A sample void function that requires some changes...

main.cpp

// db-conf CLI
//
// GitHub: https://github.com/kerig-it/db-conf


// Libraries
#include <iostream>
#include <string>
#include <vector>

// Namespaces
using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;

// CLI name
string cli_name = "db-conf";

// Output printer
void print(string str = "", bool parsing = false) {

	// Is the `parsing` argument enabled?
	if (parsing) {

		// Is the supplied string "default"?
		if (str == "default") {
			// Print the default input prompt.
			cout << cli_name << "> ";
		}
	}

	// Is the supplied string something else?
	else {
		// Print supplied string.
		cout << str << endl;
	}
}

// Help printer
void help(string arg = "") {

	// Is no argument supplied?
	if (arg.length() == 0) {
		// Print help.
		print("Available commands:");
		print(" exit\tExit the " + cli_name + " CLI.");
		print(" help\tPrint this message.");
	}

	// Is an argument supplied?
	else if (arg.length() > 0) {
		print("Support for help for individual commands to be added.");
	}
}

// Processing function
bool process(string command = "") {

	// Is a command specified?
	if (command.length() > 0) {

		// Is the command set to "exit"?
		if (command == "exit") {
			// Print feedback and exit.
			print("Exiting...");
			std::exit(0);
		}

		// Is the command set to "help"?
		else if (command == "help") {
			// Call `help` and return status.
			help();
			return true;
		}

		// Is a different command specicied?
		else {
			// Print feedback and return status.
			print("Unknown command specified: " + command);
			return false;
		}
	}
	
	// Print failure message and return `false`.
	print("No command was specified.");
	return false;
}

// Input handler function
void input(vector<string> args = { cli_name }) {

	// Is an input supplied?
	if (args.size() > 1) {
		// Process supplied arguments and call `input` recursively.
		process(args[1]);
		input();
	}

	// Is nothing supplied?
	else {
		// Print the default input prompt.
		print("default", true);

		// Define and assign via user input the variable `arg`.
		string arg;
		cin >> arg;

		// Call `input` recursively.
		input({ cli_name, arg });
	}
}

// Main function
int main(int argc, char* argv[]) {

	// Print about CLI initialisation.
	print("\nInitialising the db-conf CLI...\n");

	// Define a vector that will hold supplied arguments.
	vector<string> args;

	// "Convert" supplied arguments to a vector.
	for (int i = 0; i < argc; ++i) {
		// Add item from `argv` to `args`.
		args.push_back(argv[i]);
	}

	// Initialise CLI.
	if (argc > 1) {
		input(args);
	}
	else {
		input();
	}

	// Return success code.
	return 0;
}