Replace Conditions (if statements) with Polymorphism

README.md

From the command line create an executable that according to a prototol:

• http
• https
• ftp

Returns a partially built URL: scheme://domain:port

$ ./executable ftp
ftp://ornl.gov:20

$ ./executable https
https://ornl.gov:443

$ ./executable  http
http://ornl.gov:80

Below non OOP and OOP versions.

non_oop.cpp

/*
 * non_oop.cpp
 *
 *  Created on: Sep 4, 2015
 *      Author: [email protected]
 *
 *  Non OOP example of URL:
 *  scheme:[//domain[:port]][/]path
 */

#include <iostream>
#include <string>

std::string domain = "ornl.gov";
std::string http_port = "80";
std::string https_port = "443";
std::string ftp_port = "20";

std::string build_url(const std::string &scheme,
		const std::string &domain,
		const std::string &port) {
	return scheme + "://" + domain + ":" + port;
}

int main(int argc, char* argv[]) {
	if (argc != 2) {
		std::cerr << "Use " + std::string(argv[0]) + " [http, https, ftp]"<<std::endl;
		exit(-1);
	}
	std::string scheme = argv[1];

	if (scheme == "http"){
		std::cout << build_url(scheme,domain,http_port) <<std::endl;
	}
	else if (scheme == "https"){
		std::cout << build_url(scheme,domain,https_port) <<std::endl;
	}
	else if (scheme == "ftp"){
		std::cout << build_url(scheme,domain,ftp_port) <<std::endl;
	}
	else {
		std::cerr << "Scheme not valid. Use of these: http, https, ftp"<<std::endl;
	}
	return 0;
}

oop.cpp

/*
 * oop.cpp
 *
 *  Created on: Sep 4, 2015
 *      Author: [email protected]
 *
 *  OOP example of URL:
 *  scheme:[//domain[:port]][/]path
 */

#include <iostream>
#include <string>
#include <map>

// Abstract class: cannot be instantiated
class URL {
protected:
	std::string domain = "ornl.gov";
	std::string port;
	std::string scheme;
	std::string build_url(const std::string &scheme,
			const std::string &port) const {
		return scheme + "://" + domain + ":" + port;
	}
public:
	// Pure virtual function: i.e. must be overridden by a derived class
	virtual std::string build_url() const = 0;
	~URL() {};
};

class Http: public URL {
public:
	Http() {port = "80"; scheme = "http";}
	std::string build_url() const {
		return URL::build_url(scheme, port);
	}
};

class Https: public URL {
public:
	Https() {port = "443"; scheme = "https";}
	std::string build_url() const {
		return URL::build_url(scheme, port);
	}
};

class Ftp: public URL {
public:
	Ftp() {port = "20"; scheme = "ftp";}
	std::string build_url() const {
		return URL::build_url(scheme, port);
	}
};

/**
 * Let's imagine that I want to add a path to the URL: http://ornl.gov:80/<path>
 * I don't need to modify the code! Open/closed principle.
 * Just extend it, i.e., add new code.
 */

class HttpWithPath: public Http {
protected:
	std::string path;
public:
	HttpWithPath(std::string path) : Http(), path(path) {}
	std::string build_url() const {
		std::string default_url = URL::build_url(scheme, port);
		return default_url + "/" + path;
	}
};

/**
 * Main function
 */
std::string build_url(const URL &url) {
	return url.build_url();
}

int main(int argc, char* argv[]) {
	if (argc != 2) {
		std::cerr << "Use " + std::string(argv[0]) + " [http, https, ftp]"
				<< std::endl;
		exit(-1);
	}
	std::string scheme = argv[1];

	// UGLY!!!
	// In real world I would have here some sort of Dependency Injection (e.g. factory)
	// This just shows that we can get an object given a string
	Http http;
	Https https;
	Ftp ftp;
	std::map<std::string, URL*> choices;
	choices.insert(std::make_pair("http", &http));
	choices.insert(std::make_pair("https", &https));
	choices.insert(std::make_pair("ftp", &ftp));

	if (scheme == "http" or scheme == "https" or scheme == "ftp") {
		std::cout <<  build_url(*choices[scheme]) << std::endl;
	} else {
		std::cerr << "Scheme not valid. Use of these: http, https, ftp"
				<< std::endl;
	}
	return 0;
}