Design patterns – Factory Method – library MetaTrader 5

//FACTORY METHOD -------------------------------------------------------------------------------
// | GROUP
// |  | creational
// | INTENT 
// |  | interface for creating an object
// |  |  | defers instantiation to subclasses
// |  | subclasses decide which class to instantiate
// | BENEFITS 
// |  | aspects of the code that may vary 
// |  |  | subclass of object that is instantiated
// |  | refactoring
// |  |  | problem
// |  |  |  | creating an object by specifying a class explicitly
// |  |  | solution
// |  |  |  | create objects indirectly
// |  |  |  |  | also
// |  |  |  |  |  | Abstract Factory
// |  |  |  |  |  | Prototype
// | APPLICABILITY 
// |  | a class
// |  |  | can't anticipate the class of objects it must create
// |  |  | wants its subclasses to specify the objects it creates
// |  |  | delegates responsibility to one of several helper subclasses
// |  |  |  | localize the knowledge of which helper subclass is the delegate
// | STRUCTURE
// |  |
//                                                |         Creator        |
//                                                |------------------------|
//                                                |FactoryMethod()         |
//                                                |AnOperation()           |
//                                                | ...                    |
//                                                | product=FactoryMethod()|
//                 |Product|                      | ...                    |
//                     ^                                      ^
//                     |                                      |
//             |ConcreteProduct|<- - - - - - - -|      ConcreteCreator      |
//                                              |---------------------------|
//                                              |FactoryMethod()            |
//                                              | return new ConcreteProduct|
// |  |
#include <SRCPatternsPatterns.mqh>
namespace FactoryMethod
{
// |  | 
// | PARTICIPANTS ------------------------------------------------------------------------------
// |  | PRODUCT
// |  |  | interface of objects that the factory method creates
// |  |  |
interface Product
  {
  };
// |  |  | 
// |  | CONCRETE PRODUCT
// |  |  | implements the Product interface
// |  |  |
class ConcreteProduct:public Product
  {
public:
                     ConcreteProduct();
  };
ConcreteProduct::ConcreteProduct(void)
  {
   Print("Hash: ",&this);
  }
// |  |  | 
// |  | CREATOR
// |  |  | declares the factory method
// |  |  |  | returns an object of type Product
// |  |  |  | default implementation of the factory method
// |  |  |  |  | returns a default ConcreteProduct object
// |  |  | may call the factory method to create a Product object
// |  |  |
class Creator
  {
public:
   virtual Product*  FactoryMethod()=0;
   void              AnOperation();
                    ~Creator();
protected:
   Product*          product;
  };
Creator::~Creator(void)
  {
   delete product;
  }
void Creator::AnOperation(void)
  {
   delete product;
   Print("Creator operation: creating product with a virtual factory method.");
   product=FactoryMethod();
  }
// |  |  | 
// |  | CONCRETE CREATOR
// |  |  | overrides the factory method
// |  |  | returns an instance of a ConcreteProduct
// |  |  |
class ConcreteCreator:public Creator
  {
public:
   Product*          FactoryMethod();
  };
Product* ConcreteCreator::FactoryMethod(void)
  {
   Print("Concrete creator factory method: returning new product");
   return new ConcreteProduct;
  }
// |  |  | 
// |  | CLIENT
// |  |  | 
class Client:public ClientExample
  {
public:
   string            Output();
   void              Run();
  };
string Client::Output()
  {
   return __FUNCTION__;
  }
// |  |   
// | COLLABORATIONS ----------------------------------------------------------------------------
// |  | Creator
// |  |  | relies on its subclasses
// |  |  |  | to define the factory method
// |  |  |  |  | returns an instance of the appropriate ConcreteProduct
// |  |   
void Client::Run()
  {
   ConcreteCreator creator;
   Product* product=creator.FactoryMethod();
   creator.AnOperation();
//---
   delete product;
  }
}
// |  | 
// | OUTPUT ------------------------------------------------------------------------------------
// |  | Creational::FactoryMethod::Client::Output
// |  | Concrete creator factory method: returning new product
// |  | Hash: 138412032
// |  | Creator operation: creating product with a virtual factory method.
// |  | Concrete creator factory method: returning new product
// |  | Hash: 139460608
// |  | 
// | CONSEQUENCES ------------------------------------------------------------------------------
// |  | Code only deals with the Product interface
// |  |  | can work with any user-defined ConcreteProduct classes
// |  |  | but clients might have to subclass the Creator class
// |  |  |  | to create a particular ConcreteProduct object
// |  | Provides hooks for subclasses
// |  |  | creating objects inside a class is
// |  |  |  | more flexible than creating an object directly
// |  |  | the factory method is
// |  |  |  | not abstract
// |  |  |  | provides a reasonable default implementation
// |  | Connects parallel class hierarchies when
// |  |  | class delegates some of its responsibilities to a separate class
// |  |  | consider graphical figures that
// |  |  |  | can be manipulated interactively
// |  |  |  |  | manipulator object
// |  |  |  |  |  | implements the interaction
// |  |  |  |  |  | keeps track of any manipulation-specific state that's needed
// |  |  |  |  | figure class
// |  |  |  |  |  | provides a factory method
// |  |  |  |  |  |  | lets clients create a corresponding manipulator
// |  |  |  |  | figure subclasses
// |  |  |  |  |  | override this method to return an
// |  |  |  |  |  |  | instance of the manipulator subclass that's right for them
// | IMPLEMENTATION ----------------------------------------------------------------------------
// |  | Creator
// |  |  | abstract
// |  |  | concrete
// |  |  |  | default implementation for the factory method
// |  |  |  | the class of objects 
// |  |  |  |  | parent instantiates 
// |  |  |  |  | can be changed 
// |  | Parameterized factory methods
// |  |  | new identifiers for new kinds of products
// |  |  | associate existing identifiers with different products
// |  | Language-specific variants and issues
// |  |  | factory methods in c++ 
// |  |  |  | always virtual functions 
// |  |  |  | often pure virtual
// |  |  |  | use lazy initialization
// |  |  |  |  | creator's constructor doesn't call factory methods 
// |  | Using templates to avoid subclassing
// |  |  | template subclass of creator 
// |  |  |  | parameterized by the product class
// |  | Naming conventions
// |  |  | make use of factory methods clear
//----------------------------------------------------------------------------------------------