Data Abstraction

A “type” is a set of operations, and an “abstract data type” is a set of operations with an implementation. When we identify objects in a problem domain, the first question we should ask about them is, “What can I do with this object?” not “How is this object implemented?” Therefore, if a natural description of a problem involves employees, contracts, and payroll records, then the programming language used to solve the problem should contain Employee, Contract, and PayrollRecord types. This allows an efficient, two-way translation between the problem domain and the solution domain, and software written this way has less “translation noise” and is simpler and more correct.

In a general-purpose programming language like C++, we don’t have application-specific types like Employee. Instead, we have something better: the language facilities to create sophisticated abstract data types. The purpose of an abstract data type is, essentially, to extend the programming language into a particular problem domain.

Read the full story

Anyone who is not already familiar with design patterns may, after a brief survey of the field, come away with the impression that design patterns are a lot of marketing hype, are just some simple coding techniques, or are the playthings of computer scientists who really should get out more. While each of these impressions carries a grain of truth, design patterns are an essential component of the professional C++ programmer’s toolkit.

A “design pattern” is a recurring architectural theme that provides a solution to a common design problem within a particular context and describes the consequences of this solution. A design pattern is more than a simple description of a technique; it’s a named capsule of design wisdom gleaned from successful existing practice, written in such a way that it can be easily communicated and reused. Patterns are about programmer to programmer communication.

From a practical perspective, design patterns have two important properties. First, they describe proven, successful design techniques that can be customized in a context-dependent way to new design situations. Second, and perhaps more important, mentioning the application of a particular pattern serves to document not only the technique that is applied but also the reasons for its application and the effect of having applied it.

Read the full story