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{{Short description|Special function called to create an object}}
{{ProgLangCompare}}
In [[class-based programming|class-based]], [[object-oriented programming]], a '''constructor''' (abbreviation: '''ctor''') is a special type of [[Function (computer programming)|function]] called to [[object creation|create an object]]. It prepares the new object for use, often accepting [[Parameter (computer programming)|arguments]] that the constructor uses to set required [[member variable]]s.
A constructor resembles an [[ Most languages allow [[method overloading|overloading]] the constructor in that there can be more than one constructor for a class, with differing parameters. Some languages take consideration of some special types of constructors. Constructors, which concretely use a single class to create objects and return a new instance of the class, are abstracted by [[Factory (object-oriented programming)|factories]], which also create objects but can do so in various ways, using multiple classes or different allocation schemes such as an [[object pool]].
== Types ==
=== Parameterized constructors ===
Constructors that can take at least one argument are termed as parameterized constructors. When an object is declared in a parameterized constructor, the initial values have to be passed as arguments to the constructor function. The normal way of object declaration may not work. The constructors can be called explicitly or implicitly. The method of calling the constructor implicitly is also called the shorthand method.
<syntaxhighlight lang="cpp">
class
private:
int y;
public:
Point() = default;
Point(int x, int
x{x}, y{y} {} // Parameterized constructor
};
</syntaxhighlight>
<syntaxhighlight lang="cpp">
</syntaxhighlight>
=== Default constructors ===
If the programmer does not supply a constructor for an instantiable class, Java compiler inserts a [[default constructor]] into your code on your behalf. This constructor is known as default constructor. You would not find it in your source code (the java file) as it would be inserted into the code during compilation and exists in .class file. The behavior of the default constructor is language dependent. It may initialize data members to zero or other same values, or it may do nothing at all. In Java, a "default constructor" refer to a nullary constructor that is automatically generated by the compiler if no constructors have been defined for the class or in the absence of any programmer-defined constructors (e.g. in Java, the default constructor implicitly calls the [[Superclass (computer science)|superclass]]'s [[nullary]] constructor, then executes an empty body). All fields are left at their initial value of 0 (integer types), 0.0 (floating-point types), false (boolean type), or null (reference types)...
<syntaxhighlight lang="cpp">
class Point {
private:
int x;
int y;
public:
Point(int x = 0, int y = 0); // Default constructor.
};
</syntaxhighlight>
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=== Copy constructors ===
{{see also|Copy constructor (C++)}}
Like C++, Java also supports "Copy
While copy constructors are usually abbreviated copy ctor or cctor, they have nothing to do with class constructors used in [[.NET]] using the same abbreviation.
=== Conversion constructors ===
Conversion constructors provide a means for a compiler to implicitly create an object belonging to one class based on an object of a different type. These constructors are usually invoked implicitly to convert arguments or operands to an appropriate type, but they may also be called explicitly.
=== Move constructors ===
In C++, [[move constructor (C++)|move constructors]] take an Rvalue reference to an object of the class, and are used to implement ownership transfer of the parameter object's resources.
== Syntax ==
* [[Java (programming language)|Java]], [[C++]], [[C Sharp (programming language)|C#]], [[ActionScript]], {{nowrap|[[PHP]] 4}}, and [[MATLAB]] have a naming convention in which constructors have the same name as the class with which they are associated.
*
* In PHP 5, a recommended name for a constructor is <code>__construct</code>. For backwards compatibility, a method with the same name as the class will be called if <code>__construct</code> method can not be found. Since PHP 5.3.3, this works only for non-namespaced classes.<ref name="php5cpnstructor">[http://www.php.net/manual/en/language.oop5.decon.php Constructors and Destructors], from PHP online documentation</ref>
* In PHP 7, you should always name the constructor as <code>__construct</code>. Methods with the same name as the class will trigger an E_DEPRECATED level error.<ref name="php5cpnstructor"
* In [[Perl]], constructors are, by convention, named "new" and have to do a fair amount of object creation.
* In [[Moose perl|Moose object system]] for Perl, constructors (named ''new'') are automatically created and are extended by specifying a ''BUILD'' method.
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== Memory organization ==
In Java, C#, and VB .NET, the constructor creates reference type objects on the heap, whereas primitive types (such as <code>int</code>, <code>double</code>, etc.) are stored on the [[Stack-based memory allocation|stack]] (though some languages allow for manually allocating objects on the stack through a <code>stackalloc</code> modifier). VB .NET and C# also allow the use of the <code>new</code> operator to create value type objects, but these value type objects are created on the stack regardless of whether the operator is used or not. In these languages, object destruction occurs when the object has no references and then gets destroyed by the garbage collector.
