Constructor (object-oriented programming)

In class-based, object-oriented programming, a constructor (abbreviation: ctor) is a special type of function called to create an object. It prepares the new object for use, often accepting arguments that the constructor uses to set required member variables.

A constructor resembles an instance method, but it differs from a method in that it has no explicit return type, it is not implicitly inherited and it usually has different rules for scope modifiers. Constructors often have the same name as the declaring class. They have the task of initializing the object's data members and of establishing the invariant of the class, failing if the invariant is invalid. A properly written constructor leaves the resulting object in a valid state. Immutable objects must be initialized in a constructor.

Most languages allow 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 factories, which also create objects but can do so in various ways, using multiple classes or different allocation schemes such as an object pool.

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.

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'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)...

Copy constructors
Like C++, Java also supports "Copy Constructor". But, unlike C++, Java doesn't create a default copy constructor if you don't write your own. Copy constructors define the actions performed by the compiler when copying class objects. A Copy constructor has one formal parameter that is the type of the class (the parameter may be a reference to an object). It is used to create a copy of an existing object of the same class. Even though both classes are the same, it counts as a conversion constructor. 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 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, C++, C#, ActionScript, 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 . For backwards compatibility, a method with the same name as the class will be called if   method can not be found. Since PHP 5.3.3, this works only for non-namespaced classes.
 * In PHP 7, you should always name the constructor as . Methods with the same name as the class will trigger an E_DEPRECATED level error.
 * In Perl, constructors are, by convention, named "new" and have to do a fair amount of object creation.
 * In Moose object system for Perl, constructors (named new) are automatically created and are extended by specifying a BUILD method.
 * In Visual Basic .NET, the constructor is called " ".
 * In Python, the constructor is split over two methods, " " and " ". The  method is responsible for allocating memory for the instance, and receives the class as an argument (conventionally called " "). The   method (often called "the initialiser") is passed the newly created instance as an argument (conventionally called " ").
 * Object Pascal constructors are signified by the keyword " " and can have user-defined names (but are mostly called " ").
 * In Objective-C, the constructor method is split across two methods, " " and " " with the  method setting aside (allocating) memory for an instance of the class, and the   method handling the bulk of initializing the instance. A call to the method " " invokes both the   and the   methods, for the class instance.

Memory organization
In Java, C#, and VB .NET, the constructor creates reference type objects in a special memory structure called the "heap". Value types (such as int, double, etc.) are created in a sequential structure called the "stack". VB .NET and C# also allow the use of the new 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 C++, objects are created on the stack when the constructor is invoked without the new operator, and created on the heap when the constructor is invoked with the new operator. 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 delete operator.

Language details
Constructors are implemented in different programming languages 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 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 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). The second part is the body, which is a normal method body enclosed in curly brackets.

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.

A copy constructor has a parameter of the same type passed as const reference, for example Vector(const Vector& rhs). If it is not provided explicitly, the compiler uses the copy constructor for each member variable or simply copies values in case of primitive types. The default implementation is not efficient if the class has dynamically allocated members (or handles to other resources), because it can lead to double calls to delete (or double release of resources) upon destruction.

Example invocations:

On returning objects from functions or passing objects by value, the objects copy constructor will be called implicitly, unless return value optimization applies.

C++ implicitly generates a default copy constructor which will call the copy constructors for all base classes and all member variables unless the programmer provides one, explicitly deletes the copy constructor (to prevent cloning) or one of the base classes or member variables copy constructor is deleted or not accessible (private). Most cases calling for a customized copy constructor (e.g. reference counting, deep copy of pointers) also require customizing the destructor and the copy assignment operator. This is commonly referred to as the Rule of three.

C#
Example C# constructor:

C# static constructor
In C#, a static constructor is a static data initializer. Static constructors are also called class constructors. Since the actual method generated has the name .cctor they are often also called "cctors".

Static constructors allow complex static variable initialization. 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. Static variables are instantiated as well.

ColdFusion Markup Language (CFML)
ColdFusion Markup Language (CFML) uses a method named ' ' as a constructor method.

