Template Method

A class pattern that defines the skeleton of an algorithm as an abstract class, allowing its subclasses to provide concrete behavior

• Behavioral Pattern
• Class pattern uses generalization/inheritance

Template Method

A class pattern that defines the skeleton of an algorithm as an abstract class, allowing its subclasses to provide concrete behavior

• A fundamental technique for code reuse
• Particularly important in class libraries to factor out common behavior
• Template methods lead to Inversion of Control (IoC), that’s sometimes referred to as “the Hollywood principle,” that is, “Don’t call us, we’ll call you” [Swe85]
• The Template Design Pattern is not related to C++ templates and uses inheritance, not C++ templates

Template Method: Applicability

• Implement invariant parts of an algorithm and leave it up to subclasses to implement behavior that can vary
• Factor and localize common behavior in subclasses to avoid code duplication
• Control subclass extensions with "hook" operations

Template Method: Structure (GOF Example)

Template Method: Structure (GOF Example)

Template Method: Structure

Template Method: Structure

Template Method: Participants

• AbstractClass (e.g., Application)
• Defines primitive operations that concrete subclasses define to implement steps of an algorithm
• Implements a template method defining the skeleton of an algorithm, which calls primitive operations (in ConcreteClass), operations in AbstractClass, or anything else
• ConcreteClass (e.g., MyApplication)
• Implements the primitive operations to carry out subclass-specific steps of the algorithm

Template Method: Collaborations

• ConcreteClass relies on AbstractClass to implement invariant steps of the algorithm

Template Method: Consequences - Kinds of Operations

• primitive operations (i.e., abstract operations) must override
• hook operations - default behavior that subclasses can extend if necessary. may override and often does nothing
• concrete operations (ConcreteClass or on client classes)
• concrete AbstractClass operations (i.e., operations that are generally useful to subclasses)
• factory methods (later)

Extension via Direct Overriding

• Useful when no common parent (class) behavior

Extension via Hook Operation

• Useful when extension may not be needed
• Useful when the extension is a side-effect of standard processing

Parser.hpp

FilenameParser.hpp

UML

MemoryParser.hpp

Implementation

• The template method is the only method that calls the primitive operations; therefore, the primitive operations do not have to be public
• A template method can be a private, non-virtual member function
• Primitive operations that require overriding are declared pure virtual
• Minimize the number of primitive operations a subclass is required to implement

Known Uses

• Found in almost every abstract class

Related Patterns

• Factory Methods
• Are often called by template methods
• Strategy Pattern
• Template Method pattern uses inheritance to vary parts of an algorithm. Strategy patterns use delegation to vary the entire algorithm.

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DockerReport Template Method Design

DockerReport: Lambda & Template Method Comparison