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.
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.
Opportunistic reuse can be categorized further:
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.
A design pattern is a general solution to a recurring problem. Design patterns are more conceptual than tangible and can be modified to fit the exact need. However, abstract classes and interfaces can be reused to implement certain patterns.
Developers generally reuse large pieces of software via third-party applications and frameworks. Though frameworks are usually domain-specific and applicable only to families of applications.