Calling function based on user-input

main.cpp

#include <algorithm>
#include <string>
#include <stdexcept>
#include <vector>
#include <iostream>
#include <cmath>
#include <unordered_map>
#include <functional>
#include <iterator>

void print(const std::vector<std::string>& args) {
	std::copy(args.begin(), args.end(), std::ostream_iterator<std::string>{std::cout, "\n"});
}

void print_sqrt(const std::vector<std::string>& args) {
	if (args.size() != 1) {
		throw std::runtime_error{"sqrt must get exactly one argument"};
	}
	std::cout << std::sqrt(std::stod(args.front())) << '\n';
}
void print_square(const std::vector<std::string>& args) {
	if (args.size() != 1) {
		throw std::runtime_error{"sqare must get exactly one argument"};
	}
	const auto val = std::stod(args.front());
	std::cout << val* val << '\n';
}

void print_sum(const std::vector<std::string>& args) {
	const auto op = [](double val, const std::string& str) { return val + std::stod(str); };
	std::cout << std::accumulate(args.begin(), args.end(), 0.0, op) << '\n';
}
void print_product(const std::vector<std::string>& args) {
	const auto op = [](double val, const std::string& str) { return val * std::stod(str); };
	std::cout << std::accumulate(args.begin(), args.end(), 1.0, op) << '\n';
}

int main(int argc, char** argv) try {
	if (argc < 2) {
		std::cerr << "Error: At least one argument required.\n";
		return 1;
	}
	const auto fun = std::string{argv[1]};
	const auto args = std::vector<std::string>(argv + 2, argv + argc);
	const auto function_map =
	        std::unordered_map<std::string,
	                           std::function<void(const std::vector<std::string>&)>>{
	                {"print", print},
	                {"sqrt", print_sqrt},
	                {"square", print_square},
	                {"+", print_sum},
	                {"*", print_product}};

	const auto it = function_map.find(fun);
	if (it == function_map.end()) {
		std::cerr << "Error: No such function.\n";
		return 2;
	}
	it->second(args);
} catch (std::runtime_error& e) {
	std::cerr << "Error: " << e.what() << '\n';
	return 3;
}