
In
object-oriented programming
Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...
, a factory is an
object
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Object (abstract), an object which does not exist at any particular time or place
** Physical object, an identifiable collection of matter
* Goal, an a ...
for
creating other objects; formally, it is a
function or
method
Method (, methodos, from μετά/meta "in pursuit or quest of" + ὁδός/hodos "a method, system; a way or manner" of doing, saying, etc.), literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In re ...
that returns objects of a varying prototype or
class
Class, Classes, or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used d ...
from some method call, which is assumed to be ''new''. More broadly, a subroutine that returns a ''new'' object may be referred to as a ''factory'', as in ''factory method'' or ''factory function''. The factory pattern is the basis for a number of related
software design pattern
In software engineering, a software design pattern or design pattern is a general, reusable solution to a commonly occurring problem in many contexts in software design. A design pattern is not a rigid structure to be transplanted directly into s ...
s.
Motive
In
class-based programming
Class-based programming, or more commonly class-orientation, is a style of object-oriented programming (OOP) in which inheritance (object-oriented programming), inheritance occurs via defining ''class (computer programming), classes'' of object ( ...
, a factory is an
abstraction
Abstraction is a process where general rules and concepts are derived from the use and classifying of specific examples, literal (reality, real or Abstract and concrete, concrete) signifiers, first principles, or other methods.
"An abstraction" ...
of a
constructor of a class, while in
prototype-based programming
Prototype-based programming is a style of object-oriented programming in which behavior reuse (known as inheritance) is performed via a process of reusing existing objects that serve as prototypes. This model can also be known as ''prototypal'', ...
a factory is an abstraction of a prototype object. A constructor is concrete in that it creates objects as
instances of one class, and by a specified process (class instantiation), while a factory can create objects by instantiating various classes, or by using other allocation means, such as an
object pool
The object pool pattern is a software creational design pattern that uses a set of initialized objects kept ready to use – a "pool" – rather than allocating and destroying them on demand. A client of the pool will request an object from the ...
. A prototype object is concrete in that it is used to create objects by being
cloned, while a factory can create objects by cloning various prototypes, or by other allocation means.
A factory may be implemented in various ways. Most often it is implemented as a method, in which case it is called a ''
factory method''. Sometimes it is implemented as a function, in which case it is called a ''factory function''. In some languages, constructors are factories. However, in most languages they are not, and constructors are invoked in a way that is idiomatic to the language, such as by using the keyword
new
, while a factory has no special status and is invoked via an ordinary method call or function call. In these languages, a factory is an abstraction of a constructor, but not strictly a generalization, as constructors are not factories.
Terminology
Terminology differs as to whether the concept of a factory is a design pattern – in ''
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 fore ...
'' there is no ''factory pattern'', but instead two patterns (
factory method pattern
In object-oriented programming, the factory method pattern is a software design pattern, design pattern that uses factory methods to deal with the problem of object creation, creating objects without having to specify their exact class (computer pr ...
and
abstract factory pattern
The abstract factory pattern in software engineering is a design pattern that provides a way to create families of related objects without imposing their concrete classes, by encapsulating a group of individual factories that have a common theme w ...
) that use factories. Some sources refer to the concept as the ''factory pattern'',
Factory Pattern
'
OODesign.com
/ref> while others consider the concept a programming idiom
In computer programming, a programming idiom, code idiom or simply idiom is a code fragment having a semantic role which recurs frequently across software projects. It often expresses a special feature of a recurring construct in one or more pro ...
,[Chapter 4. The Factory Pattern: Baking with OO Goodness]
The Simple Factory defined
reserving the term ''factory pattern'' or ''factory patterns'' to more complicated patterns that use factories, most often the factory method pattern; in this context, the concept of a factory may be referred to as a ''simple factory''.[ In other contexts, particularly the Python language, the term ''factory'' is used, as in this article. More broadly, ''factory'' may be applied not just to an object that returns objects from some method call, but to a '']subroutine
In computer programming, a function (also procedure, method, subroutine, routine, or subprogram) is a callable unit of software logic that has a well-defined interface and behavior and can be invoked multiple times.
Callable units provide a ...
