In
software engineering
Software engineering is a systematic engineering approach to software development.
A software engineer is a person who applies the principles of software engineering to design, develop, maintain, test, and evaluate computer software. The term '' ...
, a circular dependency is a relation between two or more modules which either directly or indirectly depend on each other to function properly. Such modules are also known as
mutually recursive.
Overview
Circular dependencies
In software engineering, a circular dependency is a relation between two or more modules which either directly or indirectly depend on each other to function properly. Such modules are also known as mutually recursive.
Overview
Circular depen ...
are natural in many
domain model
In software engineering, a domain model is a conceptual model of the domain that incorporates both behavior and data.Fowler, Martin. ''Patterns of Enterprise Application Architecture''. Addison Wesley, 2003, p. 116. In ontology engineering, a do ...
s where certain objects of the same domain depend on each other.
However, in
software design
Software design is the process by which an agent creates a specification of a software artifact intended to accomplish goals, using a set of primitive components and subject to constraints. Software design may refer to either "all the activity ...
, circular dependencies between larger software modules are considered an
anti-pattern
An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...
because of their negative effects.
Despite this, such circular (or cyclic) dependencies have been found to be widespread among the source files of real-world software. Mutually recursive modules are, however, somewhat common in
functional programming
In computer science, functional programming is a programming paradigm where programs are constructed by Function application, applying and Function composition (computer science), composing Function (computer science), functions. It is a declar ...
, where inductive and recursive definitions are often encouraged.
Problems
Circular dependencies can cause many unwanted effects in software programs. Most problematic from a software design point of view is the ''tight
coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...
'' of the mutually dependent modules which reduces or makes impossible the separate re-use of a single module.
Circular dependencies can cause a
domino effect
A domino effect or chain reaction is the cumulative effect generated when a particular event triggers a chain of similar events. This term is best known as a mechanical effect and is used as an analogy to a falling row of dominoes. It typically ...
when a small local change in one module spreads into other modules and has unwanted global effects (program errors, compile errors). Circular dependencies can also result in infinite recursions or other unexpected failures.
Circular dependencies may also cause
memory leak
In computer science, a memory leak is a type of resource leak that occurs when a computer program incorrectly manages memory allocations in a way that Computer memory, memory which is no longer needed is not released. A memory leak may also happe ...
s by preventing certain very primitive automatic
garbage collector
A waste collector, also known as a garbageman, garbage collector, trashman (in the US), binman or (rarely) dustman (in the UK), is a person employed by a public or private enterprise to collect and dispose of municipal solid waste (refuse) and r ...
s (those that use
reference counting
In computer science, reference counting is a programming technique of storing the number of references, pointers, or handles to a resource, such as an object, a block of memory, disk space, and others.
In garbage collection algorithms, referenc ...
) from deallocating unused objects.
Causes and solutions
In very large software designs, software engineers may lose the context and inadvertently introduce circular dependencies. There are tools to analyze software and find unwanted circular dependencies.
JDepend
for Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
Circular dependencies can be introduced when implementing callback
Callback may refer to:
* Callback (comedy), a joke which refers to one previously told
* Callback (computer programming), executable code that is passed as a parameter to other code
* Callback (telecommunications), the telecommunications event th ...
functionality. This can be avoided by applying design patterns
''Design Patterns: Elements of Reusable Object-Oriented Software'' (1994) is a software engineering book describing software design patterns. The book was written by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, with a foreword ...
like the observer pattern
In software design and engineering, the observer pattern is a software design pattern in which an object, named the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by ca ...
.
See also
* Acyclic dependencies principle The acyclic dependencies principle (ADP) is a software design principle defined by Robert C. Martin that states that "''the dependency graph of packages or components should have no cycles''". This implies that the dependencies form a directed acyc ...
* Dependency hell Dependency hell is a colloquial term for the frustration of some software users who have installed software packages which have dependencies on specific versions of other software packages.
The dependency issue arises when several packages have d ...
References
{{reflist
Programming language topics
C++
Articles with example C++ code