Cheese.cfc

Create instance of a cheese.

Since ColdFusion 10, CFML has also supported specifying the name of the constructor method:

Eiffel
In Eiffel, the routines which initialize new objects are called creation procedures. Creation procedures have the following traits:


 * Creation procedures have no explicit return type (by definition of procedure).
 * Creation procedures are named.
 * Creation procedures are designated by name as creation procedures in the text of the class.
 * Creation procedures can be explicitly invoked to re-initialize existing objects.
 * Every effective (i.e., concrete or non-abstract) class must designate at least one creation procedure.
 * Creation procedures must leave the newly initialized object in a state that satisfies the class invariant.

Although object creation involves some subtleties, the creation of an attribute with a typical declaration  as expressed in a creation instruction   consists of the following sequence of steps:


 * Create a new direct instance of type.
 * Execute the creation procedure  to the newly created instance.
 * Attach the newly initialized object to the entity.

In the first snippet below, class  is defined. The procedure  is coded after the keyword.

The keyword  introduces a list of procedures which can be used to initialize instances. In this case the list includes, a procedure with an empty implementation inherited from class  , and the   procedure coded within the class.

In the second snippet, a class which is a client to  has a declarations   and   of type.

In procedural code,  is created as the origin (0.0, 0.0). Because no creation procedure is specified, the procedure  inherited from class   is used. This line could have been coded. Only procedures named as creation procedures can be used in an instruction with the  keyword. Next is a creation instruction for, providing initial values for the  's coordinates. The third instruction makes an ordinary instance call to the  procedure to reinitialize the instance attached to   with different values.

F#
In F#, a constructor can include any  or   statements defined in a class. statements define private fields and  statements execute code. Additional constructors can be defined using the  keyword.

Java
In Java, constructors differ from other methods in that:


 * Constructors never have an explicit return type.
 * Constructors cannot be directly invoked (the keyword “ ” invokes them).
 * Constructors should not have non-access modifiers.

Java constructors perform the following tasks in the following order:


 * 1) Call the default constructor of the superclass if no constructor is defined.
 * 2) Initialize member variables to the specified values.
 * 3) Executes the body of the constructor.

Java permit users to call one constructor in another constructor using  keyword. But  must be first statement.

Java provides access to the superclass's constructor through the  keyword.

A constructor taking zero number of arguments is called a "no-arguments" or "no-arg" constructor.

JavaScript
As of ES6, JavaScript has direct constructors like many other programming languages. They are written as such

This can be instantiated as such

The equivalent of this before ES6, was creating a function that instantiates an object as such

This is instantiated the same way as above.

Object Pascal
In Object Pascal, the constructor is similar to a factory method. The only syntactic difference to regular methods is the keyword  in front of the name (instead of   or  ). It can have any name, though the convention is to have  as prefix, such as in. Creating an instance of a class works like calling a static method of a class:.

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  allows to evaluate an expression immediately after the object has been built.

PHP
In PHP version 5 and above, the constructor is a method named  (notice that it's a double underscore), which the keyword   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  statement is written, you also need to send the constructor arguments for the parameters.

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 methods, 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.

Perl 5 with Moose
In the 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.

In both cases the Person class is instiated like this:

Python
In Python, constructors are defined by one or both of  and   methods. A new instance is created by calling the class as if it were a function, which calls the  and   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.

In the typical case, only the  method need be defined. (The most common exception is for immutable objects.)

Classes normally act as 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  method is permitted to return something other than an instance of the class for specialised purposes. In that case, the  is not invoked.

Raku
In 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.

The Person class is instantiated like this:

Alternatively, the named parameters can be specified using the colon-pair syntax in Perl 6:

And should you have set up variables with names identical to the named parameters, you can use a shortcut that will use the name of the variable for the named parameter:

Ruby
In Ruby, constructors are created by defining a method called. This method is executed to initialize each new instance.

Visual Basic .NET
In Visual Basic .NET, constructors use a method declaration with the name " ".