'' that returns objects, as in a ''factory function'' (even if functions are not objects) or ''factory method''. Because in many languages factories are invoked by calling a method, the general concept of a factory is often confused with the specific factory method pattern
In object-oriented programming, the factory method pattern is a software design pattern, design pattern that uses factory methods to deal with the problem of object creation, creating objects without having to specify their exact class (computer pr ...
design pattern.
Use
OOP provides polymorphism on object ''use'' by method dispatch
In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented ...
, formally subtype polymorphism via single dispatch
In computer science, dynamic dispatch is the process of selecting which implementation of a polymorphic operation (method or function) to call at run time. It is commonly employed in, and considered a prime characteristic of, object-oriented ...
determined by the type of the object on which the method is called. However, this does not work for constructors, as constructors ''create'' an object of some type, rather than ''use'' an existing object. More concretely, when a constructor is called, there is no object yet on which to dispatch.
Using factories instead of constructors or prototypes allows one to use polymorphism for object creation, not only object use. Specifically, using factories provides encapsulation, and means the code is not tied to specific classes or objects, and thus the class hierarchy or prototypes can be changed or refactored
In computer programming and software design, code refactoring is the process of restructuring existing source code—changing the '' factoring''—without changing its external behavior. Refactoring is intended to improve the design, structure, ...
without needing to change code that uses them – they abstract from the class hierarchy or prototypes.
More technically, in languages where factories generalize constructors, factories can usually be used anywhere constructors can be, meaning that interfaces that accept a constructor can also in general accept a factory – usually one only need something that creates an object, rather than needing to specify a class and instantiation.
For example, in Python, the collections.defaultdict
class has a constructor which creates an object of type defaultdict
whose default values are produced by invoking a factory. The factory is passed as an argument to the constructor, and can be a constructor, or any thing that behaves like a constructor – a callable object
A callable object, in computer programming, is any object that can be called like a function.
In different languages
In C++
* pointer to function;
* pointer to member function;
* functor;
* lambda expression.
* std::function is a templa ...
that returns an object, i.e., a factory. For example, using the list
constructor for lists:
# collections.defaultdict( efault_factory[, ...)
d = defaultdict(list)
Object creation
Factory objects are used in situations where getting hold of an object of a particular kind is a more complex process than simply creating a new object, notably if complex allocation or initialization is desired. Some of the processes required in the creation of an object include determining which object to create, managing the lifetime of the object, and managing specialized build-up and tear-down concerns of the object. The factory object might decide to create the object's class
Class, Classes, or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used d ...
(if applicable) dynamically, return it from an object pool
The object pool pattern is a software creational design pattern that uses a set of initialized objects kept ready to use – a "pool" – rather than allocating and destroying them on demand. A client of the pool will request an object from the ...
, do complex configuration on the object, or other things. Similarly, using this definition, a singleton implemented by the singleton pattern">Singleton (mathematics)">singleton implemented by the singleton pattern is a formal factory – it returns an object, but does not create new objects beyond the one instance.
Examples
The simplest example of a factory is a simple factory function, which just invokes a constructor and returns the result. In Python, a factory function f
that instantiates a class A
can be implemented as:
def f():
return A()
A simple factory function implementing the singleton pattern is:
def f():
if f.obj is None:
f.obj = A()
return f.obj
f.obj = None
This will create an object when first called, and always return the same object thereafter.
Syntax
Factories may be invoked in various ways, most often a method call (a ''factory method''), sometimes by being called as a function if the factory is a callable object (a ''factory function''). In some languages constructors and factories have identical syntax, while in others constructors have special syntax. In languages where constructors and factories have identical syntax, like Python, Perl, Ruby (programming language), Ruby, Object Pascal, and F Sharp (programming language), F#, constructors can be transparently replaced by factories. In languages where they differ, one must distinguish them in interfaces, and switching between constructors and factories requires changing the calls.
Semantics
In languages where objects are dynamically allocated, as in Java or Python, factories are semantically equivalent to constructors. However, in languages such as C++ that allow some objects to be statically allocated, factories are different from constructors for statically allocated classes, as the latter can have memory allocation determined at compile time, while allocation of the return values of factories must be determined at run time. If a constructor can be passed as an argument to a function, then invocation of the constructor and allocation of the return value must be done dynamically at run time, and thus have similar or identical semantics to invoking a factory.
Design patterns
Factories are used in various 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 fore ...
