Gain insights into software design patterns in C, explore their implementation, discover underlying principles, and learn to build a vocabulary for better communication and coding practices.

c.tar.gz

main+customer.c

main+customer.c-copy

watches

customers

observer

Reactor

Reactor-copy

observer-copy

Software design patterns are a valuable tool in any software developer's skill set. However, most design patterns are described in the context of an object-oriented programming language such as C++ or Java. The C language is sadly absent in the pattern literature. This course is here to change that by demonstrating that it is possible to use patterns in C programs and how it adds benefits to C programmers. You will learn why design patterns are more of a communication tool than technical solutions, and how patterns let you build and share a common vocabulary that simplifies communication.

The course presents a number of patterns and explains how to implement them in C. Each pattern is introduced via the problem it solves and the domains where the pattern can be used. We also discuss the underlying design principles as well as the trade-offs when applying the pattern. Finally, the course covers a set of coding idioms that will guide you when applying design patterns in C.

Software Design Patterns in C

To decouple the subject from its concrete observers and still enable the subject to notify them,
each observer must correspond to a unique instance. The First-Class ADT pattern provides a way
to realize this. However, as seen from the subject, all observers must be abstracted as one general type
sharing a common interface. In the C language, without language support for inheritance, generality is
usually spelled ```void*```. We can convert any pointer to an object to `void*` and back to its original type again without losing information. This rule makes a common interface possible.

typedef void(*ChangeTimeNotification)(void* instance,
                                      const SystemTime* newTime);
typedef struct 
{
    void* instance;
    ChangeTimeNotification notification;
} TimeObserver;

* The interface above declares a pointer to a function, which proves to be an efficient technique for
implementing dynamic behavior
* A concrete observer attaches itself, together with a pointer to a function,
at the subject. As the subject changes, it notifies the observer through the attached function.

By using ```void*``` as abstraction, we trade type-safety for flexibility. This technique also makes the programmer responsible for converting the observer back to its original type.  

A conversion back to another type than the original may have disastrous consequences. Although not very experienced in C programming, Franz Kafka provided a good example of such behavior: **“When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed in his bed into a monstrous vermin”** [(Kafka)](https://www.educative.io/courses/software-design-patterns-c/conclusion).

Gregor Samsa wasn’t type-safe. The ```TimeObserver``` structure was introduced to guard against erroneous conversions and maintain the binding between the observer and the function pointer. The implicit type conversion is encapsulated within the observer, as illustrated in the code below.

In this lesson, we will explain the implementation of the Observer pattern in the C language.

Introduction to Pattern in C

The First-Class ADT Pattern

The State Pattern

The Strategy Pattern

The Observer Pattern

The Reactor Pattern

Idiomatic Expressions

Conclusion

Implementing the Observer Pattern

Implementation Mechanism