How To Understand The Dependency Inversion Principle (Dip)

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Jacob Holt
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1 Embedded Real-Time Systems (TI-IRTS) General Design Principles 1 DIP The Dependency Inversion Principle Version:

2 Dependency Inversion principle (DIP) DIP: A. High level modules should not depend upon low level modules. Both should depend upon abstractions. B. Abstractions should not depend upon details. Details should depend upon abstractions. Ref. The Dependency Inversion Principle, article by Robert C. Martin Slide 2

3 The Copy program (1) copy read Keyboard Structure chart write Printer void copy() // a C function { int c; while ( (c=readkeyboard())!= EOF) writeprinter(c); } Slide 3

4 The Enhanced Copy program (2) Changed requirement: The output should be either to a printer or to a disk enum OutputDevice {printer, disk}; void copy(outputdevice dev) { int c; while ( (c=readkeyboard())!= EOF) if (dev== printer) writeprinter(c); else writedisk(c); } The problem is that the high level policy i.e. the Copy module is dependent upon the low level detailed modules it controls. Slide 4

5 The OO Copy program (3) Copy Reader Notice: abstract classes Writer Keyboard Reader Printer Writer The dependency has been inverted! The copy class depends upon abstractions, and the detailed reader and writer depends upon the same abstractions Slide 5

6 The OO Copy program (4) class Reader { public: virtual int read() = 0; // pure virtual operation }; class Writer { public: virtual void write(char ch) = 0; // pure virtual operation }; void Copy(Reader& r, Writer& w) { int c; while ( (c=r.read())!= EOF) w.write(c); } Slide 6

write(char ch) = 0; // pure virtual operation }; void Copy(Reader&

7 Layering Simple Layers Policy Layer Mechanism Layer Utility Layer NB! The dependency is transitive Slide 7

8 Layering Abstract Layers Policy Layer Mechanism Interface Mechanism Layer Utility Interface NB! no transitive or direct dependency between layers Utility Layer Slide 8

9 An Inversion of Ownership Notice: inversion is also an inversion of ownership We often think of utility libraries as owning their own interfaces With DIP: The clients own the abstract interfaces The servers derives from them Known as the Hollywood principle: Don t call us, we ll call you Slide 9

interfaces With DIP: The clients own the abstract interfaces The servers

10 Inverted Layers Policy Policy Layer «interface» Mechanism Interface Mechanism Mechanism Layer «interface» Utility Interface Utility Utility Layer Slide 10

11 A Simple Example (DIP) Dependency inversion can be applied where ever one object sends a message to another Button +Button(Lamp& l) +Detect() itslamp Lamp +TurnOn() +TurnOff() void Button::Detect() { bool buttonon= GetPhysicalState(); if (buttonon) itslamp->turnon(); else itslamp->turnoff(); } Slide 11

Lamp +TurnOn() +TurnOff() void Button::Detect() { bool buttonon=

12 Example: Inverted Button Model Button +Detect(): void +GetState(): bool ButtonClient +TurnOn(): void +TurnOff(): void Reusable Button Implementation Button Imp.2 Lamp Motor +Detect(): void +GetState(): bool +TurnOn(): void +TurnOff(): void +TurnOn(): void +TurnOff(): void Slide 12

13 Template Method (GoF Pattern) The GoF Template method is a good example of DIP In this pattern a high level algorithm is encoded in an abstract base class and makes use of pure virtual functions to implements its details Derived classes implements those detailed virtual functions Thus, the class containing the details depends upon the class containing the abstraction Slide 13

functions to implements its details Derived classes implements those detailed virtual

14 Template Method - GoF Structure AbstractClass TemplateMethod() PrimitiveOperation1() PrimitiveOperation(2) // Common code PrimitiveOperation1() // more common code // PrimitiveOperation2() // ConcreteClass PrimitiveOperation1() PrimitiveOperation2() Notice: The Template Method is concrete Ref. GoF Design Patterns, Gamma et. al Slide 14

PrimitiveOperation2() // ConcreteClass PrimitiveOperation1() PrimitiveOperation2()

15 DIP - conclusion The principle of dependency inversion is at the root of many of the benefits claimed for object-oriented technology. Its proper application is necessary for the creation of reusable frameworks It is also critically important for the construction of code that is resilient to change Slide 15

Its proper application is necessary for the creation of reusable frameworks

The Dependency Inversion Principle

The Dependency Inversion Principle This is the third of my Engineering Notebook columns for The C++ Report. The articles that will appear in this column will focus on the use of C++ and OOD, and will address