, specifically in creational pattern
In software engineering, creational design patterns are design patterns that deal with object creation
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Object (abstract), an object which does not ex ...
s such as the Design pattern object library. Specific recipes have been developed to implement them in many languages. For example, several '' GoF patterns'', like the ''Factory method pattern
In object-oriented programming, the factory method pattern is a software design pattern, design pattern that uses factory methods to deal with the problem of object creation, creating objects without having to specify their exact class (computer pr ...
'', the '' Builder'' or even the '' Singleton'' are implementations of this concept. The ''Abstract factory pattern
The abstract factory pattern in software engineering is a design pattern that provides a way to create families of related objects without imposing their concrete classes, by encapsulating a group of individual factories that have a common theme w ...
'' instead is a method to build collections of factories.
In some design patterns, a factory object has a method
Method (, methodos, from μετά/meta "in pursuit or quest of" + ὁδός/hodos "a method, system; a way or manner" of doing, saying, etc.), literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In re ...
for every kind of object it can create. These methods optionally accept parameter
A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...
s defining how the object is created, and then return the created object.
Applications
Factory objects are common in widget toolkit
A widget toolkit, widget library, GUI toolkit, or UX library is a library (computing), library or a collection of libraries containing a set of graphical control elements (called ''widgets'') used to construct the graphical user interface (GUI) of ...
s and software framework
In computer programming, a software framework is a software abstraction that provides generic functionality which developers can extend with custom code to create applications. It establishes a standard foundation for building and deploying soft ...
s where library
A library is a collection of Book, books, and possibly other Document, materials and Media (communication), media, that is accessible for use by its members and members of allied institutions. Libraries provide physical (hard copies) or electron ...
code needs to create objects of types which may be subclassed by applications using the framework. They are also used in test-driven development
Test-driven development (TDD) is a way of writing source code, code that involves writing an test automation, automated unit testing, unit-level test case that fails, then writing just enough code to make the test pass, then refactoring both the ...
to allow classes to be put under test.
Factories determine the ''concrete'' type of object
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Object (abstract), an object which does not exist at any particular time or place
** Physical object, an identifiable collection of matter
* Goal, an a ...
to be created, and it is here that the object is created. As the factory only returns an abstract interface to the object, the client code does not know, and is unburdened by, the concrete type of the object which was just created. However, the type of a concrete object is known by the abstract factory. In particular, this means:
* The client code has no knowledge whatsoever of the concrete data type
In computer science and computer programming, a data type (or simply type) is a collection or grouping of data values, usually specified by a set of possible values, a set of allowed operations on these values, and/or a representation of these ...
, not needing to include any header file
An include directive instructs a text file processor to replace the directive text with the content of a specified file.
The act of including may be logical in nature. The processor may simply process the include file content at the location of ...
s or class
Class, Classes, or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used d ...
declarations relating to the concrete type. The client code deals only with the abstract type. Objects of a concrete type are indeed created by the factory, but the client code accesses such objects only through their abstract interface
In computing, an abstraction layer or abstraction level is a way of hiding the working details of a subsystem. Examples of software models that use layers of abstraction include the OSI model for network protocols, OpenGL, and other graphics libr ...
.
* Adding new concrete types is done by modifying the client code to use a different factory, a modification which is typically one line in one file. This is significantly easier than modifying the client code to instantiate a new type, which would require changing ''every'' location in the code where a new object is created.
Applicability
Factories can be used when:
# Creating an object makes reuse impossible without significant duplication of code.
# Creating an object requires access to information or resources that should not be contained within the composing class.
# Managing the lifetime of generated objects must be centralized to ensure consistent behavior within an application.
Factories, specifically factory methods, are common in widget toolkit
A widget toolkit, widget library, GUI toolkit, or UX library is a library (computing), library or a collection of libraries containing a set of graphical control elements (called ''widgets'') used to construct the graphical user interface (GUI) of ...
s and software framework
In computer programming, a software framework is a software abstraction that provides generic functionality which developers can extend with custom code to create applications. It establishes a standard foundation for building and deploying soft ...
s, where library code needs to create objects of types that may be subclassed by applications using the framework.
Parallel class hierarchies often require objects from one hierarchy to be able to create appropriate objects from another.
