Single Template Method

• ConcreteClass has to subclass the entire AbstractClass
• The ConcreteClass has to include any AbstractClass dependencies
• For each abstract method, ConcreteClass must provide a definition

Multiple Template Methods

• What about if there is more than one template method?
• ConcreteClassA has to implement all AbstractClass abstract methods, i.e., primitiveOperationA1(), primitiveOperationA2(), primitiveOperationB1(), and primitiveOperationB2(), even if only interested in templateMethodA()
• Similarly for ConcreteClassB
• Only class this works fine for is ConcreteClassAB

Another Issue

• PDFReader has a dependency on PDFFormatLibrary
• CountArticles derived from PDFReader and (indirectly) has all of PDFReader's dependencies
• What if we don't want CountArticles to depend on PDFFormatLibrary?

Goal

• Separate primitive operations for a particular purpose from the rest of the class interface allows for decoupling of the rest of class
• Subclass the set of primitive operations instead of the entire class
• Reduces coupling and has multiple template methods

Solution

Solution

• AbstractHandler has only essential dependencies for the interface and does not have the dependencies of the MainClass
• ConcreteHandler is derived from AbstractHandler, so it does not share the dependencies of MainClass

Solution: Class Structure

Solution: main()

Solution: Methods

Multiple Handlers

Standard Handlers

• Can provide standard, off-the-shelf handlers
• Clients can pick these or even further subclass them
• Users can even create their own sets of off-the-shelf handlers

Advantages

• Replaces subclassing a large, complex class with subclassing a cohesive interface
• Handlers are almost always all pure virtual or empty methods
• The only dependencies that handlers have are for parameters and return types
• Allows multiple types of handlers, which the design can mix in new ways
• Allows standard handlers to be created and shared
• Easier to test, as can test individual handlers

DockerReport Template Method Design

Template Method Includes

DockerReport Handler Design

DockerReport: Template Method & Handler Comparison

Handler Includes

Decoupling YAMLParser

Decoupling YAMLParser

Decoupling YAMLParser

• ParserEngine can be a free function, a static method of a class, a method of a class, or even part of a framework
• The implementation of ParserEngine (i.e., .cpp files) are the ones that use YAMLParser
• The interface of ParserEngine (i.e., .hpp files) does not use or include YAMLParser and is completely decoupled from YAMParser
• The interface of ParserEngine (i.e., .hpp files) does use YAMLParserHandler as a parameter