In C++, objects are created on the stack when the constructor is invoked without the <code>new</code> operator, and created on the heap when the constructor is invoked with the <code>new</code> operator (which returns a pointer to the object). Stack objects are deleted implicitly when they go out of scope, while heap objects must be deleted implicitly by a destructor or explicitly by using the <code>delete</code> operator. By using the "[[Resource Acquisition is Initialization]]" (RAII) idiom, resource management can be greatly simplified.
== Language details ==<!-- see also Category:Programming language comparisons -->
Constructors are implemented in different [[programming language]]s in various ways, including:
=== C++ ===
In [[C++]], the name of the constructor is the name of the class. It returns nothing. It can have parameters like any [[Method (computer programming)|member function]]. Constructor functions are usually declared in the public section, but can also be declared in the protected and private sections, if the user wants to restrict access to them.
The constructor has two parts. First is the [[initializer list]] which follows the [[parameter (computer science)|parameter list]] and before the method body. It starts with a colon and entries are comma-separated. The initializer list is not required, but offers the opportunity to provide values for data members and avoid separate assignment statements. The initializer list is required if you have ''const'' or reference type data members, or members that do not have parameterless constructor logic. Assignments occur according to the order in which data members are declared (even if the order in the initializer list is different).<ref>https://stackoverflow.com/questions/1242830/constructor-initialization-list-evaluation-order Constructor</ref> The second part is the body, which is a normal method body enclosed in curly brackets. It is generally cheaper and better practice to use the initializer list as much as possible, and only use the constructor body for non-assignment operations and assignments where the initializer list cannot be used or is otherwise insufficient.
C++ allows more than one constructor. The other constructors must have different parameters. Additionally constructors which contain parameters which are given default values, must adhere to the restriction that not all parameters are given a default value. This is a situation which only matters if there is a default constructor. The constructor of a [[base class]] (or base classes) can also be called by a derived class. Constructor functions are not inherited and their addresses cannot be referenced. When memory allocation is required, the ''new'' and ''delete'' operators are called implicitly.
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<syntaxhighlight lang="cpp">
import std;
class PolarPoint {
private:
double
double y;
public:
PolarPoint(double r = 1.0, double theta = 0.0): // Constructor, parameters with default values.
x{r * std::cos(theta)}, y{r * std::sin(theta)} /* <- Initializer list */ {
std::println("Point: x = {}, y = {}", x, y); // Constructor body
}
};
</syntaxhighlight>
Example invocations:
<syntaxhighlight lang="cpp">
PolarPoint a;
</syntaxhighlight>
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=== C# ===
Example [[C Sharp (programming language)|C#]] constructor:
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==== C# static constructor ====
In [[C Sharp (programming language)|C#]], a ''static constructor'' is a static data initializer.<ref name="Albahari">{{cite book |last=Albahari |first=Joseph |title= C# 10 in a Nutshell |publisher= O'Reilly |isbn= 978-1-098-12195-2}}</ref>{{rp|111-112}} Static constructors are also called ''class constructors''. Since the actual method generated has the name ''.cctor'' they are often also called "cctors".<ref>{{cite web|url=http://ericlippert.com/2013/02/06/static-constructors-part-one/ |title=Fabulous Adventures in Coding |publisher=Eric Lippert |date=2013-02-06|access-date=2014-04-05}}</ref><ref>{{cite book|url=https://books.google.com/books?id=oAcCRKd6EZgC&pg=PA222 |title=Expert .NET 2.0 IL Assembler |publisher=APress |date=2006-01-01|isbn=9781430202233 |access-date=2014-04-05}}</ref>
Static constructors allow complex static variable initialization.<ref>{{Cite web|url=https://www.microsoft.com/en-us/download/details.aspx?id=55984|title=Download Visual Studio 2005 Retired documentation from Official Microsoft Download Center|website=Microsoft Store - Download Center}}</ref>
Static constructors are called implicitly when the class is first accessed. Any call to a class (static or constructor call), triggers the static constructor execution.
Static constructors are [[thread safe]] and implement a [[singleton pattern]]. When used in a [[generic programming]] class, static constructors are called at every new generic instantiation one per type.<ref name=Skeet>{{cite book |last=Skeet|first=Jon|title= C# in Depth |date=23 March 2019 |publisher= Manning |isbn= 978-1617294532}}</ref>{{rp|38}}<ref name="Albahari"/>{{rp|111}} Static variables are instantiated as well.