Factory methods are used in test-driven development
Test-driven development (TDD) is a way of writing source code, code that involves writing an test automation, automated unit testing, unit-level test case that fails, then writing just enough code to make the test pass, then refactoring both the ...
to allow classes to be put under test. If such a class Foo
creates another object Dangerous
that can't be put under automated unit test
Unit testing, component or module testing, is a form of software testing by which isolated source code is tested to validate expected behavior.
Unit testing describes tests that are run at the unit-level to contrast testing at the integration ...
s (perhaps it communicates with a production database that isn't always available), then the creation of Dangerous
objects is placed in the virtual factory method createDangerous
in class Foo
. For testing, TestFoo
(a subclass of Foo
) is then created, with the virtual factory method createDangerous
overridden to create and return FakeDangerous
, a fake object. Unit tests then use TestFoo
to test the functionality of Foo
without incurring the side effect of using a real Dangerous
object.
Benefits and variants
Besides use in design patterns, factories, especially factory methods, have various benefits and variations.
Descriptive names
A factory method has a distinct name. In many object-oriented languages, constructors must have the same name as the class they are in, which can lead to ambiguity if there is more than one way to create an object (see overloading). Factory methods have no such constraint and can have descriptive names; these are sometimes known as ''alternative constructors''. As an example, when complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the for ...
s are created from two real numbers the real numbers can be interpreted as Cartesian or polar coordinates, but using factory methods, the meaning is clear, as illustrated by the following example in C#.
public class Complex
Complex product = Complex.FromPolar(1, Math.PI);
When factory methods are used for disambiguation like this, the raw constructors are often made private to force clients to use the factory methods.
Encapsulation
Factory methods encapsulate the creation of objects.
This can be useful if the creation process is very complex; for example, if it depends on settings in configuration files or on user input.
Consider as an example a program that reads image file
An image file format is a file format for a digital image. There are many formats that can be used, such as JPEG, PNG, and GIF. Most formats up until 2022 were for storing 2D images, not 3D ones. The data stored in an image file format may be c ...
s. The program supports different image formats, represented by a reader class for each format.
Each time the program reads an image, it needs to create a reader of the appropriate type based on some information in the file. This logic can be encapsulated in a factory method. This approach has also been referred to as the Simple Factory.
Java
public class ImageReaderFactory
PHP
class Factory
interface FormatInterface
class FormatString implements FormatInterface
class FormatNumber implements FormatInterface
try catch (Error $e)
try catch (Error $e)
Limits
There are three limits associated with the use of the factory method. The first involves refactoring
In computer programming and software design, code refactoring is the process of restructuring existing source code—changing the '' factoring''—without changing its external behavior. Refactoring is intended to improve the design, structure, ...
existing code; the other two involves extending a class.
* The first limit is that refactoring an existing class to use factories breaks existing clients. For example, if class Complex were a standard class, it might have many clients with code like:Complex c = new Complex(-1, 0);
:Once it is realized that two different factories are needed, the class is changed (to the code shown earlier). But since the constructor is now private, the existing client code no longer compiles.
* The second limit is that, since the pattern relies on using a private constructor, the class cannot be extended. Any subclass must invoke the inherited constructor, but this cannot be done if that constructor is private.
* The third limit is that, if the class were to be extended (e.g., by making the constructor protected—this is risky but feasible), the subclass must provide its own re-implementation of all factory methods with exactly the same signatures. For example, if class StrangeComplex
extends Complex
, then unless StrangeComplex
provides its own version of all factory methods, the call StrangeComplex.FromPolar(1, Math.Pi); will yield an instance of Complex
(the superclass) rather than the expected instance of the subclass. The reflective programming
In computer science, reflective programming or reflection is the ability of a process to examine, introspect, and modify its own structure and behavior.
Historical background
The earliest computers were programmed in their native assembly lang ...
(reflection) features of some languages can avoid this issue.
All three problems could be alleviated by altering the underlying programming language to make factories first-class class members (see also Virtual class
In object-oriented programming, a virtual base class is a nested inner class whose functions and member variables can be overridden and redefined by subclasses of an outer class. Virtual classes are analogous to virtual functions.
The run time ty ...
).
Notes
References
*
{{Refend
Object-oriented programming
Programming language comparisons
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