ekt_bear: Discuss further the factory design pattern. When would you want to use it?

harryobas:

The factory design pattern helps to improve cohesion and promotes separation of concerns by segregating the responsibility of object creation from domain objects to a pure fabrication "factory" object.

lordZOUGA:
an instance of where it can be used?

ekt_bear: In other words:

1. A poorly-designed piece of code
2. State what the problems are
3. Use the factory design pattern to improve it

Chimanet: For instance u have a code for returning a Vehicle object, all d different Vehicles instances lyk bus, lorry, saloon implement d Vehicle interface, u can have a factory class dat does nothing than using a a static method dat has a switch statement 2 return a particular implementation of Vehicle depending on d parameter u pass 2 it. public static Vehicle getVihecle(String type){.....} method. If you pass a bus string, it switches 2 bus case statement and instantiate a bus and returns 2 u. In an oop language lyk java, u can abstract all ur database connection creation code wit dis pattern since all d different jdbc implementation of database connection implements java.sql.Connection interface, oracle connection, mysql connection, etc, so u can easily swap d implementation of d connection u r using by jst changing d parameter u pass 2 a static getConnection method of a factory class. D purpose of d factory class is jst 2 return different implementations of connection objects shikena!

lordZOUGA:
so, in fact, I can replace a factory class with a function like this in C++;
enum Transport{CAR, BUS};
Vehicle getVehicle(Transport aTrans){switch(aTrans).... Return Car();}
put it in a header file and include it in all class defnitions. Yes?. Or is there an added benefit in making it a singleton? Encapsulation?

harryobas:

A header file in C++ usually contains only function declarations and not implementations so even if u put the function in a header file u will still have to provide implementation in some sort of factory class definition.

lordZOUGA:
no. In C++, a header file can contain only function defnitions...

Chimanet: Wen u use a factory class, u jst do Factory.getVehicle(parameter), because d getVehicle method is jst lyk a global function. in java we call it a static method, u dnt need 2 create an instance of d factory class (object) in d jvm heap, cos creating an object wit d new operator in java eats up jvm memory and is expensive and cause tight coupling. So factory pattern saves memory, makes our objects loosely coupled, and our code even looks more elegant. No nid 2 b creating objects of factory classes wit new operator. Jst load d class in d jvm and use its static method. Shikena!

harryobas:

No. I still maintain that a header file contains function declarations(prototypes) and not definitions. For example the cout function is declared in the iostream header file but defined/implemented in the standard runtime library.

Chimanet: @lordzouga, from ur analogy i think wat u are trying to depicts using header file in cplusplus looks more lyk interfaces 2 me in java and csharp

lordZOUGA:
really? How do you suppose inline statements work??
Foo.h
class Foo{
int Foo;
public:
Foo(int num){Foo = num;}
getFoo(){return Foo;}
};
you think the above class won't run?

harryobas:

Are u trying to define a class inside a header file? or are u including the header file in your class definition.

lordZOUGA:
I defined the class in a header file. I could have included the #IFNDEF pre-processor directive but this is trivial...
So in main.cpp, I do this:
#include "Foo.h"
#include <iostream>
int main(){
int x = 6;

Foo myFoo(x);
int y = myFoo.getFoo();

cout << y;
return 0;


}

this works

Chimanet: Wen u use a factory class, u jst do Factory.getVehicle(parameter), because d getVehicle method is jst lyk a global function. in java we call it a static method, u dnt need 2 create an instance of d factory class (object) in d jvm heap, cos creating an object wit d new operator in java eats up jvm memory and is expensive and cause tight coupling. So factory pattern saves memory, makes our objects loosely coupled, and our code even looks more elegant. No nid 2 b creating objects of factory classes wit new operator. Jst load d class in d jvm and use its static method. Shikena!

Chimanet: Who among us is a java developer? D Calendar class, Calender.getInstance() is d clearest example of factory pattern in the java class library

harryobas:

You could also implement a factory as a singleton which creates one global instance of the factory class.