<syntaxhighlight lang="csharp">
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</syntaxhighlight>
=== ColdFusion Markup Language (CFML) ===
[[ColdFusion Markup Language]] (CFML) uses a method named '<code>init</code>' as a constructor method.
'''Cheese.cfc'''
<syntaxhighlight lang="
component {
// properties
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Create instance of a cheese.
<syntaxhighlight lang="
myCheese = new Cheese( 'Cheddar' );
</syntaxhighlight>
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Since ColdFusion 10,<ref>[https://wikidocs.adobe.com/wiki/display/coldfusionen/cfcomponent CFComponent]</ref> CFML has also supported specifying the name of the constructor method:
<syntaxhighlight lang="
component initmethod="Cheese" {
// properties
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=== Eiffel ===
In [[Eiffel (programming language)|Eiffel]], the routines which initialize new objects are called ''creation procedures''. Creation procedures have the following traits:
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* Creation procedures must leave the newly initialized object in a state that satisfies the class invariant.{{Efn|Because the inherited class invariant must be satisfied, there is no mandatory call to the parents' constructors.}}
Although object creation involves some subtleties,<ref name="eiffel standard">
* Create a new direct instance of type <code lang="eiffel">T</code>.{{Efn|The Eiffel standard requires fields to be initialized on first access, so it is not necessary to perform default field initialization during object creation.}}
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=== F# ===
In [[F Sharp (programming language)|F#]], a constructor can include any <code>let</code> or <code>do</code> statements defined in a class. <code>let</code> statements define private fields and <code>do</code> statements execute code. Additional constructors can be defined using the <code>new</code> keyword.
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=== Java ===
In [[Java (programming language)|Java]], constructors differ from other methods in that:
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Java permit users to call one constructor in another constructor using <code>this()</code> keyword.
But <code>this()</code> must be first statement. <ref>
<syntaxhighlight lang="java">
class
public X() { // Non-parameterized constructor
this(1); // Calling of constructor
System.out.println("
}
public X(int a) { // Parameterized constructor
System.out.println("
}
}
public class Example {
public static void main(String[] args) {
X x = new X();
}
}
</syntaxhighlight>
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<syntaxhighlight lang="java">
// Declaration of instance variable(s).
private int data;
// Definition of the constructor.
public
this(1);
}
// Overloading a constructor
public
data = input; // This is an assignment
}
}
class Y extends X {
private int
public Y() {
super();
data2 = 1;
}
public Y(int input1, int input2) {
super(input1);
data2 = input2
}
}
public class Example {
public static void main(String[] args) {
Y y = new Y(42, 43);
}
}
</syntaxhighlight>
A constructor taking zero number of arguments is called a "no-arguments" or "no-arg" constructor.<ref>{{cite web|url=http://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html|title= Providing Constructors for Your Classes |publisher=Oracle Corporation|date=2013|access-date=2013-12-20}}</ref>
=== JavaScript/TypeScript ===
As of ES6, [[JavaScript]] has direct constructors like many other programming languages. They are written as such
<syntaxhighlight lang="javascript">
class FooBar {
constructor(baz) {
this.baz = baz;
}
}
</syntaxhighlight>
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<syntaxhighlight lang="javascript">
const foo = new FooBar('7');
</syntaxhighlight>
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<syntaxhighlight lang="javascript">
function FooBar (baz) {
this.baz = baz;
}
</syntaxhighlight>
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This is instantiated the same way as above.
The [[TypeScript]] equivalent of this would be:
<syntaxhighlight lang="typescript">
class FooBar {
baz: string;
constructor(baz: string) {
this.baz = baz;
}
}
const foo: FooBar = new FooBar('7');
</syntaxhighlight>
=== Object Pascal ===
In [[Object Pascal]], the constructor is similar to a [[factory method]]. The only syntactic difference to regular methods is the keyword <code>constructor</code> in front of the name (instead of <code>procedure</code> or <code>function</code>). It can have any name, though the convention is to have <code>Create</code> as prefix, such as in <code>CreateWithFormatting</code>. Creating an instance of a class works like calling a static method of a class: <code>TPerson.Create('Peter')</code>.
