool & a_bool, char ** a_c_string, string & a_cpp_string, float & a_float) {
	int c;

	/*	getopt must be passed argc and argv - after all these are how command line arguments are passed to programs.

		The third parameter is a format string defining the valid command line options as well as if a specific
		option requires a parameter. This is controlled by whether or not a colon (':') comes after the option
		character. In the following example, 's' requires a parameter but 'b' does not.

		When there are no more members of argv to attempt parsing, getopt return -1.

		Notice there is no requirement to keep to any particular order of options.
	*/

	/*	Notice where optarg is declared? Me neither. It is a global char pointer brought in with getopt.h.
		It is initialized to a command line option's argument if it has one. Bad things can happen if you
		make use of optarg when a command line option does NOT have an argument.
	*/
	while ((c = getopt(argc, argv, "hf:i:bc:s:")) != -1) {
		switch (c) {
			default:
			case 'h':
				// Short circut forcing the usage to be printed by returning false from this function.
				return false;

			case 'i':
				an_int = atoi(optarg);
				break;

			case 'b':
				a_bool = true;
				break;

			case 'c':
				*a_c_string = optarg;
				break;

			case 's':
				a_cpp_string = string(optarg);
				break;

			case 'f':
				a_float = atof(optarg);
				break;
		}
	}

	/*	The s option is to be required. If the string variable still has length 0, the s option was not given.
		Note that this is just one construction of the test. I can use this construction because there is only
		one way HandleOptions can fail.
	*/
	return a_cpp_string.size() > 0;
}