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Code reuse

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Code reuse, also called software reuse, is the use of existing software, or software knowledge, to build new software. Ad hoc reuse has been practiced from the earliest days of programming. Programmers have always reused sections of code, templates, functions, and procedures. Software reuse as a recognized area of study in software engineering, however, dates only from 1968 when Douglas McIlroy of Bell Laboratories proposed basing the software industry on reusable components.

Reusable software, or software knowledge items, are called reusable assets. Assets may be designs, requirements, test cases, architectures, etc.

Perhaps the most well known reusable asset is code. Code reuse is the idea that a partial or complete computer program written at one time can be, should be, or is being used in another program written at a later time. The reuse of programming code is a common technique which attempts to save time and energy by reducing redundant work.

A very common example of code reuse is the technique of using a software library. Many common operations, such as converting information among different well-known formats, accessing external storage, interfacing with external programs, or manipulating information (numbers, words, names, locations, dates, etc.) in common ways, are needed by many different programs. Authors of new programs can use the code in a software library to perform these tasks, instead of "re-inventing the wheel", by writing fully new code directly in a program to perform an operation. Library implementations often have the benefit of being well-tested, and covering unusual or arcane cases. Disadvantages include the inability to tweak details which may affect performance or the desired output, and the time and cost of acquiring, learning, and configuring the library.

The software library is a good example of abstraction. Programmers may decide to create internal abstractions so that certain parts of their program can be re-used, or may create custom libraries for their own use. Some characteristics that make software more easily reusable are modularity, loose coupling, high cohesion, information hiding and separation of concerns.

For newly written code to use a piece of existing code, some kind of interface, or means of communication, must be defined. These commonly include a "call" or use of a subroutine, object, class, or prototype. In organizations, such practices are formalized and standardized by software product line engineering.

The general practice of using a prior version of an extant program as a starting point for the next version, is also a form of code reuse.

Some so-called code "reuse" involves simply copying some or all of the code from an existing program into a new one. While organizations can realize time to market benefits for a new product with this approach, they can subsequently be saddled with many of the same code duplication problems caused by cut and paste programming.

Many researchers have worked to make reuse faster, easier, more systematic, and an integral part of the normal process of programming. These are some of the main goals behind the invention of object-oriented programming, which became one of the most common forms of formalized reuse. A somewhat later invention is generic programming.

Another, newer means is to use software "generators", programs which can create new programs of a certain type, based on a set of parameters that users choose. Fields of study about such systems are generative programming and metaprogramming.

Types of reuse

  • Opportunistic reuse - While getting ready to begin a project, the team realizes that there are existing components that they can reuse.
  • Planned reuse - A team strategically designs components so that they'll be reusable in future projects.

Opportunistic reuse can be categorized further:

  • Internal reuse - A team reuses its own components. This may be a business decision, since the team may want to control a component critical to the project.
  • External reuse - A team may choose to buy a third-party component. Buying a third-party component typically costs the team 1 to 20 percent of what it would cost to develop internally (McConnell 1996). The team must also consider the time it takes to find, learn and integrate the component.

See also

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