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=== OCaml ===
In [[OCaml]], there is one constructor. Parameters are defined right after the class name. They can be used to initialize instance variables and are accessible throughout the class. An anonymous hidden method called <code>initializer</code> allows to evaluate an expression immediately after the object has been built.<ref>{{Cite web|url=https://ocaml.org/manual/5.3/index.html|title=OCaml - The OCaml Manual|website=ocaml.org}}</ref>
<syntaxhighlight lang="ocaml">
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=== PHP ===
In [[PHP]] version 5 and above, the constructor is a method named <code>__construct()</code> (notice that it's a double underscore), which the keyword <code>new</code> automatically calls after creating the object. It is usually used to automatically perform initializations such as property initializations. Constructors can also accept arguments, in which case, when the <code>new</code> statement is written, you also need to send the constructor arguments for the parameters.<ref name="php5cpnstructor"/>
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</syntaxhighlight>
In PHP, a class is only allowed to declare a maximum of one constructor method. Static methods, factory classes or optional constructor arguments are some ways to facilitate multiple ways to create objects of a PHP class.
=== Perl 5 ===
In [[Perl]] version 5, by default, constructors are [[factory method]]s, that is, methods that create and return the object, concretely meaning create and return a blessed reference. A typical object is a reference to a hash, though rarely references to other types are used too. By convention the only constructor is named ''new'', though it is allowed to name it otherwise, or to have multiple constructors. For example, a Person class may have a constructor named ''new'', and a constructor ''new_from_file'' which reads a file for Person attributes, and ''new_from_person'' which uses another Person object as a template.
<syntaxhighlight lang="perl">
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==== Perl 5 with Moose ====
In the [[Moose perl|Moose object system]] for Perl, most of this boilerplate can be omitted, a default ''new'' is created, attributes can be specified, and whether they can be set, reset, or are required. In addition, any extra constructor functionality can be included in a ''BUILD'' method which the Moose generated constructor will call, after it has checked the arguments. A ''BUILDARGS'' method can be specified to handle constructor arguments not in hashref / key => value form.
<syntaxhighlight lang="perl">
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=== Python ===
In [[Python (programming language)|Python]], constructors are defined by one or both of <code>__new__</code> and <code>__init__</code> methods. A new instance is created by calling the class as if it were a function, which calls the <code>__new__</code> and <code>__init__</code> methods. If a constructor method is not defined in the class, the next one found in the class's Method Resolution Order will be called.<ref name="auto">{{Cite web|url=https://docs.python.org/3/reference/datamodel.html|title=3. Data model|website=Python documentation}}</ref>
In the typical case, only the <code>__init__</code> method need be defined. (The most common exception is for immutable objects.)
<syntaxhighlight lang="
def __init__(self, value: int) -> None:
print("Initialising instance...")
self.payload: int = value
if __name__ == "__main__":
exampleInstance: ExampleClass = ExampleClass(42)
print(exampleInstance.payload)
</syntaxhighlight>
This prints:
<pre>
Creating new instance...
Initialising instance...
42
</pre>
Classes normally act as [[Factory (object-oriented programming)|factories]] for new instances of themselves, that is, a class is a callable object (like a function), with the call being the constructor, and calling the class returns an instance of that class. However the <code>__new__</code> method is permitted to return something other than an instance of the class for specialised purposes. In that case, the <code>__init__</code> is not invoked.<ref
=== Raku ===
In [[Raku (programming language)|Raku]], even more boilerplate can be omitted, given that a default ''new'' method is inherited, attributes can be specified, and whether they can be set, reset, or are required. In addition, any extra constructor functionality can be included in a ''BUILD'' method which will get called to allow for custom initialization. A ''TWEAK'' method can be specified to post-process any attributes already (implicitly) initialized.
<syntaxhighlight lang="perl6">
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=== Ruby ===
In [[Ruby (programming language)|Ruby]], constructors are created by defining a method called <code>initialize</code>. This method is executed to initialize each new instance.
<syntaxhighlight lang="rbcon">
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</syntaxhighlight>
===
[[Rust (programming language)|Rust]] does not have constructors in the sense of object-oriented programming, but often structs have a <code>new()</code> method that essentially acts as a constructor. The return type is usually indicated as <code>Self</code>.
<syntaxhighlight lang="rust">
struct Point {
x: i32,
y: i32,
}
impl Point {
pub fn new(x: i32, y: i32) -> Self {
Point { x, y }
}
}
fn main() {
let p: Point = Point::new(10, 20);
println!("Point is at ({}, {})", p.x, p.y);
}
</syntaxhighlight>
=== Visual Basic .NET ===
In [[Visual Basic .NET]], constructors use a method declaration with the name "<code>New</code>".
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== See also ==
* [[Resource acquisition is initialization]] (RAII)
* [[Allocation site]]
* [[Creational pattern]]
* [[Destructor (computer
* [[Global constructor]] in C++, and its C counterpart, [[((constructor))]] function attribute
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