C++ Programming/Code/Design Patterns - Wikibooks, open books for an open world. Programming Patterns. While not new, since the concept was already described by Christopher Alexander in its architectural theories, it only gathered some traction in programming due to the publication of Design Patterns: Elements of Reusable Object- Oriented Software book in October 1. Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, known as the Gang of Four (Go. F), that identifies and describes 2. A design pattern is neither a static solution, nor is it an algorithm. A pattern is a way to describe and address by name (mostly a simplistic description of its goal), a repeatable solution or approach to a common design problem, that is, a common way to solve a generic problem (how generic or complex, depends on how restricted the target goal is). Patterns can emerge on their own or by design. HOK is a global design, architecture, engineering and planning firm. Software design patterns are abstractions that help structure system designs. While not new, since the concept was already described by Christopher Alexander in its architectural theories, it only gathered some traction in. This is why design patterns are useful as an abstraction over the implementation and a help at design stage. With this concept, an easier way to facilitate communication over a design choice as normalization technique is given so that every person can share the design concept. Depending on the design problem they address, design patterns can be classified in different categories, of which the main categories are: Patterns are commonly found in objected- oriented programming languages like C++ or Java. They can be seen as a template for how to solve a problem that occurs in many different situations or applications. It is not code reuse, as it usually does not specify code, but code can be easily created from a design pattern. Object- oriented design patterns typically show relationships and interactions between classes or objects without specifying the final application classes or objects that are involved. Each design pattern consists of the following parts: Problem/requirement To use a design pattern, we need to go through a mini analysis design that may be coded to test out the solution. This section states the requirements of the problem we want to solve. This is usually a common problem that will occur in more than one application. Forces This section states the technological boundaries, that helps and guides the creation of the solution. Solution This section describes how to write the code to solve the above problem. This is the design part of the design pattern. It may contain class diagrams, sequence diagrams, and or whatever is needed to describe how to code the solution. Design patterns can be considered as a standardization of commonly agreed best practices to solve specific design problems. One should understand them as a way to implement good design patterns within applications. Doing so will reduce the use of inefficient and obscure solutions. Using design patterns speeds up your design and helps to communicate it to other programmers. The basic form of object creation could result in design problems or added complexity to the design. Creational design patterns solve this problem by somehow controlling this object creation. In this section of the book we assume that the reader has enough familiarity with functions, global variables, stack vs. Builder Pattern lets us defer the construction of the object until all the options for creation have been specified.#include< string> #include< iostream> #include< memory> usingnamespacestd; // . It can also return a factory for a certain group. The Factory Design Pattern is useful in a situation that requires the creation of many different types of objects, all derived from a common base type. The Factory Method defines a method for creating the objects, which subclasses can then override to specify the derived type that will be created. Thus, at run time, the Factory Method can be passed a description of a desired object (e. The pattern works best when a well- designed interface is used for the base class, so there is no need to cast the returned object. Problem We want to decide at run time what object is to be created based on some configuration or application parameter. When we write the code, we do not know what class should be instantiated. Solution Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses. In the following example, a factory method is used to create laptop or desktop computer objects at run time. Let's start by defining Computer, which is an abstract base class (interface) and its derived classes: Laptop and Desktop. Computer! First, there is a compilation benefit. If we move the interface Computer into a separate header file with the factory, we can then move the implementation of the New. Computer() function into a separate implementation file. Now the implementation file for New. Computer() is the only one that requires knowledge of the derived classes. Thus, if a change is made to any derived class of Computer, or a new Computer subtype is added, the implementation file for New. Computer() is the only file that needs to be recompiled. Everyone who uses the factory will only care about the interface, which should remain consistent throughout the life of the application. Also, if there is a need to add a class, and the user is requesting objects through a user interface, no code calling the factory may be required to change to support the additional computer type. The code using the factory would simply pass on the new string to the factory, and allow the factory to handle the new types entirely. Imagine programming a video game, where you would like to add new types of enemies in the future, each of which has different AI functions and can update differently. By using a factory method, the controller of the program can call to the factory to create the enemies, without any dependency or knowledge of the actual types of enemies. Now, future developers can create new enemies, with new AI controls and new drawing member functions, add it to the factory, and create a level which calls the factory, asking for the enemies by name. Combine this method with an XML description of levels, and developers could create new levels without having to recompile their program. All this, thanks to the separation of creation of objects from the usage of objects. Another example: #include< stdexcept> #include< iostream> #include< memory> usingnamespacestd; class. Pizza. This pattern is used, for example, when the inherent cost of creating a new object in the standard way (e. Implementation: Declare an abstract base class that specifies a pure virtual clone() method. Any class that needs a . This factory method, depending on the parameter, finds out the concrete class. On this concrete class, the clone() method is called and the object is returned by the factory method. This is a sample implementation of Prototype method. We have the detailed description of all the components here. This method requires an enum Record. Type as parameter and depending on this parameter it returns the concrete implementation of Record class./** Implementation of Prototype Method **/#include< iostream> #include< unordered. Any class that needs a . The function below demonstrates this concept: void. Do. Some. Stuff. With. AMonster(const. CPrototype. Monster*original. Monster). It is named after the singleton set, which is defined to be a set containing one element. This is useful when exactly one object is needed to coordinate actions across the system. Check list. Define a private static attribute in the . Traditional implementation uses a static member function of the Singleton class, which will create a single instance of the Singleton class on the first call, and forever return that instance. The following code example illustrates the elements of a C++ singleton class, that simply stores a single string. String. Singleton. This prevents // accidental copying of the only instance of the class. String. Singleton(const. String. Singleton& old); // disallow copy constructorconst. String. Singleton& operator=(const. String. Singleton& old); //disallow assignment operator// Note that although this should be allowed, // some compilers may not implement private destructors// This prevents others from deleting our one single instance, which was otherwise created on the heap~String. Singleton(). Configurations may need to be accessible globally, and future expansions to the application configurations may be needed. The subset C's closest alternative would be to create a single global struct. This had the lack of clarity as to where this object was instantiated, as well as not guaranteeing the existence of the object. Take, for example, the situation of another developer using your singleton inside the constructor of their object. Then, yet another developer decides to create an instance of the second class in the global scope. If you had simply used a global variable, the order of linking would then matter. Since your global will be accessed, possibly before main begins executing, there is no definition as to whether the global is initialized, or the constructor of the second class is called first. This behavior can then change with slight modifications to other areas of code, which would change order of global code execution. Such an error can be very hard to debug. But, with use of the singleton, the first time the object is accessed, the object will also be created. You now have an object which will always exist, in relation to being used, and will never exist if never used. A second common use of this class is in updating old code to work in a new architecture. Since developers may have used globals liberally, moving them into a single class and making it a singleton, can be an intermediary step to bring the program inline to stronger object oriented structure. Another example: #include< iostream> usingnamespacestd; /* Place holder for thread synchronization mutex */class.
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