#include <iostream>

void usage(char const* argv0) 
{ std::cerr << "usage: " << argv0 << " input_file output_file [iteration_count]\n"; }

int main(int argc, char** argv)
{
  if (argc < 3 || argc > 4) { usage(argv[0]); return 1; }
  // ... 
}

#include <iostream>
#include <cstdlib>

struct Args {
	int method {};		// Default value for method
	int iter { 1 };		// Default value for max-iter
};

void usage(const char* prg) {
	std::cout << "Usage: " << prg << " {-method <num>} { -iter_max <num>}\n";
}

// Return true OK, false bad
bool parse(int argc, char* argv[], Args& args) {
	bool gotmethod {};
	bool gotiter {};
	bool good {true};
	Args ags {};

	for (int a { 1 }; a < argc; a += 2)
		if (strcmp(argv[a], "-method") == 0) {
			if (gotmethod) {
				std::cout << "Duplicate -method option\n";
				good = false;
			} else {
				char* endptr;

				gotmethod = true;
				ags.method = std::strtol(argv[a + 1], &endptr, 10);
				if (*endptr != 0) {
					std::cout << "Invalid method number specified\n";
					good = false;
				}
			}
		} else if (strcmp(argv[a], "-max_iter") == 0) {
			if (gotiter) {
				std::cout << "Duplicate -max_iter option\n";
				good = false;
			} else {
				char* endptr;

				gotiter = true;
				ags.iter = std::strtol(argv[a + 1], &endptr, 10);
				if (*endptr != 0) {
					std::cout << "Invalid max_iter number specified\n";
					good = false;
				}
			}
		} else {
			std::cout << "Unknown option '" << argv[a] << "' specified\n";
			good = false;
		}

	if (good)
		args = ags;

	return good;
}

int main(int argc, char* argv[]) {
	Args args;

	if (parse(argc, argv, args)) {
		std::cout << "Method is: " << args.method << '\n';
		std::cout << "Max_iter is: " << args.iter << '\n';
	} else
		usage(argv[0]);
}

#include <iostream>
#include <string>
#include <fstream>

void help(std::string progName)
{
        std::cout << progName << " can be used as a tiny calculator, here's an example" << std::endl;
        std::cout << progName << " -v 3 -a 2 -t 4" << std::endl;
        std::cout << "which sets the current value to 3, adds 2, then multiplies the result by 4." << std::endl;

}

int main(int argc, char ** argv)
{
        double value = 0;
        std::string filePath = "";
        for(int i = 1; i < argc; i ++)
        {
                std::string arg = argv[i];
                if(arg == "-v" && i + 1 < argc)
                {
                        value = std::stod(argv[i + 1]);
                        i ++;
                }
                else if(arg == "-a" && i + 1 < argc)
                {
                        value += std::stod(argv[i + 1]);
                        i ++;
                }
                else if(arg == "-t" && i + 1 < argc)
                {
                        value *= std::stod(argv[i + 1]);
                        i ++;
                }
                else if(arg == "-f" && i + 1 < argc)
                {
                        filePath = argv[i + 1];
                        i ++;
                }
                else if(arg == "-h")
                {
                        help(argv[0]);
                }
                else
                {
                        std::cout << "Hmm, " << argv[i] << " is not viable\n";
                        help(argv[0]);
                        return 0;
                }
        }
        if(filePath.empty())
        {
                std::cout << value << std::endl;
        }
        else
        {
                std::cout << "Save output to " << filePath << std::endl;
        }

}

#include <iostream>				 //Required for cin, cout
#include <unistd.h>				 // for getopt()

using std::cout;
using std::string;

void usage(const char *argv0)
{
    cout << "This program simulates doughnut frosting erors when\n";
    cout << "making \"count\" doughnuts with \"method\" frosting method\n\n";
    cout << "Usage: " << argv0 << " -m method -c count\n";
}

int
main(int argc, char **argv)
{
    int opt;	// note that opt is an int, not a char
    int count = -1, method = -1; // You could change these to some default
				 // values if it made sense

    // Note the "=" below is intentional. It shouldn't be ==
    while ((opt = getopt(argc, argv, "m:c:")) != EOF) {
	switch (opt) {
	case 'm':
	    method = atoi(optarg); // optarg is the thing that follows -m
	    break;
	case 'c':
	    count = atoi(optarg);
	    break;
	default:
	    usage(argv[0]);
	    return 1;
	}
    }
    if (optind < argc || method<0 || count<0) {
	// There's more stuff on the command line, like "my_code.exe foobar",
	// or they didn't specify both -m and -c options, or one of them
	// was invalid like -m freddy.
	usage(argv[0]);
	return 1;
    }
    cout << "Running simulation with method " << method
	 << " and count " << count << '\n';
    return 0;
}