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Study Section
Singleton design pattern in C# is one of the most widely used design patterns. In this pattern, only one instance of a class is created
Design Patterns
Why would we want to use a Singleton? In some programs you would want a single instance of a class, for example a db connection. If you have multiple connections you may have fall into update race proplems or lockups from multiple connections trying to gain access the same db. An implementation of the singleton pattern must: ensure that only one instance of the singleton class ever exists; and provide global access to that instance. Typically, this is done by declaring all constructors of the class to be private; and providing a static method that returns a reference to the instance. We start with createing a ‘Singleton’ class, a .cpp file. #include class Singleton { static Singleton* s; std::string onlyOne; Singleton(); public: Singleton(const Singleton\u0026) = delete; Singleton\u0026 operator=(const Singleton\u0026) = delete; ~Singleton(); static Singleton* getInstance(); void setSingleton(const std::string \u0026st); std::string getSingelton(); }; You need to disable the copy constructor. Now let’s create the .h Singleton header file. #include class Singleton { static Singleton* s; std::string onlyOne; Singleton(); public: Singleton(const Singleton\u0026) = delete; Singleton\u0026 operator=(const Singleton\u0026) = delete; ~Singleton(); static Singleton* getInstance(); void setSingleton(const std::string \u0026st); std::string getSingelton(); }; Now let’s put this all together in main method in the main.cpp file #include #include \
JAVA, Servlet, Design Pattern
JAVA, Servlet, Design Pattern
SAP ABAP News \u0026 Updates
Introduction Did you ever encounter that even after the statement DEFINITION DEFERRED you still need to move the definition of your class? Or have you ever wondered that if a singleton-class has several subclasses, why all those subclasses will be singletons, or that all classes in the tree will be one singleton? Those experiences were new to me, but I encountered them and I wanted to share my amazement. This blog is a mixture of a how-to create a factory-class for singletons with inheritance, some surprises I found during coding and a request for advice. So please, please comment and ask questions. I would love to learn from my mistakes and wrong assumptions. I chose to not use test-driven approach to keep this blog more readable. The road to a solution to test several different solutions for the same problems using inheritance and a factory class was not a straight road – not at all. It was such a bumpy ride that I thought this was worthy of its own blog. This blog first appeared at our
own website. The challenge We want to perform an operation on something. In my case, I wanted to test several solutions for a certain operation on an object. So I need: ◉ Uniformity in calling the solution. ◉ Different reactions for the operation. ◉ Managed instantiation. An additional “restraint” is that this is a proof of concept. I didn’t want to clog the system with dozens of artifacts, so I created it in one program. This led to one interesting issue… Interface and inheritance – Creating the Singleton(s) First, I want to have uniformity in calling the solution. So I define an interface that states what operation should be used: INTERFACE: zlif_interface. METHODS: do_something RETURNING VALUE(zrv_text) TYPE string. ENDINTERFACE. This is the most important part of the whole solution, and now it’s done. Almost finished! Let us think now about the other parts. It would be nice if we can re-use some standard code, and only have few differences. That can be solved by creating a superclass, with several subclasses. The polymorphism of the subclasses will take care of the different behaviour. CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED. PUBLIC SECTION. INTERFACES zlif_interface. ALIASES: do_something FOR zlif_interface~do_something. ENDCLASS. CLASS zlcl_subclass_one DEFINITION INHERITING FROM zlcl_super CREATE PROTECTED. PUBLIC SECTION. METHODS: do_something REDEFINITION. ENDCLASS. CLASS zlcl_subclass_two DEFINITION INHERITING FROM zlcl_super CREATE PROTECTED. PUBLIC SECTION. METHODS: do_something