Template strategy pattern

I even deep dive into the step 2 and factor the step 2 into two more steps. In each four files I only need to change these 2 steps of step 2. Now I can 2 algorithm steps as abstract and allow subclasses to implement them.

Following is the class diagram of my initial solution. Hence this looks like a solid approach since many of the code is shared among the concrete classes and the conversion was working beautifully. After a while it exposed upon me that there was another system which customers want to incorporate. That outer system generates the output file in a completely different than the previous one.

After careful analysis I find out that I need to change the all the steps of the algorithm and for that I need to alter the abstract class. This a bad thing from design point of view because classed should be closed for modification but open for extension. As well as there can be other outer system which will one day be exposed to me and asked to develop a code for conversion of data. Therefore I needed another approach for designing my classes. In order to deal with the solution I used the strategy design pattern.

This pattern is closely related to template method. But it gives more flexibility. Strategy pattern encapsulate the behavior of an object. Strategy pattern also allows us to define a set of algorithms. Each class defines its own algorithm whose steps can be entirely different from each other.

I used this pattern because each algorithm can be extremely different than the others. This is very good for my design problems because now I can handle different data files for conversion which can be based on completely different algorithms.

Following is the class diagram of the implementation. As one can see implementing classes also depend upon the template method class. This dependency causes to change the template method if one wants to change some of the steps of the algorithm.

On the other side strategy completely encapsulates the algorithm. Improve this question. You are parametrizing a data member in DoSomething. How does it even compile? Add a comment. Active Oldest Votes. Improve this answer. John Dibling John Dibling Thanks for a great answer Let me ask you this.

How does the strategy pattern differ from policy based pattern? One thing I see from your code is a theme of varying contexts, with very similar strategy algorithms. So maybe there are two distink needs the many context for the policy pattern and the many protocals for the strategy pattern — jaybny.

I do not want to use any virtual functions, but without getting into Boost::MPI type stuff, im having a hard time designing my strategy pattern. Viktor Sehr Viktor Sehr Sign up or log in Sign up using Google.

Sign up using Facebook. Sign up using Email and Password. Yes, but it's not like strategy pattern should be used ONLY for those cases?

I mean if I only need to select algorithm's behavior at compile time should I still use strategy pattern, or it wasn't meant to be used that way? BornToCode I would say that a strategy is most useful when the choice is dynamic. Template is basically a way of building up different, related behaviors for known. You'd use some strategy though not the strategy pattern necessarily for choosing which templated behavior to employ.

For example, product inheritance - you'd create a base product, add features for different products. Strategy pattern doesn't really come into play there. Apply the pattern where it's appropriate, combine patterns where it's useful.

Show 2 more comments. The main difference between the two is when the concrete algorithm is chosen. Jeff Axelrod Both patterns support runtime selection of the algorithm used for Template Method, you would do something like if config. Sure you could do that but then you're not using the Template Pattern.

In fact, that's almost exactly what the code creating the Strategy instance will look like! Karoly Nyisztor They both can "replace behavior" and "provide extension points. You can call each subclass of the template method pattern a "strategy" too, or call each strategy class in the strategy pattern an "extension," it's just wording. So this is the right answer.

A concrete algorithm is chosen the same way for both patterns. The choice is made by invoking new ConcreteAlgorithm1 versus new ConcreteAlgorithm2. Obviously the choice happens at runtime making an algorithm choice at compile time would mean hard-coding it.

The main difference between the two is how the concrete algorithm is implemented. Is it implemented as a subclass or as a separate interface? The former is a Template.

The latter is a Strategy. The difference can be summarized as composition vs inheritance, which is a common theme of the GoF book. Show 3 more comments. I think the Class-Diagrams of both pattern are showing the differences.

Strategy Encapsulates an algorithm inside a class Link to image Template Method Defer the exact steps of an algorithm to a subclass Link to Image. Mayank Jaiswal Ludwig Wensauer Ludwig Wensauer 1, 2 2 gold badges 34 34 silver badges 42 42 bronze badges. Or is the key thing here that you have an abstraction of strategy - new way of doing something Example of a template method: Application. Example of a strategy: array.

Paul Kapustin Paul Kapustin 3, 5 5 gold badges 34 34 silver badges 45 45 bronze badges. I think this is a great answer — Calanus. Similarities Strategy and Template method patterns have a lot of similarities between them. Differences Here are some of the differences I have observed while studying these two patterns: In Strategy, the coupling between the client and strategy is more loose whereas in Template Method, the two modules are more tightly coupled.

In Strategy, mostly an interface is used though abstract class can also be used depending on the situation, and concrete class is not used whereas in Template method mostly abstract class or concrete class is used, interface is not used. In Strategy pattern, generally entire behaviour of the class is represented in terms of an interface, on the other hand, Template method is used for reducing code duplication and the boilerplate code is defined in base framework or abstract class.

In Template Method, there can even be a concrete class with default implementation. In simple words, you can change the entire strategy algorithm in Strategy pattern, however, in Template method, only some things change parts of algorithm and rest of the things remain unchanged.

In Template Method, the invariant steps are implemented in an abstract base class, while the variant steps are either given a default implementation, or no implementation at all. In Template method, the component designer mandates the required steps of an algorithm, and the ordering of the steps, but allows the component client to extend or replace some number of these steps. Inheritance versus aggregation is-a versus has-a.

It's two ways to achieve the same goal. Community Bot 1 1 1 silver badge. Himanshu P. Template Method: It's based on inheritance Defines skeleton of algorithm which can't be changed by sub classes. Ravindra babu Ravindra babu 45k 8 8 gold badges silver badges bronze badges.

Quote from the article " As one can see implementing classes also depend upon the template method class. Iulian Rosca Iulian Rosca 3 3 gold badges 14 14 silver badges 29 29 bronze badges. As in myShippingCalculator. Charlie Flowers Charlie Flowers Template Pattern: Template method is about letting subclasses redefine certain steps of the algorithm, without changing the main structure and steps of the algorithm, defined in the base class.

Strategy Pattern: Strategy pattern is about letting client selects concrete algorithms implementation at runtime. Saad Saad 8 8 silver badges 18 18 bronze badges. Strategy Design Pattern Supports composition. Provides you the flexibility to change the behavior of object at runtime. Template Method Design Pattern Favours inheritance over composition Define algorithm in your base class. Individual pieces of algorithm can be customized in child classes. Mangu Singh Rajpurohit Mangu Singh Rajpurohit 9, 3 3 gold badges 55 55 silver badges 85 85 bronze badges.

In Strategy Pattern there is no implementation provided by the base This is the reason why the base is really an interface in the class diagram The classic example is sorting. Siby Siby 1 1 silver badge 10 10 bronze badges.

If you are using a framework or library which you do not have access to the source code and you want to change some behaviors of a class, so you have to go for Template Method. That means inheritance simply. If you are developing a class and it is obvious that some parts of the logic needs to be implemented differently to handle various situations, take the Strategy pattern.

So it is extendable and also easily testable. If you are developing a class and you do not know what changes will happen in the future, divide your logic into separate and single responsible functions as much as possible. Just that. Neither Template Method nor Strategy. Amirreza Amirreza 1, 11 11 silver badges 16 16 bronze badges. The Overflow Blog. Podcast Helping communities build their own LTE networks. Podcast Making Agile work for data science.

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