REDEFINITION. ENDCLASS. I decided to omit the IMPLEMENTATION part here for better readability. The UML diagram looks like this: UML: Singleton with Inheritance As we will use these classes for operations on other objects, I want to use them as a singleton, adding a static method get_instance( ) and define a static singleton reference in the interface. I mark the classes as CREATE PROTECTED. INTERFACE: zlif_interface. METHODS: do_something RETURNING VALUE(zrv_text) TYPE string. CLASS-DATA: zgr_instance TYPE REF TO zlif_interface. CLASS-METHODS: get_instance RETURNING VALUE(zrv_instance) TYPE REF TO zlif_interface. ENDINTERFACE. CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED. PUBLIC SECTION. INTERFACES zlif_interface. ENDCLASS. CLASS zlcl_super IMPLEMENTATION. METHOD do_something. … ENDMETHOD. METHOD get_instance. IF zgr_instance IS INITIAL. zgr_instance = NEW #( ). ENDIF. zrv_instance = zgr_instance. ENDMETHOD. ENDCLASS. So far so good, right? Not really! Issue #01: I would like to implement the functionality for a singleton in my superclass, but I would like to keep it abstract as well. What could I do? I tried creating a reference to my interface. Silly me, that’s not allowed of course. I really don’t want to give up that the superclass is abstract, so the only way to go is to define the singleton-method get_instance in every subclass. . But wait! The get_instance( ) method should be a static (class) method. And static methods cannot be redefined. This is a Catch-22, isn’t it? Luckily, I am not the first one who got into this trouble. “Former member” solved this issue before me by suggesting to change the parameters of the get_instance( ) from a returning to a changing parameter in an answer to this question. Hurrah! I can keep the superclass abstract, I can even define all subclasses as CREATE PRIVATE, although I have to define all subclasses as a friend of the superclass. So the code now looks like this: INTERFACE: zlif_interface. METHODS: do_something RETURNING VALUE(zrv_text) TYPE string. CLASS-DATA: zgr_instance TYPE REF TO zlif_interface. CLASS-METHODS: get_instance CHANGING zcv_instance TYPE any. ENDINTERFACE. CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED. PUBLIC SECTION. INTERFACES zlif_interface. ALIASES: zgr_instance FOR zlif_interface~zgr_instance, do_something FOR zlif_interface~do_something, get_instance FOR zlif_interface~get_instance. ENDCLASS. CLASS zlcl_super IMPLEMENTATION. METHOD get_instance. IF zgr_instance IS INITIAL AND cl_abap_refdescr=\u003edescribe_by_data( zcv_instance )-\u003ekind = cl_abap_typedescr=\u003ekind_ref. DATA(lo_ref_descr) = CAST cl_abap_refdescr( cl_abap_refdescr=\u003edescribe_by_data( zcv_instance ) ). DATA(zlv_classname) = lo_ref_descr-\u003eget_referenced_type( )-\u003eget_relative_name( ). DATA: zlr_instance TYPE REF TO zlif_interface. CREATE OBJECT zlr_instance TYPE (zlv_classname). zgr_instance ?= zlr_instance. ENDIF. zcv_instance ?= zgr_instance. ENDMETHOD. ENDCLASS. Nice! Now we have a singleton with inheritance. I did encounter some frustrations though. I love chaining methods, so actually I would have liked this statement: CREATE OBJECT DATA(zlr_instance) TYPE ( CAST cl_abap_refdescr( cl_abap_refdescr=\u003edescribe_by_data( zcv_instance ) )-\u003eget_referenced_type( )-\u003eget_relative_name( ) ). That I can’t use the CAST keyword in the type-declaration I can understand. But I find it quite annoying that I can’t use a method to retrieve the type in the type-declaration. This yields a syntax error: CREATE OBJECT DATA(zlr_instance) TYPE \
Programming
C# CONCEPTS Gang of Four – Singleton design pattern ensures that a particular class has only one instance/object and a global access point. Photo by Sebastian Voortman from Pexels Prerequisites Basic knowledge of OOPS concepts. Any programming language knowledge. The article demonstrates the usage of singleton design patterns using the C# programming language. So, to begin with, C# View at Medium.com Learning Objectives How to code using a singleton design pattern? Getting Started Singletons cla
Software Engineering/Computer Science
There are 23 classic design patterns that are described in the original book “Design Patterns: Elements of Reusable Object-Oriented Software.” These patterns provide solutions to particular problems…
